Copepods - small marine crustaceans often referred to as the most abundant animal on Earth - form a vital link in carbon and nitrogen fluxes in the oceanic water column food webs. Copepod microbiome may be important in providing unique niches for marine bacteria and in influencing copepod health, and its activities potentially have quantitative importance in marine biogeochemical cycles. In this talk I will discuss recent studies from my lab on the composition and function of the copepod microbiome in marine subtropical and temperate waters. I show that the copepod microbiome composition experiences substantial food-driven temporal variability, yet, stable components of the microbiome also persist. The conventional paradigm on heterotrophic microbial activity in the open ocean assumes that bacteria in oxygenated surface layers use oxygen as the terminal electron acceptor in respiration. In contrast, via 15N-tracer based rate measurements, molecular approaches, and draft genomes and experiments with isolates, we have demonstrated active dissimilatory nitrate reduction under oxygenated conditions in bacteria associated with copepods. Collectively, the data suggest that copepods provide unique microhabitats for marine bacteria and host unexpected microbial metabolic activities. We have much more to learn about the importance of these dynamic microhabitats.