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1. Ocean acidification effects on in situ coral reef metabolism

   https://doi.org/10.1038/s41598-019-48407-7  

   Doo, Steve S.; Edmunds, Peter J.; Carpenter, Robert C. (August 2019, Scientific Reports)

   Abstract The Anthropocene climate has largely been defined by a rapid increase in atmospheric CO_2,causing global climate change (warming) and ocean acidification (OA, a reduction in oceanic pH). OA is of particular concern for coral reefs, as the associated reduction in carbonate ion availability impairs biogenic calcification and promotes dissolution of carbonate substrata. While these trends ultimately affect ecosystem calcification, scaling experimental analyses of the response of organisms to OA to consider the response of ecosystems to OA has proved difficult. The benchmark of ecosystem-level experiments to study the effects of OA is provided through Free Ocean CO₂Enrichment (FOCE), which we use in the present analyses for a 21-d experiment on the back reef of Mo’orea, French Polynesia. Two natural coral reef communities were incubatedin situ, with one exposed to ambient pCO₂(393 µatm), and one to high pCO₂(949 µatm). Our results show a decrease in 24-h net community calcification (NCC) under high pCO₂, and a reduction in nighttime NCC that attenuated and eventually reversed over 21-d. This effect was not observed in daytime NCC, and it occurred without any effect of high pCO₂on net community production (NCP). These results contribute to previous studies on ecosystem-level responses of coral reefs to the OA conditions projected for the end of the century, and they highlight potential attenuation of high pCO₂effects on nighttime net community calcification. 

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2. Sea‐ice microbial communities in the Central Arctic Ocean: Limited responses to short‐term pCO₂ perturbations

   https://doi.org/10.1002/lno.11690  

   Torstensson, Anders; Margolin, Andrew R.; Showalter, Gordon M.; Smith, Jr, Walker O.; Shadwick, Elizabeth H.; Carpenter, Shelly D.; Bolinesi, Francesco; Deming, Jody W. (January 2021, Limnology and Oceanography)

   Abstract The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO₂uptake. We studied how sea‐ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreakerOden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea‐ice microbial communities were exposed to elevated partial pressures of CO₂(pCO₂). We incubated intact, bottom ice‐core sections and dislodged, under‐ice algal aggregates (dominated byMelosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 μatm pCO₂for 10 d under different nutrient regimes. The results indicate that the growth of sea‐ice algae and bacteria was unaffected by these higher pCO₂levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO₂enrichment. These central Arctic sea‐ice microbial communities thus appear to be largely insensitive to short‐term pCO₂perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO₂and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea‐ice microorganisms even as the loss of Arctic sea ice continues. 

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3. Shallow coral reef free ocean carbon enrichment: Novel in situ flumes to manipulate pCO ₂ on shallow tropical coral reef communities

   https://doi.org/10.1002/lom3.10349  

   Srednick, Griffin; Bergman, Jessica L.; Doo, Steve S.; Hawthorn, Myron; Ferree, Jeffrey; Rojas, Roberto; Arias, Rogelio; Edmunds, Peter J.; Carpenter, Robert C. (February 2020, Limnology and Oceanography: Methods)

   Abstract Given the severe implications of climate change and ocean acidification (OA) for marine ecosystems, there is an urgent need to quantify ecosystem function in present‐day conditions to determine the impacts of future changes in environmental conditions. For tropical coral reefs that are acutely threatened by these effects, the metabolism of benthic communities provides several metrics suitable for this purpose, but the application of infrastructure to manipulate conditions and measure community responses is not fully realized. To date, most studies of the effects of OA on coral reefs have been conducted ex situ, and while greater ecological relevance can be achieved through free ocean carbon enrichment (FOCE) experiments on undisturbed areas of reef, such approaches have been deterred by technical challenges (e.g., spatial scale and duration, stable maintenance of conditions). In this study, we describe novel experimental infrastructure called shallow coral reef (SCoRe) FOCE to overcome these challenges and present data from a proof of concept application in Mo'orea, French Polynesia. Our objectives were to (1) implement an autonomous system that could be deployed kilometers from shore, (2) regulate the chemical (pCO₂) and physical properties of seawater over undisturbed, shallow (∼2–5‐m depth) coral reef over multiple weeks, and (3) measure the metabolic response of the coral community to the treatment conditions. We describe the design, function, and application of the SCoRe FOCE, and present data demonstrating its efficacy. This infrastructure has great potential for advancing ecologically relevant studies of the effects of changing environmental conditions on coral reefs. 

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4. Contrasting responses of photosynthesis and photochemical efficiency to ocean acidification under different light environments in a calcifying alga

   https://doi.org/10.1038/s41598-019-40620-8  

   Briggs, Amy A.; Carpenter, Robert C. (March 2019, Scientific Reports)

   Abstract Ocean acidification (OA) is predicted to enhance photosynthesis in many marine taxa. However, photophysiology has multiple components that OA may affect differently, especially under different light environments, with potentially contrasting consequences for photosynthetic performance. Furthermore, because photosynthesis affects energetic budgets and internal acid-base dynamics, changes in it due to OA or light could mediate the sensitivity of other biological processes to OA (e.g. respiration and calcification). To better understand these effects, we conducted experiments onPorolithon onkodes, a common crustose coralline alga in Pacific coral reefs, crossing pCO₂and light treatments. Results indicate OA inhibited some aspects of photophysiology (maximum photochemical efficiency), facilitated others (α, the responsiveness of photosynthesis to sub-saturating light), and had no effect on others (maximum gross photosynthesis), with the first two effects depending on treatment light level. Light also exacerbated the increase in dark-adapted respiration under OA, but did not alter the decline in calcification. Light-adapted respiration did not respond to OA, potentially due to indirect effects of photosynthesis. Combined, results indicate OA will interact with light to alter energetic budgets and potentially resource allocation among photosynthetic processes inP. onkodes, likely shifting its light tolerance, and constraining it to a narrower range of light environments. 

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5. Larval fish in a warming ocean: a bioenergetic study of temperature-dependent growth and assimilation efficiency

   https://doi.org/10.3354/meps14057  

   Shelley, CE; Johnson, DW (June 2022, Marine Ecology Progress Series)

   Rising temperatures have important consequences for somatic growth, but observed relationships between temperature and growth can vary in both magnitude and direction. The key to understanding such variation is knowing how temperature affects both the amount of energy available for growth and the efficiency with which surplus energy is assimilated into the body. We tested the hypothesis that patterns of temperature-dependent growth are driven by differential sensitivities of energy intake and expenditure to temperature. Larvae of California grunion Leuresthes tenuis were reared across a range of temperatures and 2 levels of food availability. Energy intake was measured from feeding rate, and energy expenditure was evaluated by measuring respiration and excretion rates. When food was abundant, both intake and expenditure increased with temperature, but intake increased more rapidly. These results suggest that high temperatures should lead to faster growth, and these predictions were confirmed by a separate experiment. In contrast, when food was restricted, the increase in energetic demand with temperature outpaced energy intake, suggesting a dwindling surplus of energy at high temperatures. This predicted reversal of the effects of temperature on growth was also confirmed experimentally. Finally, we compared patterns of energetics and growth to test the effects of temperature on food assimilation efficiency. When food was unlimited, assimilation efficiency decreased rapidly with temperature. When food was restricted, assimilation efficiency remained relatively high. Overall, our results emphasize the value of a bioenergetic perspective for illuminating why and how growth rates are likely to change in a warming ocean. 

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