UC San Diego

While corals rely largely on their endosymbiotic Symbiodinaceae algae for carbon, other nutrients like nitrogen and phosphorous can be obtained heterotrophically. This feeding strategy relies on digesting plankton with the aid of gastrodermal mesenterial filaments (MFs). Additionally, acid-based digestion has been proposed to be involved; however, the putative acidifying mechanism(s) are unknown. I hypothesized that the V-type H+-ATPase (VHA), an enzyme that acidifies intra and extracellular spaces in a variety of organisms, is present in coral non-filamentous gastrodermal cells, cells that do not make up the MFs, and MFs of corals. Immunostaining and confocal imaging of Stylophora pistillata coral tissue sections revealed robust VHA expression in the cnidoglanular banding of short MFs (sMFs) and long convoluted MFs (lcMFs). VHA was also expressed in the ciliated tracts of sMFs, zones of phagocytosis in lcMFs, and in the some non-filamentous gastrodermal cells. S. pistillata colonies that were starved for 21 days had less abundant apical VHA than fed corals. Coral microcolonies growing over glass coverslips were utilized to further investigate the putative role of VHA in food digestion. Optical coherence tomography imaging showed normal physiological function in microcolonies. In these microcolonies, VHA was abundant in what appeared to be a subset of gastrovascular channels that contained intact or partially degraded diatoms captured from seawater, suggesting extracellular digestion. In summary, the presence of VHA in digestive tissues in S. pistillata suggests that VHA plays multiple roles in the digestive system of corals.