Responses of the marine filamentous red alga ‘Bangia’ sp. ESS1 to recurrent changes in seawater concentration

In the intertidal zone, marine filamentous red algae of the genus Bangia generally tolerate severe fluctuations in seawater concentrations caused by rain and the inflow of river water. However, this unique ability has not been thoroughly characterized. The tolerance of the filamentous intertidal red alga ‘Bangia’ sp. ESS1 to diluted artificial seawater (ASW) were examined, which simultaneously imposed low-salinity, hypoosmotic, and low-nutrition stresses. ‘Bangia’ sp. ESS1 was highly tolerant to highly diluted seawater treatments for 2-weeks, displaying over 90 % viability in diluted ASW up to 10 % ASW. Freshwater (FW) treatment was a little stressful, but algae had a high survival rate (79.3 % viability) after 2-weeks. Under diluted ASW conditions, cell length decreased, and cell width increased, along with changes in plastid morphology and increases in membrane unsaturated fatty acid levels. During 1-week recovery period in 100 % ASW, which produced high-ionic and hyperosmotic stress, the cell viability and cell morphology of algae pre-treated with diluted-ASW were restored to levels observed in control algae maintained in 100 % ASW, whereas the FW-treated algae were only partially recovered. The recovery in 100 % ASW accelerated the production of asexual spores for clonal propagation (representing a life cycle trade-off) and resulted in an increase in the saturated fatty acid contents in membranes. These findings suggest that the dilution and restoration of artificial seawater promoted reversible morphological and physiological changes and asexual reproduction in ‘Bangia’ sp. ESS1. These changes might be required for the adaptation of the alga to variable environments based on its intrinsic high tolerance to combined ionic, osmotic, and nutrient stress.