Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery

Abstract

Ocean warming and the increased prevalence of coral bleaching events threaten coral reefs. However, the biology of corals during and following bleaching events under field conditions is poorly understood. We examined bleaching and postbleaching recovery in Montipora capitata and Porites compressa corals that either bleached or did not bleach during a 2014 bleaching event at three reef locations in Kāne‘ohe Bay, O‘ahu, Hawai‘i. We measured changes in chlorophylls, tissue biomass, and nutritional plasticity using stable isotopes (δ13C, δ15N). Coral traits showed significant variation among periods, sites, bleaching conditions, and their interactions. Bleached colonies of both species had lower chlorophyll and total biomass, and while M. capitata chlorophyll and biomass recovered 3 months later, P. compressa chlorophyll recovery was location dependent and total biomass of previously bleached colonies remained low. Biomass energy reserves were not affected by bleaching, instead M. capitata proteins and P. compressa biomass energy and lipids declined over time and P. compressa lipids were site specific during bleaching recovery. Stable isotope analyses did not indicate increased heterotrophic nutrition in bleached colonies of either species, during or after thermal stress. Instead, mass balance calculations revealed that variations in δ13C values reflect biomass compositional change (i.e., protein : lipid : carbohydrate ratios). Observed δ15N values reflected spatiotemporal variability in nitrogen sources in both species and bleaching effects on symbiont nitrogen demand in P. compressa. These results highlight the dynamic responses of corals to natural bleaching and recovery and identify the need to consider the influence of biomass composition in the interpretation of isotopic values in corals.

Trophic plasticity in a common reef‐building coral: Insights from δ13C analysis of essential amino acids

“…While we did observe correlation between our modelled AA ESS δ 13 C‐based estimates of coral heterotrophy and the commonly used bulk tissue Δ 13 C host–symbiont metric (Figure c), the AA ESS δ 13 C‐based estimates of heterotrophic nutrition provided a more quantitative assessment of coral nutrition among individual colonies. Coral bulk tissue δ 13 C values can vary as a function of the relative amounts of proteins, lipids and carbohydrates contained in the host (Wall, Ritson‐Williams, Popp, & Gates, ). Thus, by isolating a single component of coral tissue (i.e.…”

Divergent symbiont communities determine the physiology and nutrition of a reef coral across a light-availability gradient

“…However, examining the δ 13 C values in lipids between host and symbionts suggested an increase in heterotrophic carbon usage by S. pistillata in lipid synthesis below 20 m but consistently low δ 13 C-lipid values in F. favus across depths, suggesting high and invariable heterotrophic contributions to lipid biosynthesis in F. favus [48]. In bleached and post-bleaching recovered M. capitata and Porites compressa δ 13 C values showed poor relationships with bleaching history and did not suggest a greater capacity for heterotrophic feeding; instead δ 13 C values were best explained by an isotope mass balance accounting for differences in proteins:lipids:carbohydrates in coral tissues [33]. It is therefore important for uncertainties in isotope values to be further explored in symbiotic mixotrophic organisms and for caution to be applied in the interpretation of these data to infer trophic plasticity.…”

“…In the present study, changes in host and symbiont δ 13 C values appear to instead be an effect of the interaction between light-availability and symbiont community, in agreement with rapid internal cycling and a shared carbon source of the symbiont partners ( [49,61]). While M. capitata biomass and C:N was not influenced by symbiont types or environmental conditions, it is important for future studies to also consider the role of Symbiodiniaceae on coral metabolite profiles [31,90,91] which may influence tissue biochemical composition and isotope values [33,35,92,93].…”

“…Considering the δ 15 N values of heterotrophic sources in seawater (Fig. S4), corals and their symbionts most resemble the δ 15 N values of dissolved inorganic nitrogen (3.8-4.9‰ [33]) and not plankton endmembers. Therefore, M. capitata appears to have maintained high rates of photosynthesis (or P:R) as to not show systematic decreases in δ 15 N or 13 C Sk values and does not show evidence for increased heterotrophy with depth.…”

“…To cope with reductions in symbiont-derived nutrition from less-mutualistic symbionts, it has been hypothesized that coral hosts may shift towards greater heterotrophy to meet metabolic demands [30,31], as has been observed in corals under experimental thermal stress ( [32], but see in situ [33]) and on high turbidity reefs [34]. However, intraspecific changes in symbiont communities also reflect Symbiodiniaceae niche specialization, with some species being more efficient in uptake of carbon or nitrogen under low or high light and/or temperatures [17,23].…”