Effects of rapid and sustained changes in light and salinity on Heterosigma akashiwo (Raphidophyceae)

The euryhaline raphidophyte Heterosigma akashiwo is a harmful algal bloom (HAB) species found worldwide, including in coastal waters that experience inputs of freshwater from large river systems. Heterosigma akashiwo can tolerate salinities ranging from 6 to 30; however, H. akashiwo ecophysiology under covarying salinity and light regimes that represent the gradients of sediment-laden, freshwater inputs that are common in coastal waters are unknown. This research describes the responses of H. akashiwo to rapid and sustained changes in light and salinity at levels expected in estuarine and coastal regions impacted by large river inputs. We monitored changes in growth, cell size, and chlorophyll a in cultures exposed to nine salinity-light environments over 7 and 14 days and in exposed cultures returned to their original environment through rapid or incremental shifts. Heterosigma akashiwo grew at all salinity and light levels tested. Growth was primarily influenced by light following rapid salinity-light shifts but was driven by both salinity and light during incremental shifts back to the original conditions. Cell sizes were influenced by both light and salinity. Heterosigma akashiwo could sustain growth in nonideal conditions following rapid environmental changes but showed contractions in niche over longer timelines. Heterosigma akashiwo growth also returned to, or surpassed, baseline levels following removal of the two stressors. These results indicate that H. akashiwo can rapidly adapt to a wide variety of salinity-light environments and could therefore be a dominant HAB species in these river-influenced environments in the future, where such environmental fluctuations are expected.