Estim.ation of intracellular carbon and silica content of diatoms from natural assemblages using morphometric techniques’

Electron microscopic morphometric techniques were used to measure accurately the cytoplasm and frustule volumes of 12 naturally occurring planktonic diatoms in the Laurentian Great Lakes. In addition, cytoplasm volumes were measured in 6 other algal species. Estimates of carbon content were then made by converting measured cytoplasm volumes to carbon biomass. From these measurements and conversions, the average percent carbon by dry weight was found to be 19% (range lo-30%) for diatoms and 35% (range 29-43%) for the nondiatom species. These values are lower than those previously reported. This is probably due to two facts: our estimates do not include extracellular carbon that may be associated with the cells, and vacuole relative volume (% of total cell volume) ranged from 22 to 70% in diatoms and 2 to 29% for nondiatoms. The carbon content of cells with lower vacuole volumes more closely approximate those previously reported. Our estimations of percent silica by dry weight of diatoms averaged 60% (range 40-78%). Our values for silica content are closer to those reported for naturally occurring populations than to those for cultured populations. Estimation of biomass in individual phytoplankton populations in a mixed assemblage remains a complex problem. Traditionally, biovolume estimates have been used to assess the importance of certain species through conversion to organic carbon content, ash-free dry weight, or chlorophyll (Paasche 1960; Mullin et al. 1966; Strathmann 1967). The return to biovolume estimates is an attempt to escape the limitations of secondary measurement techniques such as Chl a or ATP since there is generally a poor correlation between estimates of biovolume and other independent measurements of biomass (Vollenweider et al. 1974). Sicko-Goad et al. (1977) have summarized three inherent problems associated with biovolume to biomass ‘conversion: a general lack of accuracy and precision in measuring complex, irregularly shaped microscopic algae; differing amounts of “inert” space between different divisions of algae as well as differences between species in the same division; and physiological condition of the cells may drastically affect cell