Optical discrimination of a phytoplankton species in natural mixed populations

Developing optical detection techniques for discriminating particular phytoplankon species in mixed assemblages has long been a goal of aquatic scientists. Previously, a processing algorithm for phytoplankton absorption spectra was reported that suggested detection of the red tide dinoflagellate Gym­ nodinium breve was possible. The algorithm evaluated the fourth derivative of the particulate absorption spectrum of an unknown sample and compared it to a standard fourth deriv­ ative spectrum for G. breve using a similarity index. We report here the first-time application of this technique to the detection of G. breve in natural, mixed phytoplankton communities. Pig­ ment and spectral absorption data were collected from natural blooms of G. breve in the eastern Gulf of Mexico. This di­ noflagellate is the only species of phytoplankton in the Eastern Gulf of Mexico observed to contain the pigment gyroxanthin­ diester, and it appears in constant proportion to cellular chlo­ rophyll a (Chl a) in G. breve. The in vitro absorption spectrum of gyroxanthin-diester is nearly identical to other xanthophylls (including diadinoxanthin, lutein, and 19�-hexanoyloxyfucox­ anthin) and is not singularly responsible for imparting a unique absorption signature. Quantifying gyroxanthin-diester and Chl a allowed us to estimate the fraction of the biomass in mixed populations associated with G. breve. Subsequent regression of the G. breve similarity indexes to the G. breve biomass fractions yielded a significant linear correlation. Finally, the liquid waveguide capillary cell appears to be a promising tech­ nology for automating this technique. Harmful algal blooms pose a threat that requires efforts to reduce or eliminate their negative impacts and conse­ quences (Boesch et al. 1996). In the Gulf of Mexico, toxic blooms of the dinoflagellate Gymnodinium breve Davis reg­ ularly lead to untimely restrictions on commercial and rec­ reational shellfish harvesting and deleterious effects on tour­ ism and public health (Steidinger et al. 1973). Mitigation of some harmful effects from these toxic blooms of G. breve can be achieved by early detection of this species in mixed phytoplankton communities. Currently, microscope exami­ nation of discrete water samples is the principal detection method for G. breve. Unfortunately, this method is slow, labor intensive, and intermittent. Significant benefits can be gained from an automated detection method, and optical methods hold promise for such applications (Cullen et al.