RESPONSE OF SEASONALLY COLD-WATER BACTERIOPLANKTON TO TEMPERATURE ANDSUBSTRATE TREATMENTS

Cold-water bacterioplankton response to temperature and substrate trea tments was studied in three brackish-water batch-culture experiments. The experiments were done during 1989 and 1991 using prefiltrated Balt ic Sea water. The in situ water temperature (+2 degrees C) was increas ed by 10 degrees C, and sucrose was used as a carbon source. The exper iments were run for 6-9 days. Thymidine incorporation, cell numbers an d volumes were measured. Growth rates and thymidine conversion factors were estimated. The studied bacterial community was affected by the t emperature manipulation and by the combined effect of increased temper ature and substrate addition. At the higher temperature both thymidine incorporation and cell numbers were increased. On the other hand, sub strate treatment alone was not able to change the growth pattern of th e bacterial community. Thus, the labile carbon source, at the addition level, was not the most critical factor in the regulation of cold-wat er bacterioplankton in these experiments. The bacteria were able to ta ke advantage of the added sucrose only when the experimental temperatu re was higher than in situ. Higher temperature alone, however, enabled bacteria to use other natural, probably more complicated, substances as their source of carbon. (C) 1998 Academic Press Limited.