Black carbon (BC) is the carbonaceous residue of the incomplete combustion of fossil fuels and biomass and encompasses a range of chemically heterogeneous substances from partly charred plant material to highly condensed soot aerosols. We addressed the potential role of BC aerosol deposition on marine microbial processes in the ocean by investigating the effects of BC reference material (and its exposure to simulated solar radiation) on viral and bacterial activity in batch cultures with aged seawater. Viruses and bacteria were rapidly adsorbed to BC. No difference between the effect of irradiated and non-irradiated BC on free viral parameters was observed. Bacterial leucine incorporation was higher in the BC treatments than in the BC-free controls. The stimulated bacterial production in the dark BC treatments might be caused by the reduction of viral infection due to adsorption of organic material or by direct use of BC material. Viral production was significantly lower in BC-amended treatments than in BC-free controls, and the estimated fraction of infected cells decreased with increasing BC concentration. Moreover, bacterial activity in the solar-radiation-exposed BC treatments was higher than in the dark BC treatments, indicating that radiation made BC more accessible to bacteria. Our data reveal that BC has the potential to stimulate bacterial activity in the water column, particularly after exposure to solar radiation. Rising BC levels in the atmosphere due to increasing anthropogenic emissions could have far-reaching effects, including potential stimulation of seawater heterotrophy and CO2 production, through its effects on bacteria and viruses.