Growth, pigment, and chromophoric dissolved organic matter responses of tropical Chattonella subsalsa (Raphidophyceae) to nitrogen enrichment

Blooms of the raphidophyte Chattonella subsalsa have been associated with massive fish‐kill events in several parts of the world. However, there have been few studies into physiological responses of tropical strains that could contribute to bloom outcomes. Such knowledge could provide insight into the C. subsalsa blooms recently documented within tropical coastal waters (e.g., 2010 and 2012 events in Singapore). Strains used in this study were isolated from the East Johor Straits (EJS), Singapore, an enclosed water channel frequently subjected to high levels of eutrophication. These cells were classified within the ‘global’ clade (and distinct from the ‘Adriatic Sea’ clade) based on morphology. The present study examined cellular responses to varying inputs of different forms of nitrogen (N), specifically nitrate, ammonium, and urea. Results from the study indicated that cells were unable to utilize urea as an N‐source, but grew well on a nitrate (Vₘₐₓ = 0.73 day⁻¹) and ammonium (Vₘₐₓ = 0.81 day⁻¹) supply. These growth rates were high compared to other strains from around the world, indicating that tropical C. subsalsa could exhibit elevated bloom potential within frequently eutrophic environments such as the EJS. Six pigments were detected in all cultures. These pigments were chlorophylls a and c; fucoxanthin; diadinoxanthin; violaxanthin; and β‐carotene. Chlorophyll‐a and fucoxanthin were the dominant pigments under both nitrate and ammonium regimes. Measurements of chromophoric dissolved organic matter generally increased both in molecular weight and in total content across the N‐concentration ranges. Such outcomes could have consequences for the chemical and optical conditions of the coastal environment.