Effects of Ocean Acidification on Carbon and Nitrogen Fixation in the Hermatypic Coral Galaxea fascicularis

Abstract

The supply of metabolites from symbionts to scleractinian corals is crucial to coral health. Members of the Symbiodiniaceae can enhance coral calcification by providing photosynthetically fixed carbon (PFC) and energy, whereas dinitrogen (N2)-fixing bacteria can provide additional nutrients such as diazotrophically-derived nitrogen (DDN) that sustain coral productivity especially when alternative external nitrogen sources are scarce. How these mutualistic associations benefit corals in the future acidifying ocean is not well understood. In this study, we investigated the possible effects of ocean acidification (OA; pHs 7.7 and 7.4 vs. 8.1) on calcification in the hermatypic coral Galaxea fascicularis with respect to PFC and DDN assimilation. Our measurements based on isotopic tracing showed no significant differences in the assimilation of PFC among different pH treatments, but the assimilation of DDN decreased significantly after 28 days of stress at pH 7.4. The decreased DDN assimilation suggests a nitrogenous nutrient deficiency in the coral holotiont, potentially leading to reduced coral calcification and resilience to bleaching and other stressful events. This contrasting impact of OA on carbon and N flux demonstrates the flexibility of G. fascicularis in coping with OA, apparently by sustaining a largely undamaged photosystem at the expense of N2 fixation machinery, which competes with coral calcification for energy from photosynthesis. These findings shed new light on the critically important but more vulnerable N cycling in hospite, and on the trade-off between coral hosts and symbionts in response to future climate change.

Acclimatization of a coral-dinoflagellate mutualism at a CO2 vent

“…Indeed, dinoflagellates supply glycerol as an energy source for cyanobacterial symbionts found in Montipora cavernosa , providing a steady supply of reductant (e.g., NADPH) and ATP for dinitrogen fixation in the cyanobacteria 60 . Additionally, N 2 fixation by coral-associated diazotrophs, coral calcification, reproduction, and tissue thickening are energy-intensive processes that probably compete for energy deriving from photosynthesis within the coral holobiont 66 . Thus, massive N 2 fixation, together with maintenance of coral reproduction and thickening of coral tissue occurring at the low pH Sites could be absorbing most of the photosynthetic energy from the symbiotic algae, creating an energy deficit that could possibly explain the previously observed decline in net calcification rates of this species along the same pH gradient 24 , 34 .…”

Understanding nitrogen dynamics in coral holobionts: comprehensive review of processes, advancements, gaps, and future directions

“…For acidification, which mainly focused on pCO 2 elevation, inconsistent results were documented. Some studies demonstrate a decrease in N 2 fixation rates as pCO 2 increased, explained by the reallocation of energy for calcification (Rädecker et al, 2014;Zheng et al, 2021). However, some field studies showed that N 2 fixation rates and diazotrophs in coral holobionts were higher in high pCO 2 sites than low pCO 2 sites (Biagi et al, 2020;Meunier et al, 2021b;Prada et al, 2023), such enhancement of N 2 fixation in space was attributable to the higher N requirements induced by the enhanced photosynthesis under high pCO 2 condition (Biagi et al, 2020).…”

Lineage-specific symbionts mediate differential coral responses to thermal stress

“…The extent to which corals adapt to a changing climate relies in part on the genetic variation of Symbiodiniaceae [17,18] and the environmental conditions under which the coral-dinoflagellate mutualisms develop [19,20]. Different Symbiodiniaceae strains or species are likely to differ in their intrinsic adaptive capacity, and stresstolerant traits may evolve in some opportunistic symbiont variants [21].…”