Posts Tagged 'algae'

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Ocean acidification exacerbates the inhibition of fluctuating light on the productivity of Ulva prolifera

Published 24 February 2022 Science Closed
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Highlights

  • High CO2 concentration improves the growth of Ulva prolifera.
  • Fluctuating light inhibit the growth of U. prolifera under high CO2 concentration condition.
  • Photosynthetic performance of U. prolifera was downgraded by fluctuating light and high CO2 concentration.

Abstract

Ulva prolifera, a common species of green macroalgae, is often harmful-algal-bloom causative and significantly impacts local marine ecosystems. Previous studies on the physiological characteristics of U. prolifera have been conducted under constant light (CL). However, light in the natural environment continually changes, and little is known about fluctuating light (FL). Ocean acidification (OA) has been proposed to interact with dynamic surrounding environments to affect the physiological performance of macroalgae. Therefore, we investigated the combined effects of FL (80/300, alternating between 80 μmol photons m−2 s−1 for 2.5 h and 300 μmol photons m−2 s−1 for 1.5 h, with an average light intensity of 160 μmol photons m−2 s−1 and OA (1000 ppm CO2) on U. prolifera. The results clearly showed that FL had no significant effect on the relative growth rate (RGR), whereas OA obviously improved RGR. However, under FL-OA combination conditions, RGR was inhibited significantly, accompanied by a concomitant downgraded photosynthetic performance, while the photoprotective abilities were enhanced. The results would help us accurately predict the primary productivity of macroalgae in coastal waters under future OA conditions with irradiance fluctuations.

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High-latitude calcified coralline algae exhibit seasonal vulnerability to acidification despite physical proximity to a non-calcified alga

Published 3 February 2022 Science Closed
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Highlights

  • High-latitude coralline algae face dissolution under future winter acidification.
  • Seasonal light exposure variation may be too low to impact coralline calcification.
  • Interaction with a fleshy alga may not benefit corallines’ response to acidification.

Abstract

The emergent responses of vulnerable species to global change can vary depending on the relative quality of resources available to support their productivity under increased stress, as well as the biotic interactions with other species that may alter their access to these resources. This research tested how seawater pCO2 may interact with seasonal light availability to affect the photosynthesis and calcification of high-latitude coralline algae, and whether the responses of these calcified macroalgae are modified by physical association with a non-calcified seaweed. Through an in situ approach, our study first investigated how current seasonal environmental variation affects the growth of the understory coralline algae Crusticorallina spp. and Bossiella orbigniana in Southeast Alaska’s kelp forests. We then experimentally manipulated pH to simulate end-of-century acidification scenarios, light regime to simulate seasonal light availability at the benthos, and pairings of coralline algal species with and without a fleshy red alga to examine the interactive effects of these variables on coralline productivity and calcification. Our results indicate that: 1) coralline species may face net dissolution under projected future winter pH and carbonate saturation state conditions, 2) differences in seasonal light availability in productive, high-latitude waters may not be distinct enough to modify coralline algal net calcification, and 3) association with a non-calcified red alga does not alter the response of these coralline algal species to ocean acidification scenarios. This research highlights the necessity of incorporating locally informed scenarios of environmental variability and community interactions when predicting species’ vulnerability to global change.

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The combined effects of ocean acidification and heavy metals on marine organisms: a meta-analysis

Published 20 January 2022 Science Closed
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Ocean acidification (OA) may interact with anthropogenic pollutants, such as heavy metals (HM), to represent a threat to marine organisms and ecosystems. Here, we perform a quantitative meta-analysis to examine the combined effects of OA and heavy metals on marine organisms. The results reveal predominantly additive interactions (67%), with a considerable proportion of synergistic interactions (25%) and a few antagonistic interactions (8%). The overall adverse effects of heavy metals on marine organisms were alleviated by OA, leading to a neutral impact of heavy metals in combination with OA. However, different taxonomic groups showed large variabilities in their responses, with microalgae being the most sensitive when exposed to heavy metals and OA, and having the highest proportion of antagonistic interactions. Furthermore, the variations in interaction type frequencies are related to climate regions and heavy metal properties, with antagonistic interactions accounting for the highest proportion in temperate regions (28%) and when exposed to Zn (52%). Our study provides a comprehensive insight into the interactive effects of OA and HM on marine organisms, and highlights the importance of further investigating the responses of different marine taxonomic groups from various geographic locations to the combined stress of OA and HM.

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Responses of benthic calcifying algae to ocean acidification differ between laboratory and field settings

Published 17 January 2022 Science Closed
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Accurately predicting the effects of ocean and coastal acidification on marine ecosystems requires understanding how responses scale from laboratory experiments to the natural world. Using benthic calcifying macroalgae as a model system, we performed a semi-quantitative synthesis to compare directional responses between laboratory experiments and field studies. Variability in ecological, spatial, and temporal scales across studies, and the disparity in the number of responses documented in laboratory and field settings, make direct comparisons difficult. Despite these differences, some responses, including community-level measurements, were consistent across laboratory and field studies. However, there were also mismatches in the directionality of many responses with more negative acidification impacts reported in laboratory experiments. Recommendations to improve our ability to scale responses include: (i) developing novel approaches to allow measurements of the same responses in laboratory and field settings, and (ii) researching understudied calcifying benthic macroalgal species and responses. Incorporating these guidelines into research programs will yield data more suitable for robust meta-analyses and will facilitate the development of ecosystem models that incorporate proper scaling of organismal responses to in situ acidification. This, in turn, will allow for more accurate predictions of future changes in ecosystem health and function in a rapidly changing natural climate.

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Ocean acidification induces distinct metabolic responses in subtropical zooplankton under oligotrophic conditions and after simulated upwelling

Published 11 January 2022 Science Closed
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Highlights

  • Effects of rising atmospheric CO2 on zooplankton were studied in coastal mesocosms.
  • Mesozooplankton metabolism was more affected by elevated CO2 than microzooplankton.
  • CO2-induced effects were more marked in eutrophic than in oligotrophic conditions.
  • Elevated CO2 impacts the role of zooplankton on the carbon and nitrogen cycles.

Abstract

Ocean acidification (OA) is one of the most critical anthropogenic threats to marine ecosystems. While significant ecological responses of plankton communities to OA have been revealed mainly by small-scale laboratory approaches, the interactive effect of OA-related changes on zooplankton metabolism and their biogeochemical implications in the natural environment still remains less well understood. Here, we explore the responses of zooplankton respiration and ammonium excretion, two key processes in the nutrient cycling, to high pCO2 levels in a 9-week in situ mesocosm experiment conducted during the autumn oligotrophic season in the subtropical northeast Atlantic. By simulating an upwelling event halfway through the study, we further evaluated the combined effects of OA and nutrient availability on the physiology of micro-and mesozooplankton. OA conditions generally resulted in a reduction in the biomass-specific metabolic and enzymatic rates, particularly in the mesozooplankton community. The situation reversed after the nutrient-rich deep-water addition, which initially promoted a diatom bloom and increased heterotrophic activities in all mesocosms. Under high pCO2 conditions (>800 μatm), however, the nutrient fertilization triggered the proliferation of the harmful alga Vicicitus globosus, with important consequences for the metabolic performance of the two zooplankton size classes. Here, the zooplankton contribution to the remineralization of organic matter and nitrogen regeneration dropped by 30% and 24%, respectively, during the oligotrophic period, and by 40% and 70% during simulated upwelling. Overall, our results indicate a potential reduction in the biogeochemical role of zooplankton under future ocean conditions, with more evident effects on the large mesozooplankton and during high productivity events.

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Performance of a potentially invasive species of ornamental seaweed Caulerpa sertularioides in acidifying and warming oceans

Published 29 December 2021 Science Closed
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Caulerpa, a (sub) tropical seaweed, is a notorious taxonomic group and an invasive seaweed worldwide. Similar to several species that have been introduced to benthic habitats through aquariums, Caulerpa sertularioides has also been introduced into Korean aquariums, although it is not native to the region. Thus, it is necessary to evaluate the potential of this species for invading domestic macroalgal habitats. Therefore, an indoor mesocosm experiment was conducted to examine the ecophysiological invasion risk of non-native seaweed C. sertularioides under various climate conditions and exposure to three future climate scenarios: acidification (doubled CO2), warming (5 °C increase from ambient temperature), and greenhouse (GR: combination of acidification and warming); additionally, we compared the invasion risk between future and present climates (control: 20 °C and 470 µatm CO2). High CO2 concentrations and increased temperatures positively affected the photosynthesis and growth of C. sertularioides. Photosynthesis and growth were more synergistically increased under GR conditions than under acidification and warming. Consequently, the performance of this potentially invasive species in the native macroalgal Korean habitat will be higher in the future in coastal environments. Therefore, proper management is required to prevent the geographic expansion of C. sertularioides in the Korean coastal ocean.

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Spatiotemporal variability in kelp forest and seagrass ecosystems: can local biogeochemical modification combat acidification stress?

Published 23 December 2021 Science Closed
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Anthropogenic carbon dioxide (CO2) emissions have driven widespread ocean acidification (OA). OA has reduced surface ocean pH by at least 0.1 pH units since the beginning of the industrial era and global models forecast a further decrease of 0.3 to 0.4 pH units by the end of the century. Submerged aquatic vegetation, such as kelp forests and seagrass beds, has the potential to locally ameliorate OA by removing CO2 during photosynthesis and storing it as fixed carbon. Thus, understanding the contribution of these habitats to local biogeochemistry is essential to inform coastal management and policy, especially as the impacts of anthropogenic climate change become more prevalent. The following work describes high resolution spatiotemporal variability in seagrass and kelp forest biogeochemistry (Chapters 1 and 2) and in the surface canopy extent of a giant kelp forest (Chapter 3).

In order to understand the contributions of kelp forest and seagrass metabolism to their respective local biogeochemistry, we must determine the natural variability in these systems and disentangle the physical and biological drivers of local biogeochemical variability. In Chapter 1, I deployed an extensive instrument array in Monterey Bay, CA, inside and outside of a kelp forest to assess the degree to which kelp locally ameliorates present-day acidic conditions, which we expect to be further exacerbated by OA. Interactions between upwelling exposure, internal bores, and biological production shaped the local biogeochemistry inside and outside of the kelp forest. Significantly elevated pH, attributed to kelp canopy productivity, was observed at the surface inside the kelp forest. This modification was largely limited to a narrow band of surface water, implying that while kelp forests have the potential to locally ameliorate ocean acidification stress, this benefit may largely be limited to organisms living in the upper part of the canopy. In Chapter 2, I quantified net community production (NCP) over a mixed seagrass-coral community on Ngeseksau Reef, Ngermid Bay, Republic of Palau. We observed a net heterotrophic diel signal over the deployment, but dissolved oxygen (O2) fluxes during the day were largely positive, illustrating daytime autotrophy. pH, O2, and temperature followed a clear diel pattern with maxima typically occurring in the afternoon. The relationship between tidal regime and time of day drove the magnitude of the signals observed. The case studies described in Chapters 1 and 2 emphasize the importance of high-resolution measurements (high temporal frequency as well as high horizontal and vertical spatial resolution) and consideration of the multiple drivers responsible for shaping the observed biogeochemical variability. In addition to the photosynthetic biomass (kelp and seagrass) at the center of these studies, the physical environment played an important role in dictating the signals observed, in particular water circulation and residence time.

Biogeochemical studies rarely look beyond a few deployment sites, but the ecosystem contributing to the local biogeochemical variability includes influences from beyond those discrete points. Describing the area around these discrete points is important for accurate assessment of factors driving the signals observed at those points. Remote sensing can help us capture and describe the spatial patterns of biomass contributing to changes observed in our chemical records. In Chapter 3, I established a low altitude unmanned aerial vehicle (UAV) record of giant kelp surface areal extent over 18 months on the wave-protected side of Cabrillo Point (Hopkins Marine Station) in Monterey Bay, CA. This was the same canopy responsible for elevating pH in Chapter 1; however, in this case, the kelp canopy mapping did not overlap in time with biogeochemical measurements in the kelp forest. I compared the UAV kelp classification to canopy cover determined from Landsat satellite images obtained over the same period. There was a linear relationship between the drone kelp ratio and Landsat kelp canopy fraction for spatially-matched pixels; a Landsat kelp fraction of 0.64 was equivalent to 100% kelp cover in the drone data. The level of resolution provided by UAV, compared with Landsat images, could allow more detailed mapping of kelp responses to environmental change. Future studies should pair mapping flights with biogeochemical measurements to quantify the relationship between changes in canopy area and the relative surface canopy modification of pH.

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The effectiveness of Laminaria Digitata on mitigating ocean acidification through pH analysis

Published 16 December 2021 Science Closed
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In recent years CO2 levels have been rising worldwide, causing increased acidification of ocean water. Currently the average oceanic pH is 8, and while it is known that ocean water is naturally slightly alkaline, CO2 emissions continue to rise and the ocean continues to absorb these emissions. This leads to a decrease in oceanic pH as it continues to become more acidic. The pH levels of the ocean have already fallen by 0.1 in the last 200 years. A 0.1 decrease may not seem drastic but consider the fact that the human body has a natural pH of around 7.4, and if it drops to 7.2 (just a 0.2 decrease) it may lead to death. A seemingly insignificant difference in pH level can have drastic effects on the human body, thus, the same can be considered when it comes to the ocean. Ocean acidification has already impacted many different forms of marine life and will continue to do so if no method is found to prevent the further acidification of ocean water. In the following experiment, Laminaria (kelp) was tested for capabilities of neutralizing the pH of ocean water. To combat the current ocean acidification conflict, recently, macroalgaes have been in the spotlight for its capabilities of absorbing CO2 from ocean water with remarkable results. Laminarias have proven to be particularly effective at absorbing CO2 and restoring pH levels of ocean waters, justifying its use in this experiment. Through pH analysis, the effectiveness of the Laminaria was measured over a 21 day time frame, and data was collected.

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Differences in organic carbon release between conchocelis and thalli of Pyropia haitanensis and responses to changes in light intensity and pH

Published 10 December 2021 Science Closed
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Highlights

  • DOC production rate of thallus was much higher than that of conchocelis.
  • DOC production rate of thallus tends to increase with light intensity.
  • Ocean acidification did not significantly change the DOC production rate of thallus.

Abstract

The large-scale cultivation of macroalgae has the potential to act as a carbon sink because macroalgae can release a large amount of organic carbon into the surrounding seawater. However, this needs to be evaluated on the basis of the entire life cycle under a background of changes in pH and light intensity. The present study investigated the difference in organic carbon release between conchocelis and thallus stages of the economically important red alga Pyropia haitanensis in response to three light intensities (10, 50, and 500 μmol m−2 s−1) and two pH conditions (current pH: 8.1, projected future pH: 7.5). The study found that regardless of the light intensity and pH values, the growth rates, production rates of tissue carbon, and dissolved organic carbon (DOC) of thalli tended to be higher than those of conchocelis, by more than 170%, 85%, and 106%, respectively. The DOC production rate was higher than the production rate of particulate organic carbon (POC) by at least two orders of magnitude. Positive correlations were found between growth rate and production rates of tissue carbon and growth rate and DOC production rate, but no clear relationship was found between growth and POC production. The DOC production rate of thallus tended to increase with light intensity but was not significantly influenced by ocean acidification. However, decay of tissue caused by exposure of the conchocelis to high light intensity resulted in increased POC and DOC production rates, indicating the complexity of organic carbon release by P. haitanensis. This study provides insights into the release of organic carbon during the complete life cycle of P. haitanensis, and the results can further our understanding of the carbon metabolism of this cultivated macroalgal species.

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Effects of ocean acidification on the performance and interaction of fleshy macroalgae and a grazing sea urchin

Published 9 December 2021 Science Closed
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Highlights

  • We investigated the direct and indirect effects of CO2 on an urchin and macroalgae.
  • Elevated CO2 increased production of fleshy macroalgae biomass but not photosynthesis.
  • Urchin respiration, biomass, righting time, and consumption rate were unaffected.
  • Reduced algal nutrition interacted with impaired chemosensing to preserve foraging.
  • Foraging and consumption suggest sustained trophic interactions under acidification.

Abstract

When predicting the response of marine ecosystems to climate change, it is increasingly recognized that understanding the indirect effects of ocean acidification on trophic interactions is as important as studying direct effects on organism physiology. Furthermore, comprehensive studies that examine these effects simultaneously are needed to identify and link the underlying mechanisms driving changes in species interactions. Using an onshore ocean acidification simulator system, we investigated the direct and indirect effects of elevated seawater pCO2 on the physiology and trophic interaction of fleshy macroalgae and the grazing sea urchin Lytechinus variegatus. Macroalgal (Dictyota spp.) biomass increased despite decreased photosynthetic rates after two-week exposure to elevated pCO2. Algal tissue carbon content remained constant, suggesting the use of alternative carbon acquisition pathways beneficial to growth under acidification. Higher C:N ratios driven by a slight reduction in N content in algae exposed to elevated pCO2 suggest a decrease in nutritional content under acidification. Urchin (L. variegatus) respiration, biomass, and righting time did not change significantly after six-week exposure to elevated pCO2, indicating that physiological stress and changes in metabolism are not mechanisms through which the trophic interaction was impacted. Correspondingly, urchin consumption rates of untreated macroalgae (Caulerpa racemosa) were not significantly affected by pCO2. In contrast, exposure of urchins to elevated pCO2 significantly reduced the number of correct foraging choices for ambient macroalgae (Dictyota spp.), indicating impairment of urchin chemical sensing under acidification. However, exposure of algae to elevated pCO2 returned the number of correct foraging choices in similarly exposed urchins to ambient levels, suggesting alongside higher C:N ratios that algal nutritional content was altered in a way detectable by the urchins under acidification. These results highlight the importance of studying the indirect effects of acidification on trophic interactions simultaneously with direct effects on physiology. Together, these results suggest that changes to urchin chemical sensing and algal nutritional quality are the driving mechanisms behind surprisingly unaltered urchin foraging behavior for fleshy macroalgae under joint exposure to ocean acidification. Consistent foraging behavior and consumption rates suggest that the trophic interaction between L. variegatus and fleshy macroalgae may be sustained under future acidification. However, increases in fleshy macroalgal biomass driven by opportunistic carbon acquisition strategies have the potential to cause ecological change, depending on how grazer populations respond. Additional field research is needed to determine the outcome of these results over time and under a wider range of environmental conditions.

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Who wins or loses matters: strongly interacting consumers drive seagrass resistance under ocean acidification

Published 8 December 2021 Science Closed
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Highlights

  • Ocean acidification (OA) directly facilitated algal epiphyte and seagrass.
  • Co-occurring grazers variably controlled algal overgrowth on seagrass.
  • pH-tolerant, strongly interacting grazers maintained overall grazing pressure.
  • Grazing and OA additively increased seagrass productivity.

Abstract

Global stressors are increasingly altering ecosystem resistance, resilience, and functioning by reorganizing vital species interactions. However, our predictive understanding of these changes is hindered by failures to consider species-specific functional roles and stress responses within communities. Stressor-driven loss or reduced performance of strongly interacting species may generate abrupt shifts in ecosystem states and functions. Yet, empirical support for this prediction is scarce, especially in marine climate change research. Using a marine assemblage comprising a habitat-forming seagrass (Phyllospadix torreyi), its algal competitor, and three consumer species (algal grazers) with potentially different functional roles and pH tolerance, we investigated how ocean acidification (OA) may, directly and indirectly, alter community resistance. In the field and laboratory, hermit crabs (Pagurus granosimanus and P. hirsutiusculus) and snails (Tegula funebralis) displayed distinct microhabitat use, with hermit crabs more frequently grazing in the area of high algal colonization (i.e., surfgrass canopy). In mesocosms, this behavioral difference led to hermit crabs exerting ~2 times greater per capita impact on algal epiphyte biomass than snails. Exposure to OA variably affected the grazers: snails showed reduced feeding and growth under extreme pH (7.3 and 7.5), whereas hermit crabs (P. granosimanus) maintained a similar grazing rate under all pH levels (pH 7.3, 7.5, 7.7, and 7.95). Epiphyte biomass increased more rapidly under extreme OA (pH 7.3 and 7.5), but natural densities of snails and hermit crabs prevented algal overgrowth irrespective of pH treatments. Finally, grazers and acidification additively increased surfgrass productivity and delayed the shoot senescence. Hence, although OA impaired the function of the most abundant consumers (snails), strongly interacting and pH-tolerant species (hermit crabs) largely maintained the top-down pressure to facilitate seagrass dominance. Our study highlights significant within-community variation in species functional and response traits and shows that this variation has important ecosystem consequences under anthropogenic stressors.

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Responses of the marine carbonate system to a green tide: a case study of an Ulva prolifera bloom in Qingdao coastal waters

Published 26 November 2021 Science Closed
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Highlights

  • The carbonate system was affected intensely during the Ulva prolifera bloom.
  • The calcification in the seawater affected by the Ulva prolifera bloom was enhanced.
  • The release and distribution of organic acids were controlled by the bloom.
  • Qingdao coastal waters changed from a sink of atmospheric CO2 to a strong source.

Abstract

As an environmental nuisance, Ulva prolifera green tides have occurred annually in the southern Yellow Sea since 2007. While it is expected that high levels of biological activity during these blooms can alter seawater carbonate chemistry, there has been little research on the responses of marine carbonate system to green tides. Here, the effects of the bloom on the carbonate system were examined on three cruises in June, July, and September, corresponding to the early-, late-, and after-bloom periods of the U. prolifera bloom in Qingdao coastal waters in 2018. Among these three stages, the pH (National Bureau of Standards scale), dissolved inorganic carbon (DIC), total alkalinity (TA), and partial pressure of CO2 (pCO2) were all affected by bloom, with the highest pH and lowest DIC and TA concentrations of the surface seawater occurring at the late-bloom stage. While pCO2 continuously increased from the beginning to the end of the bloom. TA increased by ∼40 μmol kg−1 between the early- and after-bloom periods likely due to the shifts in the carbonate system equilibrium caused by increased CO32− concentrations and the organic matter released by U. prolifera during decomposition. Compared to nearby areas with no U. prolifera bloom, the green tide, along with increasing temperature, reduced the pH and DIC but increased the TA and pCO2. This large-scale bloom also turned the coastal waters from being an atmospheric CO2 sink to a strong source, with the estimation of air-sea CO2 fluxes about 1.69 ± 1.70, 2.28 ± 1.16, and 7.44 ± 5.84 mmol m−2 d−1 during the early-, late-, and after-bloom periods, respectively. This bloom event also promoted the formation of CaCO3 and was an important source of low molecular weight organic acids. These new findings provide nuances for the current conversations on the role of biological processes in modulating marine carbonate system and the contribution of organic matter to alkalinity.

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Crustose coralline algae display sensitivity to near future global ocean change scenarios

Published 17 November 2021 Science Closed
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Most research investigating how ocean warming and acidification will impact marine species has focused on visually dominant species, such as kelps and corals, while ignoring visually cryptic species such as crustose coralline algae (CCA). CCA are important keystone species that provide settlement cues for invertebrate larvae and can be highly sensitive to global ocean change. However, few studies have assessed how CCA respond to low emission scenarios or conditions. In a laboratory experiment, we examined the responses of temperate CCA assemblages to combined warming and acidification projected under low, medium, and high emissions. Net calcification and net photosynthesis significantly declined in all emissions scenarios, while significant reductions in relative growth rates and increases in percentage bleaching were observed in the highest emission scenario. The negative responses of CCA to both low and medium emissions suggest that they may be adversely impacted by combined warming and acidification by 2030 if current emissions are sustained. This will have far reaching consequences for commercially important invertebrates that rely on them to induce settlement of larvae. These findings highlight the need to take rapid action to preserve these critical keystone species and the valuable services they provide.

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Marine macrophytes as carbon sinks: comparison between seagrasses and the non-native alga Halimeda incrassata in the Western Mediterranean (Mallorca)

Published 11 November 2021 Science Closed
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Seagrass species play a critical role in the mitigation of climate change by acting as valuable carbon sinks and storage sites. Another important ecosystem service of this coastal vegetation is nutrient removal. However, coastal ecosystems are under increasing pressure of global warming and associated establishment of invasive species. To elucidate the respective contributions of seagrass species Posidonia oceanica and Cymodocea nodosa and the non-native macroalga Halimeda incrassata as primary producers and nutrient sinks in coastal habitats we conducted in-situ incubations in the North-western Mediterranean Sea. Measured metabolic activity and nutrient removal as well as calcification rates in these habitats over a 24 h period in spring and summer confirmed that the endemic seagrass P. oceanica represents a valuable ecosystem with high O2 production and considerable carbon capture. The documented regression of P. oceanica meadows with higher temperatures and decline in autotrophy as measured here causes concern for the continuity of ecosystem services rendered by this habitat throughout the Mediterranean Sea with progressing climate warming. In contrast, the enhanced performance of C. nodosa and the calcifying alga H. incrassata with increasing temperatures, under expected rates of future warming is uncertain to mitigate loss of productivity in case of a potential shift in marine vegetation. This could ultimately lead to a decline in ecosystem services, decreased carbon storage and mitigation of climate change. Furthermore, this study provides a first estimate for the growth rate of H. incrassata in the Mediterranean Sea, supporting evidence for the mechanism of its rapid extension.

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Pulsed pressure: fluctuating impacts of multifactorial environmental change on a temperate macroalgal community

Published 1 November 2021 Science Closed
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Global change impacts marine organisms and communities mainly through ocean warming, acidification, deoxygenation, and changes in nutrient inputs and water circulation. To assess the ecological impacts of global change, the effects of multiple interacting environmental drivers, including their fluctuations, should be tested at different levels of biological organization. In an outdoor mesocosm study, we investigated the differential effects of three simulated upwelling events coupled with ocean warming (1–5°C above ambient) on a temperate benthic community in the Western Baltic Sea. Ocean warming, especially in summer when temperatures are close to or above the physiological optimum of many species, is likely to impose thermal stress with species-specific impacts. As the properties of deep water vary seasonally, so will the effects of upwelling. Upwelling of cooler deep water in midsummer may alleviate thermal stress, although this mitigation may be modulated by upwelling-associated shifts in other water-quality parameters such as salinity, nutrients, or late-summer hypoxia. This investigation showed that in the Western Baltic Ocean warming was rather beneficial in early and late summer but detrimental when ambient temperatures were highest in midsummer. The effects of upwelling in the absence of ocean warming were generally weakly beneficial, while this effect tended to vanish with intensifying imposed ocean warming. Hypoxia associated with the late summer upwelling impacted some of the grazer species but did not impact the macroalgae. We conclude that in coastal temperate benthic communities, ocean warming is the predominant stressor that may partially and seasonally be buffered by upwelling.

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Understanding coralline algal responses to ocean acidification: meta-analysis and synthesis

Published 29 October 2021 Science Closed
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Ocean acidification (OA) is a major threat to the persistence of biogenic reefs throughout the world’s ocean. Coralline algae are comprised of high magnesium calcite and have long been considered one of the most susceptible taxa to the negative impacts of OA. We summarize these impacts and explore the causes of variability in coralline algal responses using a review/qualitative assessment of all relevant literature, meta-analysis, quantitative assessment of critical responses, and a discussion of physiological mechanisms and directions for future research. We find that most coralline algae experienced reduced abundance, calcification rates, recruitment rates, and declines in pH within the site of calcification in laboratory experiments simulating OA or at naturally elevated CO2 sites. There were no other consistent physiological responses of coralline algae to simulated OA (e.g., photo-physiology, mineralogy, and survival). Calcification/growth was the most frequently measured parameters in coralline algal OA research, and our meta-analyses revealed greater declines in seawater pH were associated with significant decreases in calcification in adults and similar but nonsignificant trends for juveniles. Adults from the family Mesophyllumaceae also tended to be more robust to OA, though there was insufficient data to test similar trends for juveniles. OA was the dominant driver in the majority of laboratory experiments where other local or global drivers were assessed. The interaction between OA and any other single driver was often additive, though factors that changed pH at the surface of coralline algae (light, water motion, epiphytes) acted antagonistically or synergistically with OA more than any other drivers. With advances in experimental design and methodological techniques, we now understand that the physiology of coralline algal calcification largely dictates their responses to OA. However, significant challenges still remain, including improving the geographic and life-history spread of research effort and a need for holistic assessments of physiology.

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Calcium carbonate prevents Botryococcus braunii growth inhibition caused by medium acidification

Published 27 October 2021 Science Closed
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Microalgae, Botryococcus braunii in particular, have received increasing interest owing to their potential as biofuel sources. Although the fertilizer components present in wastewater are useful in reducing the cost of commercial production of microalgae, the ammonium nitrogen (NH4+-N) acidifies the medium and may inhibit the growth of B. braunii. In this study, we aimed to investigate the effectiveness of calcium carbonate in preventing the growth inhibition of B. braunii by suppressing pH decrease caused by NH4+-N. Four types of modified Chu13 media were prepared: a control, one with NH4+-N, one with calcium carbonate, and one with calcium carbonate and NH4+-N. Then, the pH, NH4+-N concentration, B. braunii growth, and hydrocarbon content were measured. We found that in the NH4+-N treatment without calcium carbonate, the pH decreased to approximately 4, and there was almost no algal growth even after 20 days. In contrast, there were no differences in the growth rate and medium pH (7–8) between the treatment with calcium carbonate and the control medium. In addition, calcium ions were leached into the medium with calcium carbonate as the culture progressed, confirming the effectiveness of calcium carbonate in maintaining a constant pH and thus preventing growth inhibition. As such, the addition of calcium carbonate effectively prevents B. braunii growth inhibition by suppressing pH decrease and is more cost-effective than aggressive pH adjustment with alkali or acid or by adding buffer solutions into the medium. This simple and inexpensive method can be used for the removal of ammonia from wastewater.

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Effects of ocean acidification on marine primary producers and related ecological processes under multiple stressors

Published 6 October 2021 Science Closed
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Although the individual effects of ocean acidification (OA), warming, solar UV radiation, deoxygenation and heavy metal pollution on marine producers are well-studied, their interactive effects are still unclear, strongly limiting our ability to project the ecological consequences of ocean climate changes. This chapter aims to provide an overview of our understanding the eco-physiological effects of OA and its combination with warming, solar UV radiation, deoxygenation and heavy metals. While OA is known to enhance photorespiration in both diatoms and green macroalgae, it enhances growth of coastal diatoms and other macroalgae that are adapted to fluctuating diel pH changes and then potentially enhances its contribution to carbon sequestration in coastal waters. OA is supposed to decrease pelagic primary productivity under multiple stressors (e.g., in combination with ultraviolet radiation, deoxygenation, warming), especially in oligotrophic waters, due to insufficient repairing or improvising processes that require both macro- and trace nutrients for syntheses of required proteins. Under influences of OA, macroalgal communities would shift toward non-calcifying species; diatoms become less abundance in phytoplankton assemblages. OA decreases calcification in algal calcifiers and exposes them to more harmful UV radiation, leading to a further decline of photosynthesis. Therefore, both the magnitude and direction of response of microalgae and macroalgae to OA largely depend on the levels of other environmental drivers (e.g., warming, deoxygenation). OA also exerts tremendous impacts on marine food webs. Total fatty acids and the ratio of long-chain polyunsaturated to saturated fatty acids of microalgae decrease, while some toxic secondary metabolites (such as phenolic compounds) accumulate under OA conditions, indicating a decline of food quality. This decline of food quality in primary producers can be transferred to secondary producers and negatively affect them (e.g., decrease in growth and reproduction). Taken together, OA can influence the biochemical compositions and contents in primary producers and their transfer to higher trophic levels and marine food webs is likely to be destabilized.

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Diel metabolism of Yellow Sea green tide algae alters bacterial community composition under in situ seawater acidification of coastal areas

Published 5 October 2021 Science Closed
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Highlights

  • Metabolism of algae mat leads to a diel pH and CO2 fluctuation in affected seawater.
  • Bacterial communities in diffusive boundary layer of the algae had a diel change.
  • Flavobacteriaceae was shown increased at night but sharp decreased at daytime.
  • Harmful algal bloom might influence coastal ocean acidification.

Abstract

Ocean acidification in coastal seawaters is a complex process, with coastal pH being affected by numerous factors including watershed and biological processes that also support metabolically diverse bacterial communities. The world’s largest macroalgal blooms have occurred consecutively in the Yellow Sea over the last 13 years. In particular, algal mats formed by Yellow Sea green tides (YSGT) significantly influence coastal environments. Herein, we hypothesized that 1) inorganic carbonate chemistry in coastal areas is altered by diel metabolism of these giant algal mats and that 2) bacterial community composition in diffusive boundary layers might be altered along diel cycles due to algal mat metabolism. In situ studies indicated that algal mat metabolism led to changes in diel pH and CO2 in affected seawaters. Such metabolic activities could intensify diel pH fluctuations in algal mat diffusive boundary layers, as noted by pH fluctuations of 0.22 ± 0.01 units, and pCO2 fluctuations of 214.62 ± 29.37 μatm per day. In contrast, pH fluctuations of 0.11 ± 0.02 units and pCO2 fluctuations of 79.02 ± 42.70 μatm were noted in unaffected areas. Furthermore, the bacterial community composition associated with diffusive algal boundary layers, including those of ambient bacteria and epiphytic bacteria, exhibited diel changes, while endophytic bacterial communities were relatively stable. Flavobacteriaceae were particularly highly abundant taxa in the ambient and epiphytic bacterial communities and exhibited increased abundances at night but sharp decreases in abundances during daytime. Flavobacteriaceae are heterotrophic taxa that could contribute to coastal area acidification at night due to the transformation of organic carbon to inorganic carbon. These results provide new insights to understand the variability in coastal ocean acidification via harmful algal blooms while providing a framework for evaluating the effects of YSGT on costal carbon cycling.

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Impacts of marine heatwaves on algal structure and carbon sequestration in conjunction with ocean warming and acidification

Published 24 September 2021 Science Closed
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As the ocean warms, the frequency, duration, intensity, and range of marine heatwaves (MHWs) increase. MHWs are becoming a severe challenge for marine ecosystems. However, our understanding in regard to their impacts on algal structure and carbon sequestration is still deficient or fragmentary, particularly when combined with ocean warming and acidification. In this paper, we reviewed the impacts of MHWs individually and combined with ocean warming and acidification on regime shift in algal community and carbon sequestration of both macroalgae and microalgae. Solid evidence shows that MHWs cause the decline of large canopy macroalgae and increase of turf-forming macroalgae in abundance, leading to the regime shift from kelp forests to seaweed turfs. Furthermore, increased grazing pressure on kelps due to tropicalization facilitates the expansion of turfs that prevent the recovery of kelps through plundering light and space. Meanwhile, MHWs could trigger microalgal blooms and the intensity of algal blooms is regulated by the severity of MHWs and nutrient availability. MHWs could lead to the decrease of carbon burial and sequestration by canopy-forming macroalgae due to depressed growth and increased mortality. The effects of MHWs on the productivity of microalgae are latitude-dependent: negative effects at low and mid-latitudes whilst positive effects at high latitudes. Ocean warming and acidification may accelerate the shift from kelps to turfs and thus decrease the carbon sequestration by canopy-forming macroalgae further. We propose that MHWs combined with ocean warming and acidification would reduce the biodiversity and facilitate the thriving of morphologically simple, ephemeral and opportunistic turfs and diatoms in coastal oceans, and phytoplankton with smaller size in open oceans. This structure shift would not be in favor of long-term carbon sequestration. Future studies could be conducted to test this hypothesis and investigate the impacts of MHWs on carbon sequestration under future ocean conditions.

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