Carbon dioxide in the atmosphere is a major greenhouse gas. Increasing atmospheric concentrations are believed to be responsible for a gradual warming of the Earth. Strategies to reduce anthropogenic CO2 emissions to the atmosphere by sequestration underground or in the deep ocean are being investigated. An international collaboration to conduct a field experiment on CO2 ocean sequestration was initiated in December 1997. The primary objective of this project is to obtain data that can be applied to assess environmental impacts of direct injection of CO2 into the deep ocean. The collaboration is being implemented by a research team comprising engineers and scientists from five nations: Japan, the US, Norway, Canada, and Australia. This paper focuses on studies being conducted as part of the field experiment to investigate the influence of decreased seawater pH from CO2 dissolution on bacterial production in the ocean. A preliminary analysis of the variation in bacterial production under different values of pH and temperature, conducted in July 1999, indicated that there was no measurable effect on the microbial population in the first 24 h of the experiment. However, there was a rapid decline in bacterial production with decreases in pH over a 96 It incubation period. In addition, the impact of reduced pH was more pronounced when the bacteria were grown at warmer temperatures, i.e. with a more rapid potential production rate. Subsequent laboratory experiments were conducted in June 2000 using pressure chambers to determine the effect of depressurization on bacterial production. For this effort, seawater samples were collected from a 600 in deep pipeline off the Kona coast of the island of Hawaii. Water was placed in a pressure chamber and allowed to stabilize for 48 h at the ambient pressure and temperature of 600 in depth at the sampling location. After 48 h, the water was depressurized and acidified with CO2 to pH values of 6.95 and 5.6. The acidified water and an un-amended sample were incubated at 5, 10 and 15degreesC for 96 h and sampled every 24 h. Similar impacts to those observed previously on the bacterial production were noted. The overall result was that longer incubation times and warmer temperatures reduced bacterial production in samples exposed to the lower pH. This observation is beneficial to develop an understanding of the long term effects of CO2 releases on basic elemental cycles that support aquatic food chains. Future work needs to investigate long term variation in the bacterial community, the impact on nutrient recycling, and the population growth efficiency. Published by Elsevier Ltd.