Photosymbiont diversity and acquisition by the sea anemone Exaiptasia spp. in an estuarine habitat of Key Largo, Florida.

Coral reef growth and recovery from bleaching depend on the onset and physiological maintenance of cnidarian-dinoflagellate symbiosis. Many cnidarian hosts can acquire new symbionts (family Symbiodiniaceae) from the environment. Symbiodiniaceae can be found at significant densities within other hosts and lesser densities as free-living populations on macroalgae, in sediments, and in the water column. The mechanisms underlining host selectivity towards symbionts are still under investigation, and even less is known about the timing and community composition when hosts uptake symbionts in their natural environments. This project studied symbiont diversity in environmental reservoirs and the progression of symbiont acquisition in bleached hosts. We first characterized various Symbiodiniaceae reservoirs in an estuarine habitat of Florida Bay at Key Largo, Florida. We then deployed bleached Exaiptasia sp. anemones in the habitat and recorded their recovery for three weeks, assessing symbiont density through cell counting and diversity via next-generation sequencing (NGS). We found that environmental reservoirs exhibit higher symbiont diversity but much lower densities than in hospite living reservoirs, highlighting the potential for healthy hosts as symbiont donors. We showed that the anemones can recover from bleaching using their residual leftover symbiont cells but recover 40% faster when also uptaking new symbionts from their environment, including non-homologous strains. We also report that symbiont shuffling can be observed in disturbed anemones in as little as three days, with decreasing symbiont density but higher symbiont diversity. Our data can guide future studies towards understanding small-scale dynamics and symbiont acquisition for cnidarians recovering from bleaching. • Environmental and in hospite symbiont communities are drastically different within Florida Bay. • Bleached anemones can recover from residual symbionts within the host. • Bleached anemones can recover using environmental reservoirs in the natural habitat. [ABSTRACT FROM AUTHOR]