UC San Diego

Coral reefs ecosystems are facing mounting threats in the Anthropocene Era, yet basic coral biology remains largely uncharacterized. An essential and unexplored physiological mechanism in corals is the transport of nitrogenous molecules such as NH3/NH4+. The goal of this thesis is to characterize the molecular and cellular mechanisms of Acropora yongei which enable nitrogen excretion and metabolic exchange between the coral animal and symbiotic Symbiodinium. This is the first study to utilize molecular and immunohistochemistry techniques to investigate the role of a Cnidarian Rh protein, a putative NH3 gas channel. Phylogenetic analysis determined A. yongei Rh (ayRh) is part of the Rhp1 protein subgroup, members of which are capable of transporting NH3 and possibly CO2 gasses along partial pressure gradients. ayRh was colocalized with the vacuolar H+-ATPase (VHA) on the symbiosome membrane constituting a novel nitrogen concentrating mechanism (NCM). ayRh-dependent NH3/NH4+ supply to Symbiodinium appears to vary on a diel cycle, possibly as a mechanism that allows coral host cells to regulate symbiont metabolism and growth. Preliminary evidence also suggests VHA may traffic with ayRh in a protein complex. Continuously high levels of ayRh are also observable in desmocytes, cells that facilitate skeletal attachment. Here, diffusion of NH3 via ayRh may play a role in regulating the pH of the calcifying environment or in integration with the aboral mesoglea. Finally, ayRh was localized to the oral ectoderm where it appears to facilitate nitrogenous waste excretion into seawater, possibly enhanced by microvilli movement akin to ciliary beating in mussels.