Introduction to Marine Biology 3rd Edition by George Karleskint, Richard Turner, James Small

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Although the roles that viruses and bacteria play in human disease might first come to mind, their roles in the sea are more diverse. Viruses control populations of other microbes, especially bloom formers. Through lysis of host cells, viruses influence marine food chains and alter biogeochemical cycles. Bacteria are important to other life in the sea on account of their wide metabolic diversity, their consumption by small eukaryotic grazers, their hastening of sedimentation, and their contribution to symbiotic relationships. In particular, the mitochondria and plastids of eukaryotes originated as marine bacteria.