Bull kelp (Nereocystis leutkeana) forests along the coast for northern California have decreased dramatically as a result of a ‘perfect storm’ of multiple environmental stressors. The disappearance of a predatory sea star and subsequent increase in purple sea urchins (Strongylocentrotus purpuratus) and the recurrence of marine heat waves have caused these once diverse ecosystems to be rapidly converted into relative species-depauperate urchin barrens. By examining the interactive effects of both a rapidly changing abiotic environment and the increase in urchin grazing pressure that is affecting this vital ecosystem, we can better understand its ultimate fate and make better-informed decisions to manage and protect it. As once large and persistent kelp forests are converted into fragmented landscapes of small kelp patches, kelp’s ability to take up dissolved inorganic carbon and reduce nearby acidity and increase both dissolved oxygen and bio-available calcium carbonate may be reduced, preventing it from serving as an environmental stress-free ‘oasis’ of reduced environmental stresses for local marine organisms and affecting ecosystem dynamics. In my first chapter, I examined whether small, fragmented kelp patches are able to retain their ability to alter local seawater chemistry to the same extent a large persistent kelp forests that have been studied previously. I found that in the canopies of small kelp patches, multiple parameters of carbonate chemistry fluctuated more than in the kelp benthos and in adjacent urchin barrens, consistent with metabolic activity by the kelp. Further, kelp fragments increased pH and aragonite saturation and decreased pCO2 during the day to a similar degree as large, intact kelp forests. These results suggest that small kelp patches could mitigate OA stress during the day and serve as spatial and temporal refugia for canopy-dwelling organisms. I also found that the benthic environment in kelp forests and adjacent urchin barrens is subject to unbuffered decreases in temperature, dissolved oxygen and pH. Thus, in chapter two, I assessed how current-day and future-predicted fluctuations in the duration and magnitude of these upwelling-associated stressors would impact the grazing, growth, and survivorship of purple urchins from kelp forest and urchin barren habitats. With upwelling predicted to increase in both intensity and duration with global climate change, understanding whether urchins from different habitats are differentially affected by upwelling-related stressors will give insight into how current and future stressors may be able to help ‘tip the scales’ and convert the increasing number of urchin barrens back into healthy productive kelp forests. I found condition-dependent susceptibility in urchins to increased magnitude and duration upwelling-related stressors. Grazing and gonadal development in kelp forest urchins was most negatively affected by distant future upwelling conditions, whereas in urchin barren urchins, grazing and survival were sensitive to exposure to upwelling in general, and also to increase in magnitudes of acidity, hypoxia, and temperature across both upwelling and non-upwelling events in the future. These results have important implications for population dynamics of urchins and their interactions with bull kelp, which could strongly affect ecosystem dynamics and transitions between kelp forests and urchin barrens. Taken together, the two chapters my thesis provide valuable insight into the potential resilience of bull kelp, a critical foundation species in northeastern Pacific coastal habitats, in the face of a rapidly changing multi-stressor environment.
Posts Tagged 'communityMF'
The ability of fragmented kelp forests to mitigate ocean acidification and the effects of seasonal upwelling on kelp-purple sea urchin interactions
Published 25 June 2020 Science ClosedTags: abundance, algae, biogeochemistry, biological response, chemistry, communityMF, echinoderms, field, laboratory, mesocosms, morphology, mortality, multiple factors, North Pacific, otherprocess, performance
External pH modulation during the growth of Vibrio tapetis , the etiological agent of Brown Ring Disease
Published 22 May 2020 Science ClosedTags: abundance, biological response, communityMF, growth, laboratory, mollusks, multiple factors, otherprocess, prokaryotes
Aims
Brown Ring Disease (BRD) is an infection of the Manila clam Ruditapes philippinarum due to the pathogen Vibrio tapetis . During BRD, clams are facing immunodepression and shell biomineralization alteration. In this paper, we studied the role of pH on the growth of the pathogen and formulated hypothesis on the establishment of BRD by V. tapetis .
Methods and Results
In this study, we monitored the evolution of pH during the growth of V. tapetis in a range of pH and temperatures. We also measured the pH of Manila clam hemolymph and extrapallial fluids during infection by V. tapetis . We highlighted that V. tapetis modulates the external pH during its growth, to a value of 7.70. During the development of BRD, V. tapetis also influences extrapallial fluids and hemolymph pH in vitro in the first hours of exposure and in vivo after 3 days of infection.
Conclusions
Our experiments have shown a close interaction between V. tapetis CECT4600, a pathogen of Manila clam that induces BRD, and the pH of different compartments of the animals during infection. These results indicate that that the bacterium, through a direct mechanism or as a consequence of physiological changes encountered in the animal during infection, is able to interfere with the pH of Manila clam fluids. This pH modification might promote the infection process or at least create an imbalance within the animal, that would favor its persistence. This last hypothesis should be tested in future experiment.
Significance and Impact of Study
This study is the first observation of pH modifications in the context of BRD and might orient future research on the fine mechanisms of pH modulation associated to BRD.
High CO2 and warming affect microzooplankton food web dynamics in a Baltic Sea summer plankton community
Published 11 May 2020 Science ClosedTags: Baltic, biological response, BRcommunity, community composition, communityMF, field, growth, laboratory, mesocosms, multiple factors, otherprocess, performance, phytoplankton, temperature, zooplankton
Aquatic ecosystems face a multitude of environmental stressors, including warming and acidification. While warming is expected to have a pronounced effect on plankton communities, many components of the plankton seem fairly robust towards realistic end-of-century acidification conditions. However, interactions of the two stressors and the inclusion of further factors such as nutrient concentration and trophic interactions are expected to change this outcome. We investigated the effects of warming and high CO2 on a nutrient-deplete late summer plankton community from the Kiel Fjord, Baltic Sea, using a mesocosm setup crossing two temperatures with a gradient of CO2. Phytoplankton and microzooplankton (MZP) growth rates as well as biomass, taxonomic composition, and grazing rates of MZP were analysed. We observed effects of high CO2, warming, and their interactions on all measured parameters. The occurrence and direction of the effects were dependent on the phytoplankton or MZP community composition. In addition, the abundance of small-sized phytoplankton was identified as one of the most important factors in shaping the MZP community composition. Overall, our results indicate that an estuarine MZP community used to strong natural fluctuations in CO2 can still be affected by a moderate increase in CO2 if it occurs in combination with warming and during a nutrient-deplete post-bloom situation. This highlights the importance of including trophic interactions and seasonality aspects when assessing climate change effects on marine zooplankton communities.
Continue reading ‘High CO2 and warming affect microzooplankton food web dynamics in a Baltic Sea summer plankton community’Model simulation of seasonal growth of Fucus vesiculosus in its benthic community
Published 10 March 2020 Science ClosedTags: algae, biogeochemistry, biological response, chemistry, communityMF, communitymodeling, crustaceans, field, growth, laboratory, mesocosms, modeling, mollusks, multiple factors, North Atlantic, performance, phytoplankton, primary production, reproduction, respiration, temperature
Numerical models are a suitable tool to quantify impacts of predicted climate change on complex ecosystems but are rarely used to study effects on benthic macroalgal communities. Fucus vesiculosus L. is a habitat‐forming macroalga in the Baltic Sea and alarming shifts from the perennial Fucus community to annual filamentous algae are reported. We developed a box model able to simulate the seasonal growth of the Baltic Fucus–grazer–epiphyte system. This required the implementation of two state variables for Fucus biomass in units of carbon (C) and nitrogen (N). Model equations describe relevant physiological and ecological processes, such as storage of C and N assimilates by Fucus, shading effects of epiphytes or grazing by herbivores on both Fucus and epiphytes, but with species‐specific rates and preferences. Parametrizations of the model equations and required initial conditions were based on measured parameters and process rates in the near‐natural Kiel Outdoor Benthocosm (KOB) experiments during the Biological Impacts of Ocean Acidification project. To validate the model, we compared simulation results with observations in the KOB experiment that lasted from April 2013 until March 2014 under ambient and climate‐change scenarios, that is, increased atmospheric temperature and partial pressure of carbon dioxide. The model reproduced the magnitude and seasonal cycles of Fucus growth and other processes in the KOBs over 1 yr under different scenarios. Now having established the Fucus model, it will be possible to better highlight the actual threat of climate change to the Fucus community in the shallow nearshore waters of the Baltic Sea.
Continue reading ‘Model simulation of seasonal growth of Fucus vesiculosus in its benthic community’
Cascading effects of climate change on plankton community structure
Published 26 February 2020 Science ClosedTags: abundance, algae, annelids, biological response, BRcommunity, communityMF, crustaceans, laboratory, multiple factors, North Atlantic, otherprocess, phytoplankton, review, temperature, zooplankton
Plankton communities account for at least half of global primary production and play a key role in the global carbon cycle. Warming and acidification may alter the interaction chains in these communities from the bottom and top of the food web. Yet, the relative importance of these potentially complex interactions has not yet been quantified. Here, we examine the isolated and combined effects of warming, acidification, and reductions in phytoplankton and predator abundances in a series of factorial experiments. We find that warming directly impacts the top of the food web, but that the intermediate trophic groups are more strongly influenced by indirect effects mediated by altered top-down interactions. Direct manipulations of predator and phytoplankton abundance reveal similar strong top-down interactions following top predator decline. A meta-analysis of published experiments further supports the conclusion that warming has stronger direct impacts on the top and bottom of the food web rather than the intermediate trophic groups, with important differences between freshwater and marine plankton communities. Our results reveal that the trophic effect of warming cascading down from the top of the plankton food web is a powerful agent of global change.
Continue reading ‘Cascading effects of climate change on plankton community structure’
Symbiont community diversity is more variable in corals that respond poorly to stress
Published 18 February 2020 Science ClosedTags: biological response, community composition, communityMF, corals, individualmodeling, laboratory, modeling, mortality, multiple factors, North Pacific, otherprocess, phytoplankton, prokaryotes, protists, temperature
Coral reefs are declining globally as climate change and local water quality press environmental conditions beyond the physiological tolerances of holobionts—the collective of the host and its microbial symbionts. To assess the relationship between symbiont composition and holobiont stress tolerance, community diversity metrics were quantified for dinoflagellate endosymbionts (Family: Symbiodiniaceae) from eight Acropora millepora genets that thrived under or responded poorly to various stressors. These eight selected genets represent the upper and lower tails of the response distribution of 40 coral genets that were exposed to four stress treatments (and control conditions) in a 10‐day experiment. Specifically, four ‘best performer’ coral genets were analyzed at the end of the experiment because they survived high temperature, high pCO2, bacterial exposure, or combined stressors, whereas four ‘worst performer’ genets were characterized because they experienced substantial mortality under these stressors. At the end of the experiment, seven of eight coral genets mainly hosted Cladocopium symbionts, whereas the eighth genet was dominated by both Cladocopium and Durusdinium symbionts. Symbiodiniaceae alpha and beta diversity were higher in worst performing genets than in best performing genets. Symbiont communities in worst performers also differed more after stress exposure relative to their controls (based on normalized proportional differences in beta diversity), than did best performers. A generalized joint attribute model estimated the influence of host genet and treatment on Symbiodiniaceae community composition and identified strong associations among particular symbionts and host genet performance, as well as weaker associations with treatment. Although dominant symbiont physiology and function contribute to host performance, these findings emphasize the importance of symbiont community diversity and stochasticity as components of host performance. Our findings also suggest that symbiont community diversity metrics may function as indicators of resilience and have potential applications in diverse disciplines from climate change adaptation to agriculture and medicine.
Adaptation to pH stress by Vibrio fischeri can affect its symbiosis with the Hawaiian bobtail squid (Euprymna scolopes)
Published 29 January 2020 Science ClosedTags: abundance, biological response, communityMF, laboratory, mollusks, multiple factors, otherprocess, protists
Many microorganisms engaged in host-microbe interactions pendulate between a free-living phase and a host-affiliated stage. How adaptation to stress during the free-living phase affects host-microbe associations is unclear and understudied. To explore this topic, the symbiosis between Hawaiian bobtail squid (Euprymna scolopes) and the luminous bacterium Vibrio fischeri was leveraged for a microbial experimental evolution study. V. fischeri experienced adaptation to extreme pH while apart from the squid host. V. fischeri was serially passaged for 2000 generations to the lower and upper pH growth limits for this microorganism, which were pH 6.0 and 10.0, respectively. V. fischeri was also serially passaged for 2000 generations to vacillating pH 6.0 and 10.0. Evolution to pH stress both facilitated and impaired symbiosis. Microbial evolution to acid stress promoted squid colonization and increased bioluminescence for V. fischeri , while symbiont adaptation to alkaline stress diminished these two traits. Oscillatory selection to acid and alkaline stress also improved symbiosis for V. fischeri , but the facilitating effects were less than that provided by microbial adaptation to acid stress. In summary, microbial adaptation to harsh environments amid the free-living phase may impact the evolution of host-microbe interactions in ways that were not formerly considered.
Biogenic acidification of Portuguese oyster Magallana angulata mariculture can be mediated through introducing brown seaweed Sargassum hemiphyllum
Published 24 January 2020 Science ClosedTags: algae, biological response, calcification, communityMF, field, laboratory, mesocosms, mollusks, multiple factors, North Pacific, physiology, respiration
Highlights
• Monoculture of oysters produces excess CO2, affecting carbon fluxes.
• Seaweed can eliminate CO2 released by oysters.
• Multi-trophic culture of oysters and seaweed can mitigate oysters monoculture negative impacts.
Abstract
The physiological responses of aquaculture organisms (e.g., oyster and seaweed) have the potential to affect seawater carbon fluxes and subsequently are affected by these seawater changes. In this study, a laboratory experiment and a field mesocosm experiment were carried out in Daya Bay, southern China. In the laboratory experiment, Portuguese oyster Magallana angulata and the brown seaweed Sargassum hemiphyllum were mono-cultured in 20-L transparent glass bottles for 24 h. Water sample were collected at four incubation time points (i.e. 0 h, 4 h, 12 h and 24 h) to examine their physiological responses across the incubation period. The results showed that the oyster calcification rate was not significantly changed among 4 h, 12 h and 24 h. On the other hand, during the 24 h incubation time, the oyster respiration rate, seawater pH, dissolved oxygen (DO), and CO32– concentration were significantly declined, but the seawater CO2 concentration was increased. For the seaweed, from 0 h to 12 h, seawater CO2 and HCO3– concentrations were significantly declined. However, the seawater pH and DO concentration were increased. In the field experiment, oyster and seaweed were cultured in mesocosm bags. The effects of different culture models of M. angulata and S. hemiphyllum (i.e. oyster monoculture, seaweed monoculture and oyster-seaweed co-culture) on seawater CO2‑carbonate system and air-sea CO2 flux (FCO2) were investigated after 24 h incubation. The results showed that DIC, HCO3– and CO2 concentrations and the partial pressure of CO2 in co-culture bags were significantly lower than the control bags (without any culture organisms) and oyster bags, indicated that S. hemiphyllum can effectively absorb the CO2 released by the oysters. The negative values of air-sea FCO2 in the co-culture bags represent a CO2 sink from the atmosphere to the sea. These results demonstrated that aquaculture organism monoculture could result in a stress for itself, and there could be an interspecies mutual benefit for both M. angulata and S. hemiphyllum in the co-culture system. The negative environmental impacts of mono-trophic oyster aquaculture in this view could be mediated with the multi-trophic inclusion of seaweed.
Predator prey interactions between predatory gastropod Reishia clavigera, barnacle Amphibalanus amphitrite amphitrite and mussel Brachidontes variabilis under ocean acidification
Published 21 January 2020 Science ClosedTags: biological response, communityMF, crustaceans, laboratory, mollusks, multiple factors, North Pacific, performance
Since the response to ocean acidification is species specific, differences in responses between predator and prey will alter their interactions, hence affect the population dynamics of both species. Changes in predator prey interactions between a predatory muricid gastropod Reishia clavigera and its prey, the barnacle Amphibalanus amphitrite amphitrite and mussel Brachidontes variabilis under three pCO2 levels (380, 950, and 1250 μatm) were investigated. The searching time for barnacles increased and the ability to locate them decreased at higher pCO2 levels. The movement speed and the prey consumption rate, however, were independent of pCO2. There was no preference towards either B. variabilis or A. amphitrite amphitrite regardless of pCO2. Exposure experiments involving multiple generations are suggested to assess transgenerational effects of ocean acidification and the potential compensation responses before any realistic predictions on the long term changes of population dynamics of the interacting species can be made.
Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes
Published 20 January 2020 Science ClosedTags: biological response, communityMF, laboratory, multiple factors, North Pacific, photosynthesis, phytoplankton, respiration, temperature
Highlights
• Increased algal densities reduce photosynthesis and respiration of Ulva conglobata.
• Algal density mediates the interactive effect of increased temperature and lowered pH.
• Altered temperature and pH oppositely affect photosynthetic rate and saturation light.
Abstract
Growing of macroalgae increases their biomass densities in natural habitats. To explore how the altered algal density impacts their photosynthetic responses to changes of environmental factors, we compared the photosynthesis versus irradiance characteristics of a marine green macroalga Ulva conglobata under low [2.0 g fresh weight (FW) L−1], medium (6.0 g FW L−1) and high biomass densities (12.0 g FW L−1), and under a matrix of temperatures (20, 25, 30 and 35 °C) and pH levels (7.8, 8.2 and 8.6). Increased algal densities decreased the photosynthetic O2 evolution rate among all combined temperature and pH treatments, in parallel with the decrease of light-utilizing efficiency (α, the initial slope) and maximum photosynthetic rate (Pmax) and the increase of light saturation point (EK). Rising temperature interacted with lowered pH to increase the α under low but not under high algal densities. Rising temperature increased the Pmax and decreased the EK under low algal density, but not under high density. Lowered pH promoted the Pmax and EK under all three algal densities. The increased temperature enhanced the dark respiration (Rd) and light compensation point (EC), while the altered pH showed a limited effect. Moreover, the increased algal density reduced the Rd, and had a limited effect on the EC. In addition, our results indicate that changing algal densities caused the complex photophysiological changes in responses to the temperature and pH changes, and these complex responses resolved into a close relation between Rd and Pmax across the matrix of temperatures and pH levels.
Experimental acidification increases susceptibility of Mercenaria mercenaria to infection by Vibrio species
Published 14 January 2020 Science ClosedTags: abundance, biological response, communityMF, laboratory, mollusks, morphology, mortality, multiple factors, North Atlantic, otherprocess, physiology, prokaryotes, reproduction
Highlights
• Clams in high pCO2/low pH were more susceptible to infection by pathogenic Vibrios.
• Growth and abundance of Vibrio spp. were greater under high pCO2/low pH.
• Clams reared under high pCO2/low pH seemed to have a broad tolerance range for pH.
• Long-term effect of acidification and susceptibility to vibriosis is understudied.
Abstract
Ocean acidification alters seawater carbonate chemistry, which can have detrimental impacts for calcifying organisms such as bivalves. This study investigated the physiological cost of resilience to acidification in Mercenaria mercenaria, with a focus on overall immune performance following exposure to Vibrio spp. Larval and juvenile clams reared in seawater with high pCO2 (∼1200 ppm) displayed an enhanced susceptibility to bacterial pathogens. Higher susceptibility to infection in clams grown under acidified conditions was derived from a lower immunity to infection more so than an increase in growth of bacteria under high pCO2. A reciprocal transplant of juvenile clams demonstrated the highest mortality amongst animals transplanted from low pCO2/high pH to high pCO2/low pH conditions and then exposed to bacterial pathogens. Collectively, these results suggest that increased pCO2 will result in immunocompromised larvae and juveniles, which could have complex and pernicious effects on hard clam populations.
Antioxidant responses of triangle sail mussel Hyriopsis cumingii exposed to harmful algae Microcystis aeruginosa and high pH
Published 18 November 2019 Science ClosedTags: biological response, communityMF, laboratory, mollusks, morphology, multiple factors, North Pacific, physiology
Highlights
• The comprehensive effects of toxic cyanobacteria and high pH on mussels were assessed.
• Interaction between cyanobacteria and high pH on physiological indicator were found.
• Compare to high pH, toxic M. aeruginosa induce more severe oxidative stress response.
• Toxic algae or high pH exposure history showed latent effects on Hyriopsis cumingii.
Abstract
In lakes and reservoirs, harmful algal blooms and high pH have been deemed to be two important stressors related to eutrophication, especially in the case of CO2 depletion caused by dense blooms. However, the effects of these stressors on the economically important shellfish that inhabit these waters are still not well-understood. This study evaluated the combined effects of the harmful algae Microcystis aeruginosa (0%, 50%, and 100% of total dietary dry weight) and high pH (8.0, 8.5 and 9.0) on the antioxidant responses of the triangle sail mussel H. cumingii. The mussels were exposed to algae and high pH for 14 d, followed by a 7-day depuration period. Reactive oxygen species (ROS) in the mussel hemolymph, antioxidant and detoxifying enzymes, such as glutathione-S-transferase (GST), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and malondialdehyde (MDA) in the digestive glands were analyzed during the experimental period. GST, SOD and GPx activity levels and the content of GSH increased following exposure to toxic M. aeruginosa, whereas CAT activity was inhibited. pH showed no significant effects on the immune defense mechanisms and detoxification processes. However, a high pH could cause increased ROS and MDA levels, resulting in oxidative injury. After a 7-day depuration period, exposure to toxic M. aeruginosa or high pH resulted in latent effects for most of the examined parameters. The treatment group exposed to the highest pH (9.0) displayed an increased oxidation state compared with the other pH treatments (8.0 and 8.5) for the same concentrations of toxic M. aeruginosa. The trends observed for ROS, MDA, GPx, GST, SOD and GSH levels indicated that a high density of toxic algae could result in severe and continuous effects on mussel health.
Combination of ocean acidification and warming enhances the competitive advantage of Skeletonema costatum over a green tide alga, Ulva linza
Published 15 November 2019 Science ClosedTags: algae, biological response, communityMF, growth, multiple factors, photosynthesis, phytoplankton, respiration, temperature
Highlights
• Coculture did not affect growth rate of U. linza but decreased it for S. costatum.
• Elevated CO2 relieved the inhibitory effect of U. linza on growth of S. costatum.
• At elevated CO2, higher temperature increased the growth rate of S. costatum.
• At elevated CO2, higher temperature reduced the growth rate of U. linza.
• Coculture did not affect respiration of U. linza but stimulated it for S. costatum.
Abstract
Red tide and green tide are two common algal blooms that frequently occur in many areas in the global oceans. The algae causing red tide and green tide often interact with each other in costal ecosystems. However, little is known on how future CO2-induced ocean acidification combined with temperature variation would affect the interaction of red and green tides. In this study, we cultured the red tide alga Skeletonema costatum and the green tide alga Ulva linza under ambient (400 ppm) and future CO2 (1000 ppm) levels and three temperatures (12, 18, 24 °C) in both monoculture and coculture systems. Coculture did not affect the growth rate of U. linza but significantly decreased it for S. costatum. Elevated CO2 relieved the inhibitory effect of U. linza on the growth of S. costatum, particularly for higher temperatures. At elevated CO2, higher temperature increased the growth rate of S. costatum but reduced it for U. linza. Coculture with U. linza reduced the net photosynthetic rate of S. costatum, which was relieved by elevated CO2. This pattern was also found in Chl a content, indicating that U. linza may inhibit growth of S. costatum via harming pigment synthesis and thus photosynthesis. In monoculture, higher temperature did not affect respiration rate of S. costatum but increased it in U. linza. Coculture did not affect respiration of U. linza but stimulated it for S. costatum, which was a signal of responding to biotic and/abiotic stress. The increased growth of S. costatum at higher temperature and decreased inhibition of U. linza on S. costatum at elevated CO2 suggest that red tides may have more advantages over green tides in future warmer and CO2-enriched oceans.
Reef dissolution : rates and mechanisms of coral dissolution by bioeroding sponges and reef communities
Published 9 October 2019 Science ClosedTags: biological response, calcification, communityMF, corals, dissolution, laboratory, multiple factors, North Atlantic, nutrients, respiration
For coral reefs to persist, the rate of CaCO3 production must be greater than the rate of erosion to enable positive growth. Negative impacts of global change (ocean acidification and warming) and local stressors (eutrophication, overfishing) on accretion co-occur with positive effects of these changes on bioerosion capacity and chemical dissolution by excavating euendolithic organisms. This is especially relevant for reefs characterised with low calcifying rates as they will tip faster into net loss. The Caribbean reefs suffered from a decrease by up to 80% in scleractinian coral cover in the past 50 years, their configuration bears very little resemblance with reefs pre1980s, in terms of benthic composition, coral cover and structural complexity. Specifically, excavating sponges can contribute up to 90% of the total macroborer activity on coral reefs and their rates of bioerosion are positively affected by pCO2. The overarching aim of this thesis was to quantify and understand the accretion and loss terms of coral reef communities with a focus on the interactions of anthropogenic ocean acidification and eutrophication with bioerosion by coral-excavating sponges.The use of incubations was central in this piece of work. Changes in the chemical composition of the water overlying sponges and reef communities indicate the relative contribution of metabolic processes such as net calcification/dissolution and net respiration/production. However, we first used fluorescence microscopy to investigate the underlying mechanisms of CaCO3 dissolution by excavating sponges. It revealed that they promote CaCO3 dissolution by decreasing pH at the sponge/coral interface. The high [H+] at this site is achieved through delivery of low-pH vesicles by the etching cells. The enzyme carbonic anhydrase, which is responsible for significantly increasing the speed of the reversible reaction H2O+CO2↔H++HCO3−, has been shown to be associated to the sponge’s etching processes and is therefore thought to play a role in the dissolution of CaCO3. By blocking its activity whilst incubating sponges and analysing the rate of dissolution, CA was found to play an important role in speeding up protonation of HCO3− ions at the dissolution site, enabling CO2 to diffuse out of the etching area. When exposed to different ranges of ocean acidification and eutrophication, bioerosion rates increased with both variables but no synergistic relation was revealed. Incubations performed at the community level around Saba and Curacao yielded net community calcification (NCC) rates which were lower than those reported for reef flats worldwide. Still, Saba coral reefs are considered relatively pristine sites compared to the average within the wider Caribbean. Around Curaçao, incubations on reef assemblages dominated by coral yielded even lower NCC rates. Incubations of other benthic assemblages that currently characterized shallow Caribbean reef substrate (such as bioeroding sponges, benthic cyanobacterial mats and sand) all resulted in net dissolution. For both Saba and Curaçao, results suggest that reef calcification on these sites is barely able to compensate the CaCO3 losses due to dissolution from other opportunistic benthic residents. With the ongoing global and local pressures, the delicate balance between CaCO3 accretion and loss is likely to tip.
Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures
Published 8 October 2019 Science ClosedTags: algae, biological response, BRcommunity, communityMF, corals, growth, laboratory, mesocosms, morphology, multiple factors, North Pacific, photosynthesis, physiology, primary production, respiration, temperature
Highlights
• The combined effect of OA and rising temperatures stimulated the growth of macroalgae.
• OA resulted in higher coral calcification rates when corals were co-incubated with seagrass.
• Macroalgal growth was lower in seagrass-containing mesocosms.
• Coral and macroalgal, but not seagrass, growth suffered at 31°C under OA conditions.
• Seagrass helped to stabilize the system’s metabolism in response to projected climate change stressors.
Abstract
Ocean acidification (OA) and warming currently threaten coastal ecosystems across the globe. However, it is possible that the former process could actually benefit marine plants, such as seagrasses. The purpose of this study was to examine whether the effects of the seagrass Thalassia hemprichii can increase the resilience of OA-challenged coral reef mesocosms whose temperatures were gradually elevated. It was found that shoot density, photosynthetic efficiency, and leaf growth rate of the seagrass actually increased with rising temperatures under OA. Macroalgal growth rates were higher in the seagrass-free mesocosms, but the calcification rate of the model reef coral Pocillopora damicornis was higher in coral reef mesocosms featuring seagrasses under OA condition at 25 and 28°C. Both the macroalgal growth rate and the coral calcification rate decreased in all mesocosms when the temperature was raised to 31°C under OA conditions. However, the variation in gross primary production, ecosystem respiration, and net ecosystem production in the seagrass mesocosms was lower than in seagrass-free controls, suggesting that the presence of seagrass in the mesocosms helped to stabilize the metabolism of the system in response to simulated climate change.
The combined effects of ocean acidification with fleshy macroalgae and filamentous turfs on tropical crustose coralline algae
Published 28 August 2019 Science ClosedTags: algae, biological response, calcification, communityMF, field, laboratory, mesocosms, morphology, photosynthesis, respiration, South Pacific
Global climate change induces multiple stressors on tropical coral reefs that threaten their persistence. Ocean acidification decreases calcification in most dominant reef builders, such as crustose coralline algae (CCA). Climate change also has the potential to increase the biomass of fleshy macroalgae and filamentous turf in coral reef ecosystems. While fleshy macroalgae and turf may shade, abrade, and have otherwise negative consequences on CCA metabolism, their high rates of photosynthesis may mitigate OA locally through carbon uptake, resulting in a local increase in pH. This thesis explored the effects of OA, combined with the presence of either fleshy macroalgae or algal turfs, on Lithophyllum kotschyanum, an abundant species of CCA in Moorea, French Polynesia. In a mesocosm study, three canopy types, clear mimics, dark mimics, and S. pacificum, were crossed with two CO2 levels, ambient (400 μatm) and elevated (1000 μatm). The clear, dark, and S. pacificum canopies resulted in stepwise decreases in calcification of L. kotschyanum. This response suggests that shading and likely flow moderation decrease CCA calcification. To separate the effects of fleshy macroalgal metabolism from the effects of its physical structure, a subsequent mesocosm and field experiment were performed. In the mesocosm study, a header tank that provided S. pacificum-treated seawater to treatment tanks was used to determine the metabolic effect of S. pacificum on L. kotschyanum. In the field, S. pacificum canopies were attached to 20 30 cm grids, upstream from CCA samples. Data from the mesocosm study support a positive effect of carbon uptake by S. pacificum, but the metabolic effect did not translate into the field. Because S. pacificum was placed in closer proximity to CCA samples in the field than in lab, the difference in L. kotschyanum calcification between the mesocosm and field experiment may be due to physical effects of the canopy in the field, such as shading. The combined results of these two studies suggest that upstream macroalgal communities have the potential to mitigate the negative effects of OA to downstream calcifiers, but will not benefit understory calcifiers. Finally, a mesocosm experiment was conducted to address the combined effects of OA and the presence of epiphytic turf algae on host CCA. In a factorial experiment, L. kotschyanum samples with and without epiphytic turf algae were placed in flow through tanks where pCO2 was ambient (400 μatm) or elevated (1000 μatm). Results indicated a significant effect of elevated pCO2 on CCA calcification and a negative effect of turf presence, despite a higher pH in the presence of turf during light incubations. This indicates that any benefit of higher daytime pH within the DBL of L. kotschyanum was outweighed by the negative effects, such as shading, nighttime anoxia and low pH. Overall, these studies indicate that fleshy macroalgae and filamentous turfs can raise seawater pH locally, but any benefit of this effect is outweighed by the negative effects of fleshy macroalgae and turf presence. The only instance during which CCA may incur a net benefit from fleshy macroalgae occurs when calcifiers are situated downstream of a dense macroalgal community, entirely unaffected by its physical structure. Ultimately, fleshy macroalgae and turf affect CCA negatively, regardless of OA treatment.
Functional loss in herbivores drives runaway expansion of weedy algae in a near-future ocean
Published 13 August 2019 Science ClosedTags: algae, biological response, BRcommunity, communityMF, fish, mesocosms, mollusks, morphology, multiple factors, performance, South Pacific, temperature
Highlights
• Elevated CO2 and warming increased productivity of turf algae.
• Elevated CO2 increased per capita feeding rates of gastropods.
• Ocean warming reduced grazer diversity, density, and biomass.
• As a result, ocean warming drove a fourfold expansion of weedy algal species.
Abstract
The ability of a community to absorb environmental change without undergoing structural modification is a hallmark of ecological resistance. The recognition that species interactions can stabilize community processes has led to the idea that the effects of climate change may be less than what most considerations currently allow. We tested whether herbivory can compensate for the expansion of weedy algae triggered by CO2 enrichment and warming. Using a six-month mesocosm experiment, we show that increasing per capita herbivory by gastropods absorbs the boosted effects of CO2 enrichment on algal production in temperate systems of weak to moderate herbivory. However, under the combined effects of acidification and warming this compensatory effect was eroded by reducing the diversity, density and biomass of herbivores. This loss of functionality combined with boosted primary productivity drove a fourfold expansion of weedy algal species. Our results demonstrate capacity to buffer ecosystems against CO2 enrichment, but loss of this capacity through ocean warming either in isolation or combined with CO2, driving significant algal turf expansion. Identifying compensatory processes and the circumstances under which they prevail could potentially help manage the impacts of ocean warming and acidification, which are further amplified by local disturbances such as habitat loss and herbivore over-exploitation.
The harmful algae, Cochlodinium polykrikoides and Aureococcus anophagefferens, elicit stronger transcriptomic and mortality response in larval bivalves (Argopecten irradians) than climate change stressors
Published 17 June 2019 Science ClosedTags: biological response, communityMF, laboratory, molecular biology, mollusks, mortality, multiple factors, phytoplankton, temperature
Global ocean change threatens marine life, yet a mechanistic understanding of how organisms are affected by specific stressors is poorly understood. Here, we identify and compare the unique and common transcriptomic responses of an organism experiencing widespread fisheries declines, Argopecten irradians (bay scallop) exposed to multiple stressors including high pCO2, elevated temperature, and two species of harmful algae, Cochlodinium (aka Margalefidinium) polykrikoides and Aureococcus anophagefferens using high‐throughput sequencing (RNA‐seq). After 48 hr of exposure, scallop transcriptomes revealed distinct expression profiles with larvae exposed to harmful algae (C. polykrikoides and A. anophagefferens) displaying broader responses in terms of significantly and differentially expressed (DE) transcripts (44,922 and 4,973; respectively) than larvae exposed to low pH or elevated temperature (559 and 467; respectively). Patterns of expression between larvae exposed to each harmful algal treatment were, however, strikingly different with larvae exposed to A. anophagefferens displaying large, significant declines in the expression of transcripts (n = 3,615; 87% of DE transcripts) whereas exposure to C. polykrikoides increased the abundance of transcripts, more than all other treatments combined (n = 43,668; 97% of DE transcripts). Larvae exposed to each stressor up‐regulated a common set of 21 genes associated with protein synthesis, cellular metabolism, shell growth, and membrane transport. Larvae exposed to C. polykrikoides displayed large increases in antioxidant‐associated transcripts, whereas acidification‐exposed larvae increased abundance of transcripts associated with shell formation. After 10 days of exposure, each harmful algae caused declines in survival that were significantly greater than all other treatments. Collectively, this study reveals the common and unique transcriptional responses of bivalve larvae to stressors that promote population declines within coastal zones, providing insight into the means by which they promote mortality as well as traits possessed by bay scallops that enable potential resistance.
Grazers increase the sensitivity of coralline algae to ocean acidification and warming
Published 26 March 2019 Science ClosedTags: algae, biological response, calcification, communityMF, laboratory, mesocosms, photosynthesis, primary production, respiration, temperature
Highlights
- Stimulation of the primary production and calcification of corallines by grazing
- Different response of maerl between winter and summer conditions
- High vulnerability of corallines to ocean acidification in the presence of grazers
Abstract
Coralline algae are expected to be adversely impacted by ocean acidification and warming. Most research on these algae has involved experiments on isolated species, without considering species interactions, such as grazing. This myopic view is challenging because the impact of climate change on coralline algae will depend on the direct impacts on individual coralline species and the indirect effects of altered interactions with other species. Here, we tested the influence of grazing on the response of the coralline alga Lithothamnion corallioides to near-future ocean acidification and warming. Two three-month experiments were performed in the winter and summer seasons in mesocosms under crossed conditions of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C) in the presence and absence of grazers. In the winter, L. corallioides photosynthesis decreased with rising temperature in the presence of grazers, while calcification increased. It is likely that increased calcification may act as a structural protection to prevent damage from grazing. However, increasing calcification rates in the presence of grazers may be detrimental to other physiological processes, such as photosynthesis. In the summer, L. corallioides primary production, respiration, and calcification were higher in the presence of grazers than in their absence. Light calcification rates were reduced under high pCO2 in the presence of grazers only. Moreover, dark calcification rates were more adversely affected by pCO2 increase in the presence of grazers. Through their feeding activity, grazers may alter the structural integrity of thalli and increase the sensitivity of coralline algae to ocean acidification. Our results indicate that both season and grazing play a key role in the response of L. corallioides to acidification and warming. Seasonal variations and species interactions are thus critical to consider to make ecologically relevant predictions of the effects of future environmental changes.
Obligate ectosymbionts increase the physiological resilience of a scleractinian coral to high temperature and elevated pCO2
Published 20 December 2018 Science ClosedTags: biological response, calcification, communityMF, corals, crustaceans, laboratory, multiple factors, South Pacific, temperature
Invertebrate ectosymbionts within the coralla of scleractinians enhance host fitness through protection from corallivores and nutrient addition. Here, we explore the ectosymbiotic relationship between the coral Pocillopora verrucosa and the crab Trapezia serenei and the shrimp Alpheus spp., to test for effects on coral calcification under contrasts of seawater temperature (27.7 °C and 29.9 °C) and pH (ambient, 8.0 and reduced, 7.7). Regardless of temperature, ectosymbionts depressed calcification by 55% (vs without ectosymbionts) at ambient pH; however, ectosymbionts only depressed calcification under ambient pH but not at reduced pH. These results suggest that P. verrucosa grows fastest at ambient pH without ectosymbionts, but when ectosymbionts are present, colonies are protected from further declines in calcification at reduced pH. This implies that there may be a change from a currently parasitic ectosymbiont–coral relationship to a commensal relationship that could increase fitness advantages for corals hosting crustacean ectosymbionts under ocean acidification conditions.
