VIRUSES AND THE MICROBIAL LOOP

The abundance of viral-like particles in marine ecosystems ranges from <10(4) ml(-1) to >10(8) ml(-1). Their distribution in time and space parallels that of other biological parameters such as bacterial abunda nce and chlorophyll a. There is a lack of consensus between methods us ed to assess viral activity, i.e., rate of change in viral abundance ( increase or decrease). The highest rates, 10-100 days(-1), are observe d in experiments with short sampling intervals (0.2-2 h), while lower rates, on the order of 1 day(-1), are observed in experiments with lon ger sampling intervals (days). Few studies have been carried out, but viruses appear, at least in some cases, to have a significant impact o n carbon and nutrient flow in microbial food webs. Viruses have also b een demonstrated to exert a species specific control of both bacteria and phytoplankton populations in natural waters.