Organic carbon fluxes mediated by corals at elevated pCO2 and temperature

Abstract

Increasing ocean acidification (OA) and seawater temperatures pose significant threats to coral reefs globally. While the combined impacts of OA and seawater temperature on coral biology and calcification in corals have received significant study, research to date has largely neglected the individual and combined effects of OA and seawater temperature on coralmediated organic carbon (OC) fluxes. This is of particular concern as dissolved and particulate OC (DOC and POC, respectively) represent large pools of fixed OC on coral reefs. In the present study, coral-mediated POC and DOC, and the sum of these coral-mediated flux rates (total OC, TOC = DOC + POC) as well as the relative contributions of each to coral metabolic demand were determined for 2 species of coral, Acropora millepora and Turbinaria reniformis, at 2 levels of pCO 2 (382 and 741 μatm) and seawater temperatures (26.5 and 31.0°C). Independent of temperature, DOC fluxes decreased significantly with increases in pCO 2 in both species, resulting in more DOC being retained by the corals and only representing between 19 and 6% of TOC fluxes for A. millepora and T. reniformis. At the same time, POC and TOC fluxes were unaffected by elevated temperature and/or pCO 2. These findings add to a growing body of evidence that certain species of coral may be less at risk to the impacts of OA and temperature than previously thought.

Living in a high CO2 world: a global meta‐analysis shows multiple trait‐mediated fish responses to ocean acidification

“…Certain effects, such as an increase in fish activity (boldness) and changes in fish lateralization, do not have an immediate bearing on ecological “fitness,” because in nature they represent a trade‐off in which, for example, bold individuals can find food faster (Mamuneas et al. ) but are more exposed to predation (Ward et al. ) than shy individuals.…”

“…Other works showed that elevated temperature has a mitigating or no effect on the response of scleractinian corals to OA (Langdon andAtkinson 2005, Edmunds 2011). For instance, moderate increases in pCO 2 and warming may enhance coral calcification, whereas higher pCO 2 and thermal conditions may lead August 2018 FISHES RESPONSE TO OCEAN ACIDIFICATION 331 to decreasing calcification rates (Castillo et al 2014, Levas et al 2015. This variability in response to OA in invertebrates is mirrored by the effect of temperature and OA on fish.…”

Can heterotrophic uptake of dissolved organic carbon and zooplankton mitigate carbon budget deficits in annually bleached corals?

“…In recent years, dissolved organic carbon (DOC) has been increasingly recognized as another source of fixed carbon for corals and may play an important role in coral resistance to bleaching. Healthy corals are typically net producers of DOC (Haas et al 2010;Naumann et al 2010Naumann et al , 2012Levas et al 2015) via the release of mucus and/or dissolved organic materials that account for losses of 5-45 % of photosynthetically fixed C (Crossland et al 1980;Edmunds and Davies 1986;Crossland 1987;Bythell 1988;Ferrier-Pages et al 1998;Tanaka et al 2009), although DOC can be utilized by some corals (Naumann et al 2010;Tremblay et al 2012). However, temperature stress and bleaching may at times influence coral net DOC fluxes.…”

Heterotrophy promotes the re-establishment of photosynthate translocation in a symbiotic coral after heat stress

“…First, they can alter the taxonomic composition of their endosymbiont communities toward a greater abundance of more thermally-resistant symbionts32, but this can have negative consequences for their growth2433. Second, corals can increase their heterotrophic feeding capacity283435, wherein energy and nutrients acquired through plankton feeding are either directly used by the host for its own needs2836 or translocated to the symbionts to improve their growth and photosynthetic efficiency3738. However, not all coral species are able to increase their heterotrophic capacity283439, and the effectiveness of this strategy to promote bleaching-tolerance of corals in general is therefore uncertain.…”

“…mucus) into the surrounding environment2340. However, evidence in the literature indicates that bleaching can either stimulate the loss of organic matter394142 or its uptake35. Finally, corals can increase translocation of photosynthates by the remaining symbionts to sustain the host metabolism43 or translocate carbon to their symbionts to enhance their growth and photosynthesis, which in turn will increase carbon translocation38.…”