Biomass of thraustochytrid protoctists in coastal water

Abundance, cellular contents of carbon and nitrogen, and C/N ratio of planktonic thraustochytrids in the Seto Inland Sea and the coastal Hyuga-nada, Japan, were determined.Thraustochytrid cells were stained with acrflavine and counted dlrectly by epifluorescence rnicroscopy.Thraustochytrids were found in the water column at a density of 2.5 X 103 to 4.5 X 104 cells I-', with an overall average of 1.6 X 104 cells 1-l.In order to estimate the impact of thraustochytrid biomass on the coastal plankton community, cellular carbon and nitrogen contents of a cultured thraustochytrid were determned.The carbon and nitrogen for 10 pm cell contents were estimated to be 1.65 X 10-4 pg C cell-' and 1.58 X 1W5 pg N cell-', respectively.The cellular C/N ratio of 10.4 was higher than that known for pelagic bacteria.The estimated population biomass of thraustochytrids varied from 0.41 to 7.44 pg C I-', with an overall average of 2.60 pg C 1-'; and 0.039 to 0.71 pg N I-', with an overall average of 0.23 pg N I-'.The thraustochytrid carbon biomass corresponded to about 3.4 to 29% of total planktonic decomposers [bacteria and thraustochytrids).It is suggested that the contribution of thraustochytrids to the microbial food chain and carbon cycling through organic degradation and production may not be negligible.