Microphotometric assessment of spectral absorption and its potential application for characterization of harmful algal species

A comparison was made of microphotometric measurements and spectrophotometr ic measurements of particulate spectral absorption of four algal species, i ncluding the chlorophyte Dunaliella tertiolecta Butcher; a nontoxic dinofla gellate, Amphidinium carterae Hulburt; a diatom, Chaetoceros gracilis Schut t; and a toxic dinoflagellate, Gymnodinium breve Steidinger, Particulate sp ectral absorption of monospecies cultures was estimated as the product of t he average absorption efficiency factor, Q(a), determined by microphotometr y, the cellular cross-sectional area, and the cell number density. Estimate s of particulate spectral absorption from microphotometric measurements wer e, in most cases, within one standard deviation of values determined from s pectrophotometric measurements of algal suspensions. Estimates of Q(a)(675) were shown to be consistent with values reported in previous studies for c ells of similar size and pigmentation and were consistent with theoretical predictions. Absorption spectra of mixtures of C. gracilis and G. breve wer e numerically decomposed into contributions by absorption signatures of mon ospecies cultures using either spectrophotometric or microphotometric measu rements as the basis for end members. Modeled contributions assigned to eit her species displayed trends consistent with the actual proportions contrib uted to the spectrum by each algal culture. However, the technique was sens itive to measurement variability, which reduced the level of agreement betw een modeled and actual contributions. The utility of this approach for iden tification of algal taxa will depend on the degree to which algal spectral absorption signatures differ and the capabilities for acquiring high-resolu tion data with low signal-to-noise ratios.