ABUNDANCE OF THRAUSTOCHYTRIDS IN COASTAL PLANKTON

The abundance of thraustochytrids was investigated in and off the Seto Inland Sea, Japan. Thraustochytrid cells were stained with acriflavin e and counted directly by epifluorescence microscopy. Thraustochytrids were present in the water column at a density of 2.1 x 10(3) to 5.6 x 10(4) cells l(-1), with an overall average of 1.0 x 10(4) cells l(-1) , which was about 10(-5) to 10(-6) of the bacterioplankton abundance. In contrast to the low abundance of thraustochytrids, their biovolume was estimated to compose a not negligible fraction of marine microbiot a, corresponding to about 3 to 43% of the bacterioplankton biovolume. Thus, it is suggested that thraustochytrids play an ecological role as alternative food sources for picoplankton feeders, as well as being a ctive degraders and consumers in aquatic microbial food chains.