Biomass of thraustochytrid protoctists in coastal water

Abundance, cellular contents of carbon and nitrogen, and C/N ratio of plank tonic thraustochytrids in the Seto Inland Sea and the coastal Hyuga-nada, J apan, were determined. Thraustochytrid cells were stained with acriflavine and counted directly by epifluorescence microscopy. Thraustochytrids were f ound in the water column at a density of 2.5 x 10(3) to 4.5 x 10(4) cells l (-1), with an overall average of 1.6 x 10(4) cells l(-1). In order to estim ate the impact of thraustochytrid biomass on the coastal plankton community , cellular carbon and nitrogen contents of a cultured thraustochytrid were determined. The carbon and nitrogen for 10 mu m cell contents were estimate d to be 1.65 x 10(-4) mu g C cell(-1) and 1.58 x 10(-5) mu g N cell(-1), re spectively. The cellular C/N ratio of 10.4 was higher than that known for p elagic bacteria. The estimated population biomass of thraustochytrids varie d from 0.41 to 7.44 mu g C l(-1), with an overall average of 2.60 mu g C l( -1); and 0.039 to 0.71 mu g N l(-1), with an overall average of 0.23 mu g N l(-1). The thraustochytrid carbon biomass corresponded to about 3.4 to 29% of total planktonic decomposers (bacteria and thraustochytrids). It is sug gested that the contribution of thraustochytrids to the microbial food chai n and carbon cycling through organic degradation and production may not be negligible.