Changes in free-living zooxanthellae abundance during summer in a temperate Japanese coastal sea (Takashima, Japan)

Free-living or exogenous zooxanthellae play a crucial role in coral survival; however, their ecological dynamics remain largely unexplored. This study focused on tracking the abundance of free-living zooxanthellae (Clades A to F) in seawater around the temperate coastal sea (Takashima, Japan) from summer to early autumn using quantitative polymerase chain reaction (qPCR). In addition, we examined the influence of temperature, light conditions (photosynthetic photon flux density [PPFD]), dissolved inorganic nitrogen (DIN), and coral health on the abundance of zooxanthellae. Our findings revealed an increase in the abundance of free-living zooxanthellae from late summer to early autumn, irrespective of clade. A significant negative correlation was observed between the ribosomal RNA (rRNA) gene copy number, amplified using a universal primer, and coral health scores. Conversely, there was a positive correlation between the rRNA gene copy number and seawater temperature. The relationship between PPFD and DIN was not significantly correlated with rRNA gene copy number. Notably, the rRNA gene copy number, amplified using the Clade C primer, was the most abundant and exhibited a significant positive correlation with seawater temperature, whereas it was negatively correlated with coral health scores. The increase in the abundance of free-living zooxanthellae, particularly Clade C, in seawater was facilitated by rising seawater temperatures and the release of zooxanthellae cells from host corals, with the latter likely being the predominant factor. To our knowledge, this is the first study to monitor temporal changes in the abundance of free-living zooxanthellae in seawater.