Effects of temperature, salinity, and depth on Symbiodiniaceae lineages hosted by Palythoa tuberculosa near a river mouth

The diversity of symbiotic dinoflagellate Symbiodiniaceae hosted by anthozoans is known to be driven by the environment where the hosts are found. This study examined how environmental variations (Palythoa tuberculosa. We monitored the dominant Symbiodiniaceae lineages within tagged P. tuberculosa colonies near the Hija River mouth and adjacent coastal reefs at Mizugama, Okinawa, Japan, between July 2016 and April 2018. Seven sites were chosen based on depth and distance from the river mouth, with 5 tagged colonies at each site. Water parameters of tidal pools (TPs) at the river mouth, especially temperature, were significantly different from other sites. Surprisingly, P. tuberculosa colonies at TPs were more resilient to bleaching during summers compared to colonies at other shallow sites. We observed different Cladocopium psbAncr lineages hosted by the tagged P. tuberculosa, with TPs colonies usually hosting one Cladocopium lineage (designated as lineage 4). Colonies from the deep sites and other shallow sites hosted mostly Cladocopium lineages 1 (generalist) and 2 (riverine specialist). Throughout the study period, the shallow colonies (included TPs) recorded higher rates of switching dominant Cladocopium lineages (mostly to lineage 1), whereas most deep colonies did not switch their dominant Cladocopium lineages. Our results show that Cladocopium lineage 1, previously reported as a generalist lineage in terms of environmental parameters, could also be an opportunist lineage during periods of host stress. Our study confirms that Cladocopium lineage flexibility likely helps the resilience of P. tuberculosa in such variable environments.