Shallow CO2 vents are used as natural laboratories to study biological responses to ocean acidification, and so it is important to determine whether pH is the primary driver of bacterial processes and community composition, or whether other variables associated with vent water have a significant influence. Water from a CO2 vent (46 m, Bay of Plenty, New Zealand) was compared to reference water from an upstream control site, and also to control water acidified to the same pH as the vent water. After 84 h, both vent and acidified water exhibited higher potential bulk water and cell-specific glucosidase activity relative to control water, whereas cell-specific protease activities were similar. However, bulk vent water glucosidase activity was double that of the acidified water, as was bacterial secondary production in one experiment, suggesting that pH was not the only factor affecting carbohydrate hydrolysis. In addition, there were significant differences in bacterial community composition in the vent water relative to the control and acidified water after 84 h, including the presence of extremophiles which may influence carbohydrate degradation. This highlights the importance of characterizing microbial processes and community composition in CO2 vent emissions, to confirm that they represent robust analogues for the future acidified ocean. [ABSTRACT FROM AUTHOR]