Heterotrophic bacteria are ubiquitous and abundant in the World Ocean,often dominating the biomass and carbon cycling within planktonic food webs,and mediating much of the biogeochemical nutrient-cycling processes.Until recently,marine bacteria have been considered as a single group with unknown diversity and variation.However,it is now recognized that the abundance and activity of specific phylogenetic groups within the bacterial assemblage and their role in the cycling,transformation,and remineralization of dissolved organic matter may vary both spatially and temporally.