Ocean warming increases the incidence of coral bleaching, which reduces or eliminates the nutrition corals receive from their algal symbionts, often resulting in widespread mortality. In contrast to extensive knowledge on the thermal tolerance of coral-associated symbionts, the role of the coral host in bleaching patterns across species is poorly understood. Here, we applied a Bayesian analysis of carbon and nitrogen stable isotope data to determine the trophic niche overlap between corals and their symbionts and propose benchmark values that define autotrophy, heterotrophy, and mixotrophy. The amount of overlap between coral and symbiont niche was negatively correlated with polyp size and bleaching resistance. Our results indicated that as oceans warm, autotrophic corals lose their competitive advantage and thus are the first to disappear from coral reefs.

Turbid coral reefs experience high suspended sediment loads and low-light conditions that vertically compress the maximum depth of reef growth. Although vertical reef compression is hypothesized to further decrease available coral habitat as environmental conditions on reefs change, its causative processes have not been fully quantified. Here, we present a high-resolution time series of environmental parameters known to influence coral depth distribution (light, turbidity, sedimentation, currents) within reef crest (2–3 m) and reef slope (7 m) habitats on two turbid reefs in Singapore. Light levels on reef crests were low [mean daily light integral (DLI): 13.9 ± 5.6 and 6.4 ± 3.0 mol photons m–2 day–1 at Kusu and Hantu, respectively], and light differences between reefs were driven by a 2-fold increase in turbidity at Hantu (typically 10–50 mg l–1), despite its similar distance offshore. Light attenuation was rapid (KdPAR: 0.49–0.57 m–1) resulting in a shallow euphotic depth of <11 m, and daily fluctuations of up to 8 m. Remote sensing indicates a regional west-to-east gradient in light availability and turbidity across southern Singapore attributed to spatial variability in suspended sediment, chlorophyll-a and colored dissolved organic matter. Net sediment accumulation rates were ∼5% of gross rates on reefs (9.8–22.9 mg cm–2 day–1) due to the resuspension of sediment by tidal currents, which contribute to the ecological stability of reef crest coral communities. Lower current velocities on the reef slope deposit ∼4 kg m2 more silt annually, and result in high soft-sediment benthic cover. Our findings confirm that vertical reef compression is driven from the bottom-up, as the photic zone contracts and fine silt accumulates at depth, reducing available habitat for coral growth. Assuming no further declines in water quality, future sea level rise could decrease the depth distribution of these turbid reefs by a further 8–12%. This highlights the vulnerability of deeper coral communities on turbid reefs to the combined effects of both local anthropogenic inputs and climate-related impacts.

No abstract

The role of diazotrophs in coral physiology and reef biogeochemistry remains poorly understood, in part because N2 fixation rates and diazotrophic community composition have only been jointly analyzed in the tissue of one tropical coral species. We performed field-based 15N2 tracer incubations during nutrient-replete conditions to measure diazotroph-derived nitrogen (DDN) assimilation into three species of scleractinian coral (Pocillopora acuta, Goniopora columna, Platygyra sinensis). Using multi-marker metabarcoding (16S rRNA, nifH, 18S rRNA), we analyzed DNA- and RNA-based communities in coral tissue and skeleton. Despite low N2 fixation rates, DDN assimilation supplied up to 6% of the holobiont’s N demand. Active coral-associated diazotrophs were chiefly Cluster I (aerobes or facultative anaerobes), suggesting that oxygen may control coral-associated diazotrophy. Highest N2 fixation rates were observed in the endolithic community (0.20 µg N cm−2 per day). While the diazotrophic community was similar between the tissue and skeleton, RNA:DNA ratios indicate potential differences in relative diazotrophic activity between these compartments. In Pocillopora, DDN was found in endolithic, host, and symbiont compartments, while diazotrophic nifH sequences were only observed in the endolithic layer, suggesting a possible DDN exchange between the endolithic community and the overlying coral tissue. Our findings demonstrate that coral-associated diazotrophy is significant, even in nutrient-rich waters, and suggest that endolithic microbes are major contributors to coral nitrogen cycling on reefs.

Surface water samples of size-selected seston (0.7-20 lm) were collected from April 2013 to September 2013 at three similar coarse-sand benthic habitats. Additionally, seston sampling was performed at a fixed location throughout a complete tidal cycle (2014). A combination of fatty acid (FA), isotope, and flow cytometry analyses were used to determine the quality and quantity of nano-and pico-sized particulate organic matter (POM). High variability was found between fatty acid replicate samples. Similar temporal patterns were observed at two sheltered sites, while the exposed site displayed less pronounced seasonal changes. Lower concentrations of 16C and 18C polyunsaturated fatty acids were found during low tide sampling. Globally, POM was dominated by picoeukaryotes, with concentrations exceeding 50,000 cells mL 21 , and (16:4x3 1 18:3x3)/Rx3 is proposed as novel biomarker of picoeukaryotes in this region.

No abstract

Coral reef productivity depends on fast nutrient cycling, mediated largely by enzymatic breakdown of organic matter. Alkaline phosphatases hydrolyse phosphomonoesters and are one of the key enzymes involved in marine phosphorus cycling. They are expressed by a plethora of marine organisms including both planktonic microbes and metazoans such as corals, often in response to phosphate limitation, and are potentially important for coral P nutrition and reef biogeochemical cycling. However, most alkaline phosphatase activity (APA) data are from open-ocean environments, and the rates and drivers of APA in coastal waters are not well understood. Here, we measured APA both in the water column and associated with three coral species at reefs in Singapore, where the monsoonal ocean current reversal creates strong seasonal changes in dissolved nutrient availability. Water column APA was consistently high, averaging 9 ± 10 nmol l-1 h-1, but was not correlated with dissolved phosphate or other biogeochemical parameters. Experimental phosphate addition did not reduce seawater APA but addition of labile organic carbon did increase seawater APA, indicating that the increase in APA was driven by heterotrophic activity rather than phosphate stress. Coral APA ranged from 12–163 µmol m-2 h-1 depending on species and was equivalent to the APA in several meters of overlying water. While most coral APA was associated with the coral holobiont rather than the coral mucus, corals released 12 – 55 µmol h-1 per m2 of mucus-associated APA into the water column, which is potentially significant for water column DOP cycling.

M. (2022). Monsoon-driven biogeochemical dynamics in an equatorial shelf sea: time-series observations in the Singapore Strait.