Relationships between cell volume and the carbon and nitrogen content of marine photosynthetic nanoplankton

Unialgal cultures were used to investigate relationships between cell volume and the carbon and nitrogen content of nondiatomaceous marine nanophytoplankton. Cell dimensions were determined by image‐analyzed epifluorescence microscopy and particulate C and N by high‐temperature dry combustion. Volumes were calculated by direct integration with published algorithms (biovolume), but could be estimated equally well from linear dimensions as prolate spheres. Preservation with 0.5% glutaraldehyde reduced cell volumes 29% on average. Correlations were highly significant between biovolume of preserved cells and C and N contents. Nonlinear regression models appeared most appropriate because smaller cells contained more C and N per unit volume than did larger cells. Suggested general C densities for estimating cell C from preserved volume were 0.36 pg µ m −3 for 10 1 µ m 3 cells, 0.24 pg µ m −3 for 10 2 µ m 3 cells, and 0.16 pg C µ m −3 for 10 3 µm 3 cells. Previous regression models substantially underestimated the C densities of nanophytoplankton of 10 1 –10 3 µ m 3 . The explanation for these differences includes the method of determining mean population volumes, the use of preservatives, and the occurrence of significant vacuolar volume in larger phytoplankton.