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

Abstract

Cell carbon and nitrogen densities (fg μm −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 μm −3 for C; 113 to 401 fg N μm −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 (μm 3 ml −1) into the flagellate C and N concentration (μg 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 μm −3 for growing cells similar to Paraphysomonas sp. and 260 fg C μm −3 for stationary growth cells in nature. N density and C:N were not significantly different between logarithmic and stationary growth phase.