While far less conspicuous than the ocean’s animals and plants, microorganisms make up almost 90% of all living biomass in the sea. These marine microorganisms play fundamentally important roles in controlling the health and function of the ocean. They form the foundation of the marine food web and therefore control the productivity of ocean ecosystems. By regulating important marine chemical cycling processes, these microorganisms also play a major role in governing the ocean’s strong influence on global climate. However, the microbiology of the ocean is changing. Anthropogenic impacts, including coastal pollution and climate change are leading to sometimes dramatic shifts in the composition and ecological functions of marine microbial communities. Shifting ocean currents, caused by climate change, are leading to potentially important changes in the distribution of important ‘key stone microbes’, which will likely have profound flow on effects for ocean productivity that will reverberate right up the marine food chain. The increased frequency of Marine Heat Wave events is leading to increased abundance and impacts of marine pathogens that threaten the health of marine organisms, including important fishery and aquaculture species. There is even growing evidence that these events will lead to increased incidence of pathogenic marine microbes that threaten human health. The increasing occurrence of heavy rainfall events and associated coastal run-off is leading to microbial contamination of marine ecosystems, which sometimes leads to inputs of pathogens with heightened resistance to antibiotics. In this presentation, I will provide an introduction to the microbiology of marine environments and discuss the ways that environmental shifts are leading to fundamentally important changes in the identities, roles and impacts of microbes within the ocean.