Carbon and nitrogen densities of the cultured marine heterotrophic flagellate Paraphysomonas sp

Cell carbon and nitrogen densities (fg mu m(-3)) and C:N ratios of the heterotrophic (bacterivorous) flagellate Paraphysomonas sp. at different growth phases were studied in batch co-cultures with their bacterial food. Flagellate growth phase was observed during daily cell counts using image analyzed 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 determine 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 bacteria in the flagellate-bacteria cultures. The total flagellate C and N concentrations were determined by difference between the calculated C and N of bacteria alone and of mixed bacteria and flagellates. C and N densities of flagellates were determined by dividing the total flagellate biovolume (mu m(3) ml(-1)) into the flagellate C and N concentration (mu g ml(-1)). A one-tailed t-test showed a significant difference between C density of Paraphysomonas 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 growing cells similar to Paraphysomonas sp. and 260 fg C mu m(-3) for stationary growth cells in nature. N density and C:N were not significantly different between logarithmic and stationary growth phase. (C) 1998 Elsevier Science B.V. All rights reserved.