Threatened Caribbean Coral Is Able to Mitigate the Adverse Effects of Ocean Acidification on Calcification by Increasing Feeding Rate

Abstract

Global climate change threatens coral growth and reef ecosystem health via ocean warming and ocean acidification (OA). Whereas the negative impacts of these stressors are increasingly well-documented, studies identifying pathways to resilience are still poorly understood. Heterotrophy has been shown to help corals experiencing decreases in growth due to either thermal or OA stress; however, the mechanism by which it mitigates these decreases remains unclear. This study tested the ability of coral heterotrophy to mitigate reductions in growth due to climate change stress in the critically endangered Caribbean coral Acropora cervicornis via changes in feeding rate and lipid content. Corals were either fed or unfed and exposed to elevated temperature (30°C), enriched pCO2 (800 ppm), or both (30°C/800 ppm) as compared to a control (26°C/390 ppm) for 8 weeks. Feeding rate and lipid content both increased in corals experiencing OA vs. present-day conditions, and were significantly correlated. Fed corals were able to maintain ambient growth rates at both elevated temperature and elevated CO2, while unfed corals experienced significant decreases in growth with respect to fed conspecifics. Our results show for the first time that a threatened coral species can buffer OA-reduced calcification by increasing feeding rates and lipid content.

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

“…Nevertheless, when photosynthesis was reduced in both fed and unfed corals at high temperature, calcification was also reduced by 61–65% compared to control temperature at day 28, and by 46–53% at day 56. These results suggest that heterotrophic feeding does not alter the sensitivity of corals to heat stress, as previously observed in corals under acidification stress63646566. Our observation is also in contrast to other studies demonstrating that supply of inorganic nutrients can reduce calcification sensitivity to both heat stress and to ocean acidification66676869.…”

Shifts in coral reef biogeochemistry and resulting acidification linked to offshore productivity

“…Thus, our finding of enhanced calcification during periods of increased heterotrophy runs counter to some modeling and field-based studies (10,19). However, numerous experiments have shown that if corals, the dominant reef calcifiers, are well fed (such as by zooplankton) or have nutritionally replete diets, they will have greater rates of both tissue and skeletal growth (20)(21)(22)(23)(24). Some results have even demonstrated that corals may be able to counteract reduced rates of calcification resulting from OA by increasing feeding rates (23).…”

“…Given the experimental evidence linking increased heterotrophy to higher calcification rates in corals (20)(21)(22)(23)(24) and the statistical significance of our cross-correlation analysis, we hypothesize that lateral advection of offshore blooms as well as nutrient upwelling, both of which were exacerbated during the winters of 2010 and 2011, possibly due to the NAO state, provided external pulses of nutrition to the reef. These pulses of nutrition enabled short-term shifts in reef NEC and NEP toward increasing calcification and heterotrophy, respectively, and it was these biogeochemical shifts that ultimately caused the observed changes in seawater pH and Ω aragonite (Fig.…”

“…However, numerous experiments have shown that if corals, the dominant reef calcifiers, are well fed (such as by zooplankton) or have nutritionally replete diets, they will have greater rates of both tissue and skeletal growth (20)(21)(22)(23)(24). Some results have even demonstrated that corals may be able to counteract reduced rates of calcification resulting from OA by increasing feeding rates (23). Furthermore, increased heterotrophy and elevated tissue growth in corals have been found to cause overall respiration rates to increase (21).…”

Two threatened Caribbean coral species have contrasting responses to combined temperature and acidification stress

“…Pandolfi et al () pointed out that, although lab, mesocosm, and field studies indicate that the impact of acidification is consistently negative (Kroeker et al ), the relationship with saturation state is highly variable and often nonlinear. Sensitivity to decreases in pH seems to be reduced when corals are given weeks, rather than days, to acclimate (Marubini et al ; Reynaud et al ; Ries et al ) when they have abundant food (Ries et al ; Edmunds ; Towle et al ), or are supplied with high levels of inorganic nutrients (Langdon and Atkinson ; Ezzat et al ). This broad range of variability in sensitivity to pH may partly be explained by differences in experimental methodology, but it is likely that there are real differences between species.…”