Optical characteristics of the harmful dinoflagellate Alexandrium tamarense in response to different nitrogen sources

The chlorophyll a specific absorption coefficient aph* and absorption ratio (aph ratio) of Alexandrium tamarense at five concentrations of nitrate, ammonium and urea over a range from 6 to 100 μM were examined. The experimental results were compared to two coastal diatom species and a prymnesiophyte to identify differences in absorption ratios. Cells exposed to increasing nitrate concentrations were characterized by an increase in aph* at 443, 490, 510, 555 and 675 nm. However, ammonium and urea induced low aph* values at their lowest and highest concentrations. The aph relative to 510 or 555 nm was constant regardless of the concentration of the N source, but dependent on the N source. Oxidized N induced a lower aph ratio than the reduced form. Comparisons of the aph ratio among taxonomic groups revealed significant differences. The aph ratio of A. tamarense was 20–30 and >50% lower than those of two diatoms and a prymnesiophyte, respectively. The aph ratio of the present study could assist in increasing the capability for detecting harmful species such as A. tamarense.