Award: OCE-1029569

It is well known that sunlight is the energy source driving the biological production of organic material using carbon dioxide in marine systems and most other habitats in the biosphere. Less well known and understood is the role of sunlight in converting that organic material back to carbon dioxide. Two general mechanisms are possible. First, sunlight may have a direct effect on organic material via "photochemical reactions" to produce carbon dioxide, but also carbon monoxide and small organic compounds which eventually may be degraded by microbes to carbon dioxide. Second, some microbes, known as photoheterotrophs, are able to gather energy from sunlight which may help them degrade organic material back to carbon dioxide. Both photochemical processes and photoheterotrophs were examined by this project in the Delaware estuary. This marine environment was chosen because of past work showing the importance of photochemical reactions in affecting organic material and of other work demonstrating the high abundance of two types of photoheterotrophic bacteria in the estuary. Estuaries are important test beds for exploring general questions in biogeochemistry and in the carbon cycle because they are the interface between the land and the oceans. This project found that production and consumption rates of carbon monoxide and selected organic compounds ("carbonyl" compounds) are very high in the estuary. Although production is by photochemical reactions, we found that consumption was dominated by microbial processes. However, because concentrations of carbon monoxide and the organic compounds are low, the consumption rates are also low compared to microbial growth. Still, consumption rates of carbon monoxide were higher during the day when photohetrotrophs would be most active. Direct measurements did not indicate any difference in photoheterotrophic activity between day and night, but there was an overall correlation with this activity and sunlight levels in the estuary. Some of the carbonyl compounds seem to come from atmosphere, in contrast to previous work indicating that marine systems are a source of these compounds. These compounds and others have important roles in atmospheric chemistry and ultimately the global carbon cycle. Probably because of the contribution of light energy, the cell size of photoheterotrophs is larger and they grow faster in the Delaware estuary than other microbes. The research of this project answered some important questions about photochemistry and photoheterotrophs but the main link between the two remains to be demonstrated. Last Modified: 02/19/2015 Submitted by: David J Kieber