Stress tolerance alteration in the freshwater cnidarian green hydra (Hydra viridissima) via symbiotic algae mutagenesis

Abstract

Symbiotic organisms such as corals are threatened by changing climate because they are sensitive to stress, and may be unable to adapt quickly due to the long host generation time. Instead of selecting for stress tolerant hosts, manipulating their symbiotic microbes has been proposed because microbes affect holobiont phenotypes and they have rapid life cycles. Because it is time-consuming to isolate stress tolerant symbionts from the wild even when appropriate types exist, mutation and selection of symbionts is a promising alternative approach. Using green hydra (Hydra viridissima) and symbiotic algae (Chlorella variabilis NC64A), we found symbiont mutagenesis in vitro altered their UV-B resistance as well as that of holobionts receiving mutated algae. In addition, hydra UV-B tolerance was positively correlated to that of the algae they were hosting, as the hydra associated with UV-B tolerant algal strains exhibited higher UV-B resistance. However, chronic low-level UV-B selection decreased algal resistance to acute high-level UV-B, which was unexpected, and did not affect UV-B resistance of holobionts. The variations in algal UV-B tolerance and hydra UV-B tolerance were largely due to mutagenesis rather than selection. Our results suggest symbiont mutagenesis and trait-based identification may be more effective than assisted evolution in holobiont phenotype alteration, and it highlights the need to characterize symbiont traits in vitro that are correlated to stress tolerance they can confer to hosts, which may have application in conservation, agriculture and forestry.