Intra-class variability in the carbon, pigment and biomineral content of prymnesiophytes and diatoms

Chlorophyll (chl) and carotenoid pigment data has significantly advanced our understanding of the distribution and class composition of phytoplankton biomass. However, the conversion of this data into quantitative and reliable estimates of biomass necessitates empirical carbon and pigment measurements on individual species. We have studied the carbon, pigment (chl and carotenoid) and biomineral (silicate and calcite) content of 20 prymnesiophytes and diatoms as key representatives of ecologically important phytoplankton in marine temperate waters. Batch cultures were sampled for each analysis in triplicate during early and late growth periods. To enable intra-class comparisons, pigment/chl a ratios are presented as are cellular constituent densities derived by normalising concentrations with cellular counts and volumes. For both prymnesiophytes and diatoms in early growth, chl a and carbon density were found to decrease from 8 to <2 fg μm^-3 and from 0.5 to <0.1 pg μm^-3 respectively as cell volume increased from 20 to >1000 μm^-3. Pigment densities often decreased for the late growth period whereas carbon concentrations increased rapidly (up to 6-fold), resulting in a decrease in chl a/carbon ratio from an average 0.02 to 0.01. Regressions of POC and biovolume indicated that between 40 and 80% of the total POC for cells harvested during late growth was due to non-viable material. In addition, we found that chemotaxonomic marker pigments did not correlate with biominerals for either diatoms (fucoxanthin with silicate), or prymnesiophytes (19'hexanoyloxyfucoxanthin with calcite). Our empirical data, which are presented for a wider range of species than previously available, strengthen the basis upon which quantitative estimations of phytoplankton biomass in aquatic ecosystems are reliant.