UC San Diego

Photosymbioses between cnidarian hosts and Symbiodiniaceae algae enable the success of coral reef ecosystems. Algae remain photosynthetically active in symbiosis capturing light energy to fix CO2, evolve O2, and produce photosynthates which are transferred to hosts as substrate for growth and metabolism. Host cells phagocytose free-living algae and sequester them in arrested phagosomes, termed “symbiosomes,” enabling control over symbionts’ metabolisms promoting the production of photosynthates while regulating symbiont population growth. The underlying cellular mechanisms are mostly unknown and only a V-type H+-ATPase (VHA)-dependent carbon-concentrating mechanism (CCM) is documented in corals. My thesis sought to identify whether the CCM is widespread in cnidarian photosymbioses, establish the CCM’s importance under various environmental conditions, and identify nitrogen transport mechanisms. Chapter 2 describes the cloning of a candidate coral NH3/NH4+ (total ammonia, Tamm) channel (ayRhp1) from Acropora yongei. Heterologous expression in Xenopus oocytes and flux assays determined ayRhp1 is a dual NH3/CO2 gas channel. Immunofluoresence Airyscan confocal microscopy (IACM) revealed ayRhp1 is present in the symbiosome membrane where, together with VHA, it concentrates Tamm in the symbiont’s microenvironment. Furthermore, ayRhp1 is preferentially present in the symbiosome membrane during the day; presumably to provide algae with Tamm to ensure photosynthate production during photosynthesis. Chapter 3 tested whether Cassiopea jellyfish, which host their symbiotic algae in motile cells called amebocytes, employ a CCM like that from coral host cells. Western blotting and AICM revealed that Cassiopea amebocytes express both VHA and carbonic anhydrase (CA) and that both proteins localized to the symbiosome membrane. Respirometry using VHA and CA inhibitors revealed significant reductions in O2 evolution rate, a proxy for photosynthetic activity, suggesting that host VHA and CA are functionally coupled in the CCM. Chapter 4 explored the prevalence of the CCM in the coral Stylophora pistillata after acclimation to various environmental light conditions. While VHA protein abundance correlated with increased irradiance, AICM found limited symbiosomal VHA and respirometry experiments did not detect functional evidence for CCMs. Instead, VHA was prominently observed in calcifying cells, where it may contribute to pH regulation for biomineralization. Together, my thesis advances our mechanistic understanding of nutrient exchange in cnidarian photosymbioses.