AUTHOR=Ip Yuen K. , Chew Shit F. 

TITLE=Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 8 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.627722

DOI=10.3389/fmars.2021.627722

ISSN=2296-7745

ABSTRACT=Giant clams can grow to large sizes despite living in oligotrophic waters of the tropical Indo-Pacific, because they maintain a mutualistic relationship with phototrophic dinoflagellates (zooxanthellae) that provide them with photosynthates. The host clam needs to absorb exogenous inorganic carbon (Ci), nitrogen (N) and phosphorus (P), and furnish them to the symbionts, which cannot access the ambient seawater because they are living extracellularly inside a tubular system located mainly in the colorful outer mantle. In light, photosynthesizing symbionts need greater supplies of these nutrients than in the dark. Hence, the uptake rates of these exogenous nutrients by the host must be greater in light than in darkness, implying that the host’s transporters involved need to be regulated by some kind of light-responsive mechanisms. Furthermore, the growth and development of the host must also be light-dependent, as it only receives photosynthates from the symbionts in light. Consequently, giant clams display many light-dependent phenomena related to phototrophy, antioxidative defense, biomineralization, as well as absorption of exogenous Ci, N and P. These phenomena involve enzymes and transporters in several organs of the host, whereby their gene and protein expression levels are upregulated during illumination. This review aims to examine molecular mechanisms of light-dependent physiological phenomena in intact giant clam-dinoflagellate associations, and to highlight differences between giant clams and scleractinian corals in those regards. As the population of giant clams in nature are dwindling due to climate change and ocean acidification, a good understanding of their light-dependent processes may generate new ideas to improve their growth and survival in the dynamic and rapidly changing environmental conditions.