The cytoplasm of living cells comprises components that are considerably more dense than water (proteins, ~1300; carbohydrates, ~1500; nucleic acids ~1700 kgm-3), so that the average density of live cells is rarely less than ~1050 kg m-3. Inclusions such as polyphosphate bodies (~2500 kg m-3) and exoskeletal structures of cal-cite and, especially, the opaline silica of diatom frustules (~2600 kgm-3) increase the average density still further. Some of the excess density can be offset by the presence of oils and lipids that are lighter than water, the lightest having a density in the order of 860 kg m-3 (Sargent, 1976). However, these oils rarely account for more than 20% of the cell dry mass. Without adaptation, most freshwater phytoplankters are bound to be heavier than the medium and naturally sink! The corresponding deduction in respect of marine phytoplankton suspended in sea water (pw generally ~1030 kgm-3) is made with rather less confidence, where the scope for regulation of pc can be more purposeful.

The list in Table 2.3 is an abbreviated version of one that was included in Reynolds (1984a). It is intended to illustrate the range of densities rather than the comprehensiveness of the data. In particular, it is easy to distinguish the gas-vacuolate Cyanobacteria which, in life, have densities of <1000kgm-3 that enable them to float,

Table 2.3 Some representative determinations of phytoplankton densities


pc (range, in kgm-3)




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