MARINE VIRAL ECOLOGY - INCORPORATION OF BACTERIOPHAGE INTO THE MICROBIAL PLANKTONIC FOOD-WEB PARADIGM

In the decade since the microbial loop was defined by Azam et al. (Mar. Ecol. Prog. Ser., 59, 1-17, 1983), the importance of the interaction between microbial organisms and the larger planktonic animals has been a subject of controversy. Until recently, grazing was considered to be the major fate of bacterial production. Now, however, viruses are seen to have an important role in microbial processes. We describe how growth and recycling parameters affect the transfer of bacterial production through a microbial loop model that includes viruses. The loop is very inefficient for all reasonable conditions, but its relative importance as a source of mesozooplankton nutrition is variable. The model demonstrates that in mesotrophic coastal waters, the microbial loop is unlikely to supply more than a minor component of mesozooplankton nutrition, a proposition that is supported by accumulating evidence. For oligotrophic pelagic waters, the model indicates that in the absence of viruses the microbial loop, despite its low efficiency, may provide an important resource for mesozooplankton. Bacterial production, without viral mortality, is also relatively important in the case of direct exploitation by salps. Under these conditions, bacteria account for 10-30% of mesozooplankton nutrition. With high levels of bacteriophage activity, zooplankton production is generally reduced by 5-15%. We thus conclude that bacteriophages could significantly affect mesozooplanktonic and, hence, exploitable marine production.