Microalgal pigment ratios in relation to light intensity: implications for chemotaxonomy

Abstract

Pigment-based chemotaxonomy relies upon the assertion that the pigment arrays of mixed microalgal communities (phytoplankton, periphyton, microphytobenthos) can be deconvo- luted in ways to provide an estimate of taxon-specific chlorophyll a (chl a) as a proxy for biomass. This relies on having valid chl a:biomarker pigment ratios. However, pigment ratios change for a variety of reasons. We examined the effects of only light intensity (photon flux density, PFD) on the pigment ratios of 10 species in the phyla Cyanobacteria, Chlorophyta, Chromophyta (Bacillariophyceae), Haptophyta, and Pyrrophyta. Chl a:marker pigment versus PFD (30-45 through 1800 µmol quanta m -2 s -1 ) data revealed distinct trends for photosynthetic accessory pigments (PAP) versus photopro- tective pigments (PPP). Chl a:PAP class pigments (chl b, fucoxanthin, peridinin, echinenone) exhib- ited relative stability (possibly stoichiometry) with increasing PFD, while chl a:PPP class pigment trends revealed large increases in the relative amounts of the PPP pigments (zeaxanthin, myxoxan- thophyll, canthaxanthin, scytonemins). Total chl a cell -1 often decreases in concert with increasing PFD and partially or totally explains the decrease in chl a:PPP ratios. We verified that all 'xanthophyll cycle' pigments expressed large and erratic ratios and therefore did not predict chl a levels. We con- clude that light (400 to 700 nm) measurements taken in the field can offer valuable ancillary data for adjusting pigment-based chemotaxonomic formulae for the study of microalgal communities.