Calculation of the cell carbon content of four taxa of diatom: biovolume and spectrophotometry

Abstract

Background. Phytoplankton biomass is an essential parameter in aquatic ecosystems; two of the most widely used
indicators for estimating phytoplankton biomass are chlorophyll-a (Chl-a) by spectrophotometry and biovolume (BV).
Goals. To show how different results are when estimating phytoplankton biomass by means of the spectrophotometric
and BV methods. Methods. Biomass by Chl-a and BV were estimated in four cultures of marine diatoms. For estimating
BV, we used live cells and acid Lugols solution fixed cells. Results. Biomass values were noticeably different: the BV for
Thalassiosira hispida and Skeletonema costatum underestimates ?41% of the biomass with respect to values obtained by spectrophotometry. In contrast, the BV for Pseudo-nitzschia sp. and Cylindrotheca closterium overestimates the biomass by 4% and 25%, respectively. In the case of estimated biovolume, the results indicate that fixer acid Lugols solution modified the size of the cell significantly in C. closterium and Pseudo-nitzschia sp. (p<0.05), but not for S. costatum and T. hispida (p>0.05). Two equations are provided for biomass calculation when studying live (pgC/cell = -1.5567 + 0.1428 (BV)) or acid Lugols solution fixed (pgC/cell = -5.0126 + 0.1644 (BV)) diatom cells. Conclusions. Although BV is useful, it can have drawbacks because it produces values above and below those estimated by spectrophotometry, in addition to the fact that acid Lugol´s solution modifies cell size. We would advise considering the available information based in Chl-a or carbon of different species.