Bacterial photosynthesis in surface waters of the open ocean

The oxidation of the global ocean by cyanobacterial oxygenic photosynthesis , about 2,100 Myr ago(1), is presumed to have limited anoxygenic bacterial photosynthesis to oceanic regions that are both anoxic and illuminated(2,3) . The discovery of oxygen-requiring photosynthetic bacteria about 20 years ago(4) changed this notion, indicating that anoxygenic bacterial photosynth esis could persist under oxidizing conditions. However, the distribution of aerobic photosynthetic bacteria in the world oceans, their photosynthetic competence and their relationship to oxygenic photoautotrophs on global sca les are unknown. Here we report the first biophysical evidence demonstratin g that aerobic bacterial photosynthesis is widespread in tropical surface w aters of the eastern Pacific Ocean and in temperate coastal waters of the n orthwestern Atlantic. Our results indicate that these organisms account for 2-5% of the photosynthetic electron transport in the upper ocean.