Investigation of the effect of GFP-type proteins on photoinduced proton flux in reef corals

Projections of how reef corals will respond to climactic changes such as ocean acidification are limited by our understanding of their fundamental physiology, such as the mechanisms of how proton flux and pH are regulated. Green fluorescent protein (GFP)-type proteins present within corals have been hypothesized to have multiple biological roles, such as light regulation and photoprotection of symbiotic zooxanthellae. A previously experimentally unexplored role of GFP-type proteins within corals is proton pumping under light excitation. The aim of this study was to investigate the effect of GFP-type proteins on coral proton flux and change in pH in response to light illumination. This was achieved by comparing fluorescent and non-fluorescent samples of Euphyllia glabrescens and Favia pallida in response to different illumination wavelengths. Non-invasive microelectrode ion flux estimation (MIFE) measured proton flux and pH at the tissue level, and confocal microscopy and the pH sensitive fluorescent probe SNARF-1 AM was used to determine pH at the cellular level. Cytosolic pH of fluorescent host cells of E. glabrescens significantly decreased by 0.16 pH units in response to 60 minutes of blue light illumination. Furthermore, no response in cytosolic pH from non-fluorescent cells or under red illumination was observed. At the tissue level, blue and red wavelength illumination of F. pallida polyps resulted in approximately double the proton flux and pH change in high fluorescent areas than that of low fluorescent areas. This light activated acidification of cells and larger proton flux observed in fluorescent samples could be generated by GFP-type proteins as a mechanism to counteract the large increase in pH generated by zooxanthellae photosynthesis, potentially increasing both calcification and photosynthetic rates. The observed increase in proton flux in the presence of GFP-type proteins would enable reef corals greater capacity to modulate their pH, increasing their resilience to ocean acidification