Low-nutrient environments are abundant and widespread across Earth, in terrestrial, freshwater, and marine environments, and from a microbial perspective may thus be regarded as the norm rather than an exception. Consequently, an extraordinary variety of adaptations have evolved among microorganisms to survive, and even reproduce, when nutrients are scarce. Here we first present an overview of the physicochemical and biological conditions that determine the transition from nutrient rich coastal environments to the low-nutrient (oligotrophic) situation that dominates the open ocean. We thereafter present genetic, physiological and metabolic adaptations important for marine bacteria to compete successfully in open ocean environments, noting that SAR11 group bacteria (Alphaproteobacteria) and cyanobacterial Prochlorococcus are among the most abundant organisms in the open ocean. Distinct adaptations are necessary for adjusting to limited availability of dissolved organic carbon, the macronutrients nitrogen and phosphorous, or micronutrients like iron (and other trace metals) or vitamins. Moreover, a multitude of distinct bacterial groups use photosystems based on rhodopsins or bacteriochlorophyll to harvest energy from sunlight. The spatial and temporal distribution of microorganisms, as influenced by nutrient availability, can be expected to have major importance in determining biogeochemical cycles of elements necessary for life and the flux of energy through ecosystems. © 2019 Elsevier Inc. All rights reserved.