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Mesozooplankton as a factor in the microbial food web and its direct/indirect trophic interactions in the oligotrophic open ocean are discussed as studied in the subtropical North Pacific using cruise grazing experiments with different concentration treatments of the mixed mesozooplankton community. Increasing the concentration of mesozooplankton raised growth rates of heterotrophic bacteria, and mesozooplankton fed diverse microzooplankton and nanoheterotrophs and their predation indirectly raised net phytoplankton/heterotroph growth rates

Lack of knowledge about bacterial respiration (BR) limits ability to understand the role of bacteria in the carbon cycle of aquatic ecosystems. In the transformation of organic matter, heterotrophic bacteria have two major roles, that of making new bacterial biomass, bacterial secondary production (BP), and that of respiring organic carbon (C) to inorganic, that is BR. A lot has been learned about BP for planktonic bacteria, but little is known about BR. The ecological and physiological bases of the regulation of BGE have been reviewed (BGE = (BP)/(BP + BR)). BGE varies systematically with BP and ecosystem trophic richness. Planktonic bacteria seem to maximize carbon utilization rather than BGE. Maintenance energy costs seem to be highest in oligotrophic systems in consequence.