Mesophotic corals in Hawaii maintain autotrophy to survive low-light conditions: Stable isotope dataset

In mesophotic coral ecosystems, reef-building corals and their
photosynthetic symbionts can survive with less than 1% of surface
irradiance. How depth-specialist corals rely upon autotrophically and
heterotrophically-derived energy sources across the mesophotic zone
remains unclear. We analyzed the stable carbon (δ13C) and nitrogen (δ15N)
isotope values of a Leptoseris community from the ʻAuʻau
Channel, Maui, Hawaiʻi (65-125 m) including four coral host species living
symbiotically with three algal haplotypes. We characterized the isotope
values of hosts and symbionts across species and depth to compare trophic
strategies. Symbiont δ13C was consistently 0.5‰ higher than host δ13C at
all depths. Mean colony host and symbiont δ15N differed by up to 3.7‰ at
shallow depths and converged at deeper depths. These results suggest that
both heterotrophy and autotrophy remained integral to colony survival
across depth. The increasing similarity between host and symbiont δ15N at
deeper depths suggests that nitrogen is more efficiently shared between
mesophotic coral hosts and their algal symbionts to sustain autotrophy.
Isotopic trends across depth did not generally vary by host species or
algal haplotype, suggesting that photosynthesis remains essential to
Leptoseris survival and growth despite low light availability in the
mesophotic zone.