The bioavailability of chemical compounds in the marine environment fundamentally influences the growth and physiology of microorganisms. Organic and inorganic chemicals that are produced by some marine plankton can be consumed by other plankton for energy production, growth, or to initiate essential physiological processes. Cultures of the diatom Thalassiosira pseudonana, a model phytoplankton species, and Pelagibacter ubique, the most abundant heterotrophic bacteria in the ocean, were used to study how microbial chemical interactions influence the physiology of these two groups of co-occurring marine plankton that have substantial roles in the carbon cycle. Specifically, the production and consumption of volatile organic compounds (VOCs) were studied in monocultures and co-cultures of these plankton. In co-culture, Pelagibacter benefited from a wide variety of VOCs produced by T. pseudonana, and heterotrophic consumption of VOCs taxed carbon fixation in the diatom by promoting the loss of diffusible VOCs from the primary producer. In this interaction, VOCs were shown to be a large fraction (ca. 20%) of fixed carbon that is transferred to heterotrophic bacteria, suggesting that VOC cycling by phytoplankton and bacteria is a significant component of the global carbon cycle. Acetone and isoprene, VOCs with important roles in atmospheric processes, were two of the VOCs utilized by Pelagibacter in co-culture. Pelagibacter metabolized acetone and isoprene at rates sufficient to explain discrepancies in measured fluxes of these compounds between the ocean and atmosphere, demonstrating that heterotrophic bacteria have significant control over the emission of VOCs from the ocean. While working with these cultures, T. pseudonana was shown to initiate the essential sexual reproduction phase of the diatom life cycle in the presence of ammonium. This chemical, released by other plankton to maintain elemental homeostasis, is a cue that initiates cell size restoration and introduces genetic diversity in centric diatom populations. The experiments and results herein give new information about how the chemical environment controls physiology in plankton, and that changes in physiology can further modify the chemical environment. These processes and feedbacks establish a complex network of microbe-mediated chemical interactions that have important implications for global biogeochemical processes.