Growth and physiological responses of a tropical toxic marine microgalga Heterosigma akashiwo (Heterokontophyta: Raphidophyceae) from Singapore waters to varying nitrogen sources and light conditions

Strains of Heterosigma akashiwo have been identified in Singapore waters, but there have been few studies detailing the biology of such tropical strains. Management of such species is important for the integrity of aquaculture industries, as well as for food safety and public health. Coastal modifications may, however, be altering the productivity of such species. This study investigated the effects of two anthropogenic factors—nitrogen (N)-pollution and light-shading—upon a single strain of H. akashiwo which was isolated from Singapore waters. The study observed that H. akashiwo strains grew well under the pulsed supply of three N-sources—ammonium, nitrate, and urea. Growth rate values suggest its ability to out-grow diatom species, which typically dominate coastal environments. The light absorption capacity highlighted potential response dissimilarity under the different N-regime conditions. Light experiments also demonstrated the ability of H. akashiwo to tolerate decreased light conditions, which allow it to thrive at depth. The ability to accumulate intracellular stores of N-nutrients was also demonstrated and can be important for sustaining cell productivity. This study therefore indicated how poor management of coastal environments may enhance the bloom potential of such toxic bloomforming species.