Response of seasonally cold-water bacterioplankton to temperature and substrate treatments

Cold-water bacterioplankton response to temperature and substrate treatments was studied in three brackish-water batch-culture experiments. The experiments were done during 1989 and 1991 using prefiltrated Baltic Sea water. The in situ water temperature (+2 degrees C) was increased by 10 degrees C, and sucrose was used as a carbon source. The experiments were run for 6-9 days. Thymidine incorporation, cell numbers and volumes were measured. Growth rates and thymidine conversion factors were estimated. The studied bacterial community was affected by the temperature manipulation and by the combined effect of increased temperature and substrate addition. At the higher temperature both thymidine incorporation and cell numbers were increased. On the other hand, substrate treatment alone was not able to change the growth pattern of the bacterial community. Thus, the labile carbon source, at the addition level, was not the most critical factor in the regulation of cold-water bacterioplankton in these experiments. The bacteria were able to take advantage of the added sucrose only when the experimental temperature 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.