Aquatic microbiology for ecosystem scientists: New and recycled paradigms in ecological microbiology

In all ecosystems, bacteria are the most numerous organisms and through the m flows a large fraction of annual primary production. In the past decade w e have learned a great deal about some of the factors that regulate bacteri a and their activities, and how these activities, in turn, alter ecosystem- level processes. Here I review three areas in which recent progress has bee n made with particular reference to pelagic ecosystems: the problem of bact erial cell dormancy; the effect of solar radiation on organic matter labili ty; and, the maintenance of net heterotrophy. In a system in which grazing is the major source of mortality for bacteria, bacterial cell dormancy is s omething of a paradox. I argue that the degree to which bacteria are grazed by flagellates (highly selective grazers) versus other grazers (cladoceran s, bivalves) may explain the degree and variation in the proportion of acti ve cells which recent evidence shows to be large. Another factor affecting bacterial activity that has come to the fore in recent years is solar radia tion. Irradiation, especially in the ultra-violet range has long been thoug ht of as simply deleterious to some bacteria. A wealth of newer evidence sh ows that refractory dissolved organic compounds may be converted into micro bially labile compounds by solar radiation in several wavebands. This inter action between irradiation and organic matter (photolysis) may explain, in part, how dissolved organic carbon (C) may be refractory in the dark enviro nment of the soil but become labile in the illumunated surface waters of la kes or rivers. The newer evidence shows that aquatic ecosystems, at least o ligotrophic ones, are significantly subsidized by terrestrially-produced or ganic matter. I review here multiple lines of evidence that suggest that fr eshwater ecosystems are predominantly systems which respire more organic C than they produce by photosynthesis, and are therefore net heterotrophic. W hile net heterotrophy is an interesting exception for terrestrial ecosystem s, it appears to be commonplace for aquatic systems and represents an impor tant linkage between terrestrial and aquatic ecosystems.