Fluorescence and absorption characteristics of dissolved organic matter excreted by phytoplankton species of western Bay of Bengal under axenic laboratory condition

Phytoplankton exudates are major source of dissolved organic matter (DOM) in the Sea. We examined the absorption and fluorescence characteristics of Tetraselmis spp, Cylindrotheca closterium and Odontella aurita exudates in an axenic culture experiment. The objective was to test the ability of phytoplankton to produce fluorescent DOM (FDOM) apart from protein fluorescent matter in the absence of bacteria, and if so how does the fluorescence efficiency expressed as fluorescence-absorption coefficient ratio of different FDOM components compare. Through our study, the ratio of absorption coefficients at 350 nm and 440 nm (a350:a440 ratio) has the potential to be a unique descriptor of the growth condition of algae. It was minimum (4.12 ± 0.59, n = 14) in the growth phase for the three species. The spectral slope (S300-440) indicates that higher molecular weight compounds are present in exudates when phytoplankton grows than during lag or decay. Protein (tryptophan) like fluorescence was contained in exudates throughout the life cycle. It constituted the most abundant component also, ca., ~ 69% of total fluorescence in Tetraselmis spp, and C. closterium, and ~ 62% in the larger celled O. aurita. The remaining fluorescence was made up of humic like fluorophores of the UV (two varieties), Visible and Marine types, all of which showed maxima at the end of exponential stage. The accumulated fluorescence at the close of the experiment surviving photochemical degradation was maximum for the visible humic fluorophore in Tetraselmis sp. (~ 187%) and for the protein like fluorophore in O. aurita (71%).