Dynamics of thraustochytrid protists in the water column of the Arabian Sea

Thraustochytrids, a group of osmoheterotrophic chromistan protists are ubiq uitous in the sea. However, little is known of their ecological role, parti cularly in oceanic waters. To obtain an insight into their dynamics in this realm, we investigated thraustochytrids and bacteria in the water column, up to 2000 m in the central Arabian Sea. Four seasons, namely the end of th e southwest summer monsoon (September/October 1993), the summer pre-monsoon (April/May 1994), the end of the northeast winter monsoon (February/March 1995) and the peak period of the southwest monsoon (July/August 1995) were studied. Thraustochytrids were estimated using the acriflavine direct detec tion method (AfDD) and bacteria by the acridine orange direct counts (AODC) . Thraustochytrids were present in substantial numbers throughout the 150 m water column in all the stations investigated during the end of the biolog ically productive summer and winter monsoons (0 to 1313 x 10(3) and 3.7 to 183 x 10(3) cells l(-1) water respectively). Their populations in the upper 150 m were positively related to bacteria and particulate organic matter d uring these 2 periods, the relationship being significant during the end of the winter monsoon. They were infrequent during the low productive summer pre-monsoon period (present in < 29% of the total 41 samples), a period of high bacterial abundance, as well as in the peak-productive southwest summe r monsoon (present in 10 of 19 samples). Vertical distribution was seasonal ly variable. Thraustochytrids were regularly detected in the Arabian Sea ox ygen minimum zone (OMZ) at 200 to 1000 m. Similar to the upper 150 m, thrau stochytrids below 200 m were least: frequent in the summer pre-monsoon (pre sent in 49% of samples), although high numbers, up to 266 x 10(3) l(-1) wer e found at 1000 m in 1 station. Their numbers in the OMZ ranged from 0 to 3 8 x 10(3) l(-1) during the end of the summer and winter monsoons. Both thra ustochytrids and bacteria showed a distinct peak at 250 to 500 m in the OMZ during these 2 seasons. Dense populations of thraustochytrids were detecte d even at depths up to 2000 m. We hypothesise that seasonal increases of th raustochytrid populations are related to particles following phytoplankton degradation at the end of the biologically productive seasons. Our estimate s suggest that during the 3 seasons, maximum thraustochytrid C biomass reac hed values ranging from 36 to 217% of bacterial C biomass in the upper 150 m and 34.5 to 56% of bacterial C biomass between 200 and 2000 m.