Estimating the contribution of microalgal taxa to chlorophyll a in the field-variations of pigment ratios under nutrient- and light-limited growth
Montoya, Joseph P.
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Cellular concentrations of chlorophylls and carotenoids were measured in nutrient- and light-limited cultures of marine microalgae to determine the utility of accessory pigments as proxies for the biomass of specific groups of microalgae in the ocean. In most species, concentrations of chlorophyll a (chl a) and photosynthetically active pigments varied linearly with growth rate in nitrate-limited continuous cultures or with the logarithm of the irradiance in light-limited and light-sufficient batch cultures, as has been observed before. Rates of pigment-concentration change as a function of irradiance or growth rate did not covary with rates of maximum growth. Concentrations of carotenoids covaried with chl a in most species analyzed; intraspecies variations of chl a-carotenoid ratios were usually smaller than variations of chl a:b or chl a:c ratios. These results were used to critically evaluate the assumptions underlying iterative methods used to determine the contribution of different algal taxa to chl a from ratios of chl a and accessory pigments. Estimates based on chl a:b or chl a:c ratios are prone to error because these ratios can vary by up to an order of magnitude among species and within species as a function of irradiance, thus violating an assumption of the iterative methods. Instead, such methods should rely on ratios of chl a and photosynthetically active carotenoids. Using simple models and field data from the Chesapeake Bay, USA, we showed that iterative methods are either prone to error when different populations of microalgae co-vary or do not give discrete solutions. As an alternative we suggest methods that rely more strongly on empirically determined pigment ratios.