Cell carbon and nitrogen densities (fg mu m(-3)) and C:N ratios of the hete
rotrophic (bacterivorous) flagellate Paraphysomonas sp. at different growth
phases were studied in batch co-cultures with their bacterial food. Flagel
late growth phase was observed during daily cell counts using image analyze
d epifluorescence microscopy. At logarithmic and stationary growth phases,
the flagellate-bacteria co-cultures were filtered onto GF/F filters for CHN
analyses. Parallel cultures of bacteria alone were similarly analyzed to d
etermine carbon and nitrogen relationships to bacterial cell volume. These
bacterial C and N densities (range 393 to 1390 fg C mu m(-3) for C; 113 to
401 fg N mu m(-3) for N) were used to calculate C and N concentration of ba
cteria in the flagellate-bacteria cultures. The total flagellate C and N co
ncentrations were determined by difference between the calculated C and N o
f bacteria alone and of mixed bacteria and flagellates. C and N densities o
f flagellates were determined by dividing the total flagellate biovolume (m
u m(3) ml(-1)) into the flagellate C and N concentration (mu g ml(-1)). A o
ne-tailed t-test showed a significant difference between C density of Parap
hysomonas sp. at logarithmic and stationary growth phase at a significance
level of 0.05. We suggest using cell C density of 430 fg C mu m(-3) for gro
wing cells similar to Paraphysomonas sp. and 260 fg C mu m(-3) for stationa
ry growth cells in nature. N density and C:N were not significantly differe
nt between logarithmic and stationary growth phase. (C) 1998 Elsevier Scien
ce B.V. All rights reserved.