Levels of autotrophy and heterotrophy in mesophotic corals near the end photic zone

Mesophotic corals live at similar to 30-150 m depth and can sustain metabolic processes under light-limited conditions by enhancing autotrophy through specialized photoadaptations or increasing heterotrophic nutrient acquisition. These acclimatory processes are often species-specific, however mesophotic ecosystems are largely unexplored and acclimation limits for most species are unknown. This study examined mesophotic coral ecosystems using a remotely operated vehicle (Ashmore Reef, Western Australia at 40-75m depth) to investigate the trophic ecology of five species of scleractinian coral (from genera Leptoseris, Pachyseris, and Craterastrea) using stable isotope analyses (delta C-13 and delta N-15) of host and symbiont tissues and protein concentration. Trophic strategies were analyzed between species and between overall corals sampled above and below the end-photic point, where light is only 1% of surface irradiance. Results showed species-specific differences in resource use. Leptoseris hawaiiensis, L. scabra, and P. speciosa had similar Delta C-13 values (delta C-13 host - delta C-13 symbiont) approaching zero (< 0.5 parts per thousand) which indicated greater dependence on symbiont autotrophy. In contrast, Leptoseris glabra and Craterastrea levis had higher Delta C-13 values (1.4 to 3.5 parts per thousand) which indicated a greater reliance on external carbon sources. The latter two species also demonstrated tight nitrogen recycling within the holobiont, exhibiting low Delta N-15 values (host delta N-15 - symbiont delta N-15 =< 0.5 parts per thousand), compared to more autotrophic species (Delta N-15 = >1.2 parts per thousand). Some species demonstrated the ability to maintain metabolic processes despite substantially reduced light availability (0.5 - 2% of surface irradiance). This research challenges our knowledge of acclimation limits for many scleractinian corals and contributes novel information for Ashmore Reef, the Western Australia region and mesophotic ecosystems in general, and critically examines common methods used to interpretate trophic ecology with bulk stable isotopes delta C-13 and delta N-15.