Carbon- and Nitrogen-to-Volume Ratios of Bacterioplankton Grown under Different Nutritional Conditions

Carbon- and nitrogen-to-volume (C/V and N/V) ratios were determined for freshwater bacterial assemblages grown in lake water filtrate or in water enriched with nutrients (aqueous extract of lake seston, glucose, arginine, phosphate, or ammonium). Biovolume was measured by epifluorescence microphotography, and carbon and nitrogen biomasses were measured with a CHN analyzer. Despite large variations of nutritional conditions (i.e., the composition and concentration of the dissolved organic carbon) and different mean cell sizes of the bacterial assemblage (0.17 to 1.8 mum per cell), the C/V, N/V, and carbon-to-nitrogen weight ratios varied little (C/V ratio, 0.14 pg of C per mum [standard deviation, 0.057; n = 15]; N/V ratio, 0.027 pg of N per mum [standard deviation; 0.011, n = 15]; carbon-to-nitrogen weight ratio, 5.6 [standard deviation, 2.2, n = 15]). An average C/V ratio of 0.12 pg of C per mum that was derived from natural and cultured bacterial assemblages is proposed as an appropriate conversion factor for estimation of the biomass of freshwater bacteria.