Thraustochytrids: A neglected agent of the marine microbial food chain

Abstract

The base of the pelagic microbial food chain consists of bacterioplankton and mycoplankton (mostly thraustochytrids), the latter having drawn less attention. In a rather simplistic formulation, bacterioplankton is regarded as the harvester of autochthonous (marine) dissolved organic material exuded from phytoplankton, while mycoplankton is seen to be the scavenger of refractory dissolved organic material which is often allochthonous (riverine). This study presents a case study report on the abundance of thraustochytrids (mycoplankton) and bacterioplankton in the discharge area of River Shimanto, Northwestern Pacific, Japan. Thraustochytrid cells were stained with acriflavine and counted directly by epifluorescence microscopy. Thraustochytrids were found in the surface water at a density of 0.9 to 3.6 x 10 3 cells l -1 , with an overall average of 2.5 x 10 3 cells l -1 . Distribution of thraustochytrids was correlated to (or controlled by) river discharge as depicted by lowered salinity. On the other hand, abundance of bacterioplankton, 1.2 to 1.4 x 10 9 cells l -1 , showed no significant correlation with river discharge. Thraustochytrids may grow on terrestrial organic matter in the riverine input, which is often refractory to degradation, and play a role in enhancing carbon cycling in the estuarine and coastal areas.