Posts Tagged 'nutrients'

Impact of temperature, CO2, and iron on nutrient uptake by a late-season microbial community from the Ross Sea, Antarctica

The Southern Ocean is rapidly changing as a result of rising sea surface temperatures, elevated CO2 concentrations, and modifications to iron sources and sinks. The Southern Ocean has seasonally high rates of primary production, making it critical to determine how changes will impact biogeochemical rate processes in this important sink for CO2. During the austral summer, we measured nitrogen and carbon uptake rates by a late-season Ross Sea microbial community under different potential climate change conditions. A natural microbial assemblage was collected from the ice edge, and grown using a semi-continuous culturing followed by a continuous culturing ‘ecostat’ approach. The individual and combined impacts of temperature elevation and iron addition were tested during both approaches, and CO2 level was also manipulated during the continuous experiment. Nutrient concentrations and biomass parameters were measured throughout both experiments. During the continuous experiment we also measured uptake rates of nitrate (NO3-) and dissolved inorganic carbon (DIC) by 2 size classes (0.7-5.0 and >5.0 µm) of microorganisms. Of the parameters tested, temperature elevation had the largest impact, significantly increasing NO3- and DIC uptake rates by larger microorganisms. Iron addition was also important; however, the magnitude of its impact was greater when temperature was also changed. These results indicate that NO3- and DIC uptake rates may increase as sea surface warming occurs in the Southern Ocean, and thus have important implications for estimating new production and potential carbon uptake and eventual export to the deep sea.

Continue reading ‘Impact of temperature, CO2, and iron on nutrient uptake by a late-season microbial community from the Ross Sea, Antarctica’

Bottom-up effects on biomechanical properties of the skeletal plates of the sea urchin Paracentrotus lividus (Lamarck, 1816) in an acidified ocean scenario

Highlights

  • Biomechanical properties of sea urchin test have a great importance in their individual fitness.
  • Combined effect of decreased pH and macroalgal diet highlights potential cascading effects.
  • No direct short-term effect of decreased pH and macroalgal diet on plate mechanical properties.
  • Longer term exposure needed to observe substantial differences on skeletal plate structure.

Abstract

Sea urchins, ecologically important herbivores of shallow subtidal temperate reefs, are considered particularly threatened in a future ocean acidification scenario, since their carbonate structures (skeleton and grazing apparatus) are made up of the very soluble high-magnesium calcite, particularly sensitive to a decrease in pH. The biomechanical properties of their skeletal structures are of great importance for their individual fitness, because the skeleton provides the means for locomotion, grazing and protection from predators. Sea urchin skeleton is composed of discrete calcite plates attached to each other at sutures by organic ligaments. The present study addressed the fate of the sea urchin Paracentrotus lividus (Lamarck, 1816) skeleton in acidified oceans, taking into account the combined effect of reduced pH and macroalgal diet, with potential cascading consequences at the ecosystem level. A breaking test on individual plates of juvenile specimens fed different macroalgal diets has been performed, teasing apart plate strength and stiffness from general robustness. Results showed no direct short-term effect of a decrease in seawater pH nor of the macroalgal diet on single plate mechanical properties. Nevertheless, results from apical plates, the ones presumably formed during the experimental period, provided an indication of a possible diet-mediated response, with sea urchins fed the more calcified macroalga sustaining higher forces before breakage than the one fed the non-calcified algae. This, on the long term, may produce bottom-up effects on sea urchins, leading to potential shifts in the ecosystem equilibrium under an ocean acidified scenario.

Continue reading ‘Bottom-up effects on biomechanical properties of the skeletal plates of the sea urchin Paracentrotus lividus (Lamarck, 1816) in an acidified ocean scenario’

Divergent responses of Atlantic cod to ocean acidification and food limitation

In order to understand the effect of global change on marine fishes, it is imperative to quantify the effects on fundamental parameters such as survival and growth. Larval survival and recruitment of the Atlantic cod (Gadus morhua) was found to be heavily impaired by end‐of‐century levels of ocean acidification. Here, we analysed larval growth among 35‐36 days old surviving larvae, along with organ development and ossification of the skeleton. We combined CO2‐treatments (ambient: 503 μatm, elevated: 1179 μatm) with food availability in order to evaluate the effect of energy limitation in addition to the ocean acidification stressor. As expected, larval size (as a proxy for growth) and skeletogenesis were positively affected by high food availability. We found significant interactions between acidification and food availability. Larvae fed ad libitum showed little difference in growth and skeletogenesis due to the CO2 treatment. Larvae under energy limitation were significantly larger and had further developed skeletal structures in the elevated CO2 treatment compared to the ambient CO2 treatment. However, the elevated CO2 group revealed impairments in critically important organs, such as the liver, and had comparatively smaller functional gills indicating a mismatch between size and function. It is therefore likely that individual larvae that had survived acidification treatments, will suffer from impairments later during ontogeny. Our study highlights important allocation trade‐off between growth and organ development, which is critically important to interpret acidification effects on early life‐stages of fish.

Continue reading ‘Divergent responses of Atlantic cod to ocean acidification and food limitation’

The impacts of iron limitation and ocean acidification on the cellular stoichiometry, photophysiology, and transcriptome of Phaeocystis antarctica

Phaeocystis antarctica is an integral player of the phytoplankton community of the Southern Ocean (SO), the world’s largest high-nutrient low-chlorophyll region, and faces chronic iron (Fe) limitation. As the SO is responsible for 40% of anthropogenic CO2 uptake, P. antarctica must also deal with ocean acidification (OA). However, mechanistic studies investigating the effects of Fe limitation and OA on trace metal (TM) stoichiometry, transcriptomic, and photophysiological responses of this species, as well as on the Fe chemistry, are lacking. This study reveals that P. antarctica responded strongly to Fe limitation by reducing its growth rate and particulate organic carbon (POC) production. Cellular concentrations of all TMs, not just Fe, were greatly reduced, suggesting that Fe limitation may drive cells into secondary limitation by another TM. P. antarctica was able to adjust its photophysiology in response to Fe limitation, resulting in similar absolute electron transport rates across PSII. Even though OA-stimulated growth in Fe-limited and -replete treatments, the slight reduction in cellular POC resulted in no net effect on POC production. In addition, relatively few genes were differentially expressed due to OA. Finally, this study demonstrates that, under our culture conditions, OA did not affect inorganic Fe or humic-acid-like substances in seawater but triggered the production of humic-acid-like substances by P. antarctica. This species is well adapted to OA under all Fe conditions, giving it a competitive advantage over more sensitive species in a future ocean.

Continue reading ‘The impacts of iron limitation and ocean acidification on the cellular stoichiometry, photophysiology, and transcriptome of Phaeocystis antarctica’

Response of subtropical phytoplankton communities to ocean acidification under oligotrophic conditions and during nutrient fertilization

The subtropical oceans are home to the largest phytoplankton biome on the planet. Yet, little is known about potential impacts of ocean acidification (OA) on phytoplankton community composition in the vast oligotrophic ecosystems of the subtropical gyres. To address this question, we conducted an experiment with 9 in situ mesocosms (~35 m3) off the coast of Gran Canaria in the eastern subtropical North Atlantic over a period of 9 weeks. By establishing a gradient of pCO2 ranging from ~350 to 1,025 μatm, we simulated carbonate chemistry conditions as projected until the end of the twenty-first century. Furthermore, we injected nutrient-rich deep water into the mesocosms halfway through the experiment to simulate a natural upwelling event, which regularly leads to patchy nutrient fertilization in the study region. The temporal developments of major taxonomic groups of phytoplankton were analyzed by flow cytometry, pigment composition and microscopy. We observed distinct shifts in phytoplankton community structure in response to high CO2, with markedly different patterns depending on nutrient status of the system. Phytoplankton biomass during the oligotrophic phase was dominated by picocyanobacteria (Synechococcus), which constituted 60–80% of biomass and displayed significantly higher cell abundances at elevated pCO2. The addition of deep water triggered a substantial bloom of large, chain-forming diatoms (mainly Guinardia striata and Leptocylindrus danicus) that dominated the phytoplankton community during the bloom phase (70–80% of biomass) and until the end of the experiment. A CO2 effect on bulk diatom biomass became apparent only in the highest CO2 treatments (>800 μatm), displaying elevated concentrations especially in the stationary phase after nutrient depletion. Notably, these responses were tightly linked to distinct interspecific shifts within the diatom assemblage, particularly favoring the largest species Guinardia striata. Other taxonomic groups contributed less to total phytoplankton biomass, but also displayed distinct responses to OA treatments. For instance, higher CO2 favored the occurrence of prymnesiophyceae (Phaeocystis globosa) and dictyochophyceae, whereas dinoflagellates were negatively affected by increasing CO2. Altogether, our findings revealed considerable shifts in species composition in response to elevated CO2 and/or lower pH, indicating that phytoplankton communities in the subtropical oligotrophic oceans might be profoundly altered by ocean acidification.

Continue reading ‘Response of subtropical phytoplankton communities to ocean acidification under oligotrophic conditions and during nutrient fertilization’

The effects of nutrient addition and ocean acidification on tropical crustose coralline algae

As the global population increases, the occurrence of multiple anthropogenic
impacts on valuable coastal ecosystems, such as coral reefs, also increases. These
stressors can be global and long-term, like ocean acidification (OA), or local and short term, like nutrient runoff in some areas. The combination of these stressors can  potentially have additive or interactive effects on the organisms in coral reef
communities. Among the most important groups of organisms on coral reefs are crustose coralline algae (CCA), calcifying algae that cement the reef together and contribute to the global carbon cycle. This thesis studied the effects of nutrient addition and OA on Lithophyllum kotschyanum, a common species of CCA on the fringing reefs of Mo’orea, French Polynesia. Two mesocosm experiments tested the individual and interactive effects of OA and short-term nitrate and phosphate addition on L. kotschyanum. These experiments showed that nitrate and phosphate addition together increased photosynthesis, OA had interactive effects with nutrient addition, and after nutrient addition ended, calcification and photosynthetic rates changed in unpredictable ways in different OA and nutrient treatments. Because the results of the first two experiments showed impacts of nutrients even after addition stopped, two more mesocosm experiments were conducted to study the changes in photosynthesis and calcification over hourly time scales more relevant to a single nutrient pulse event. These two experiments revealed the existence of diurnal variation in light-saturated photosynthetic rate, but not calcification rate, under ambient and elevated pCO2. This pattern of increased maximum photosynthesis in the middle of the day can have important implications for how the time of nutrient runoff events during the day impacts CCA physiology. Finally, a field experiment was conducted to determine the effects of short- and long-term nutrient addition on L. kotschyanum. The results showed that a series of short-term nutrient additions did not increase photosynthesis or calcification rates above those in ambient nutrient conditions, but continual nutrient enrichment for 6 weeks increased photosynthetic rates. This increase in photosynthesis under only long-term enrichment shows the need for consideration of specific nutrient addition scenarios on coral reefs when predicting how the community will be affected.

Continue reading ‘The effects of nutrient addition and ocean acidification on tropical crustose coralline algae’

Plankton community respiration and ETS activity under variable CO2 and nutrient fertilization during a mesocosm study in the subtropical North Atlantic

The enzymatic electron transport system (ETS) assay is frequently used as a proxy of respiratory activity in planktonic communities. It is thought to estimate the maximum overall activity of the enzymes associated with the respiratory ETS systems in both eukaryotic and prokaryotic organisms. Thus, in order to derive actual respiration rates (R) from ETS it is necessary to determine empirical R/ETS conversion algorithms. In this study we explore the temporal development of R and ETS activity in natural plankton communities (from bacteria to large phytoplankton) enclosed in mesocosms, treated with different CO2 concentrations. The experiment lasted 30 days, during which abrupt changes in community structure and biomass occurred through a sharp transition from oligotrophy (phase I) to highly eutrophic conditions (phase II) after nutrient-induced fertilization (day 18). R and ETS did not show any response to CO2 under oligotrophic conditions, but R increased significantly more in the two high CO2 mesocosms after fertilization, coinciding with a sharp rise in large phytoplankton (mostly diatoms). R and ETS were significantly correlated only during the eutrophic phase. The R/ETS ranged more than 3 fold in magnitude during the experiment, with phase-averaged values significantly higher under oligotrophic conditions (0.7-1.1) than after nutrient fertilization (0.5-0.7). We did not find any significant relationship between R/ETS and community structure or biomass, although R correlated significantly with total biomass after fertilization in the four mesocosms. Multiple stepwise regression models show that large phytoplankton explains most of the variance in R during phases I (86%) and II (53%) and of ETS (86%) during phase II, while picophytoplankton contributes up to 73% to explain the variance in the ETS model during phase I. Our results suggest that R/ETS may be too variable in the ocean as to apply constant values to different communities living under contrasting environmental conditions. Controlled experiments with natural communities, like the present one, would help to constrain the range of variability of the R/ETS ratio, and to understand the factors driving it.

Continue reading ‘Plankton community respiration and ETS activity under variable CO2 and nutrient fertilization during a mesocosm study in the subtropical North Atlantic’


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Ocean acidification in the IPCC AR5 WG II

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