Biogeochemical niches and trophic plasticity of shallow and mesophotic corals recovering from mass bleaching

Abstract

The differential survival of corals and their recovery from oceanic thermal stress is related not only to coral bleaching susceptibility but also physiological plasticity including trophic strategy. We investigated postbleaching event recovery of three morphologically diverse, reef-building coral species from shallow (10 m) and mesophotic (30 m) reefs across the Maldives archipelago. Trophic plasticity was evaluated by carbon and nitrogen contents and stable isotopes of coral hosts and their algal symbionts including isotopic niche position and size. Seawater temperatures recorded before, during, and after the mass coral bleaching event revealed significant thermal stress in both shallow and mesophotic reefs although cumulative warming appeared lower in mesophotic reefs (9.0 vs. 5.9 degree heating weeks, respectively). Ten months following thermal stress, increased nitrogen stable isotope (δ 15 N) values in all three coral host species without concurrent changes in water column δ 15 N indicated enhanced heterotrophic feeding during thermal stress and/or recovery. Host C:N decreased in all species post-bleaching event while host carbon content (%C) decreased in shallow, more autotrophic populations of two species, suggesting prolonged stress. Host carbon stable isotope (δ 13 C) values were lower in mesophotic, more heterotrophic populations of two species compared to shallow counterparts. Biogeochemical and isotopic niches both showed different niche specialization among the three species. Consistent shifts in host isotopic niche positions support a conserved host response mechanism (heterotrophy) to oceanic thermal stress, but shallow and mesophotic corals still had not fully recovered almost one year after the bleaching event.

Significant Differences in Coral Trophic Status Between Nearshore and Offshore Reefs Recorded by δ15N of Coral Symbiotic Zooxanthellae and Host Tissue in the South China Sea in the SCS

“…In addition, △ h−z 15 N values of corals in Xisha and Nansha were also significantly higher than that in Sanya ( p < 0.001, Figure 4c). The extent of coral △ h−z 15 N increase in Xisha and Nansha (1.49% and 1.70%, respectively) relative to Sanya were close to or even greater than that (0.3∼1.3%) caused by the increase in heterotrophic predation in the previously field bleaching study (Radice et al., 2022). Therefore, we suggest that relatively high δ 15 N h and △ h−z 15 N values in corals from the offshore Xisha and Nansha reefs are due to offshore corals have a relatively higher heterotrophic predation intensity compared to corals in the nearshore Sanya.…”

“…For example, Radice et al. (2022) reported that in Maldives archipelago, the δ 15 N h of coral Galaxea fascicularis increased by 0.6% after 10 months of thermal stress due to the increase of heterotrophic predation intensity. The indoor feeding experiment results of Rangel et al.…”

“…Similar changes in δ 15 N h have been found in previous field studies and laboratory feeding experiments. For example, Radice et al (2022) reported that in Maldives archipelago, the δ 15 N h of coral Galaxea fascicularis increased by 0.6% after 10 months of thermal stress due to the increase of heterotrophic predation intensity. The indoor feeding experiment results of Rangel et al (2019) showed that δ 15 N h of coral Porites lutea increased by 0.5% after feeding POM.…”

Marine heatwaves modulate the genotypic and physiological responses of reef‐building corals to subsequent heat stress

“…Adult Pocillopora and Stylophora that had experienced the 2016 and 2017 heatwaves at Lizard Island had distinct (non‐overlapping) coral host isotopic niches and also significantly greater protein content than juvenile conspecifics that had not experienced back to back bleaching, which may be a result of heterotrophic feeding as observed in experimental work (Ferrier‐Pagès et al., 2003 ; Krueger et al., 2020 ). Differential trophic strategies may be related to: (i) the history of environmental stress (Hughes & Grottoli, 2013 ; Radice et al., 2022 ; Schoepf et al., 2015 ), (ii) inherent species‐specific biology (Conti‐Jerpe et al., 2020 ; Grottoli et al., 2006 ), and/or (iii) differences in nutritional resources between regions (Fox et al., 2018 ). Large plankton δ 15 N values (>300 μm) were similar in both locations (before timepoint) indicating similar access to at least some oceanic resources while variability in smaller size fractions may represent a mixture of lagoonal and oceanic nutrient subsidies.…”

Heterotrophy in marine animal forests in an era of climate change

“…Food availability is a critical parameter affecting the distribution, composition, and productivity of MAFs (Portilho‐Ramos et al ., 2022). To cope with nutrient‐poor oligotrophic environments, MAF builders have evolved a variety of nutritional strategies, ranging from heterotrophy to almost complete autotrophy, depending on the contribution of symbiotic partners in mixotrophic organisms (Rossi et al ., 2019; Radice et al ., 2022). Mixotrophy, common in shallow warm‐water MAFs, is a particularly successful strategy because it expands the metabolic portfolio of animals (O'Malley, 2015).…”