RNA/DNA ratios and DNA concentrations as indicators of growth rate and biomass in planktonic marine organisms

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Abstract
RNA and DNA concentrations were measured by a modification of the ethidium bromide method in batch cultures of marine phytoplankton and in natural plankton populations (1-333 .mu.m) from Dabob Bay, Washington State, USA, in spring and summer of 1981. In the cultures the DNA/C ratio remained nearly constant during all phases of growth and growth rate was linearly related to the RNA/DNA ratio. The DNA/C ratio and the relationship between growth rate and RNA/DNA ratio for cultures was used to estimate the amount of living biomass and the growth rates for plankton populations of Dabob Bay. The amount of living C was never < 57% of the total particulate C and average growth rates ranged from < 0.3 divisions d[day]-1 to > 4 divisions d-1. Some of the potential problems with such calculations, for example, occurrence of detrital DNA, variability in the DNA/C ratio for different groups of organisms, and differences in the relationship between growth rate and RNA/DNA ratio, are discussed. These problems may not be major obstacles to using DNA concentrations and RNA/DNA ratios for answering major questions in biological oceanography.