Intellectual Merit

A major environmental achievement of the 20^th century was the implementation of the Montreal Protocol and its subsequent amendments that phased out the production and consumption of human-made ozone-depleting substances (ODSs) (e.g., anthropogenic chlorofluorocarbons; CFC). Since the implementation of the Protocol, global emissions and atmospheric concentrations of anthropogenic ODSs have declined by more than 90%. Consistent with these reductions, the recovery of the ozone layer is already detectable in some areas of the world. However, the recovery still lags the reductions of reactive halogen gas loading. Today, the 1987 Montreal Protocol still regulates the production and consumption of anthropogenic ODSs. Nonetheless, unregulated and natural sources of halogenated hydrocarbons (e.g., halomethanes) continue to destroy atmospheric ozone. In contrast to CFC, the emissions of these short-lived halomethanes (e.g., CH₃Br, CH₃I) are overwhelmingly dominated by natural sources and their atmospheric concentrations are currently increasing. In contrast to their human-made counterparts, their sources and transport to the atmosphere have not been properly identified. As a result, the atmospheric halomethane budget remains open with sinks still outweighing all known sources.

The goal of this project was to establish a link between the synthesis of halomethanes by heterotrophic bacteria and the environmental availability of different methylating agents found in the water column (e.g. different chemical forms of vitamin B₁₂ (cyano, methyl, adenosyl, and hydroxyl) as well as the amino acid methionine) in a coastal upwelling system.

Although the synthesis of halomethanes has previously been observed in marine algae, no study to date had established the role of widely distributed taxa of marine heterotrophic bacteria on the synthesis of these biogenic halogenated volatile halocarbons in the ocean. We hypothesized that bacterial halomethane production could be more significant than previously thought, as they are the most abundant organisms in the surface ocean and halogenating enzymes are ubiquitous in marine heterotrophic bacteria. To test this hypothesis, we used a complementary approach combining the study of marine bacteria in cultures with environmental observations and field amendment experiments. 

The experimental laboratory results using marine bacteria in pure cultures showed that the synthesis of halomethanes is a generalized process in a wide range of marine heterotrophs. Furthermore, halomethane production was growth-rate dependent in all the strains we studied. We further evaluated the potential production rates of these biogenic gases in response to global warming. Our estimates showed that a rise of 3 degrees Celsius in temperature would translate into a 35%-84% increase in halomethane production by 2100. Overall, our results indicate that marine heterotrophic bacteria are significant producers of these climate-relevant gases and that their contribution to the atmospheric halogen budget could increase in the future due to surface ocean warming, potentially impacting the ozone layer recovery.

The field research component of this project provided the first temporal distributions of these ozone-depleting gases and established the relevance of highly productive coastal areas for halomethanes production. The highest levels of these gases were detected during the upwelling season when high biological activity triggered their synthesis. Consistent with their biogenic origin, halomethanes were undetectable below 200 meters, where biological activity was very low. During our study, we found that the temporal changes in the levels of halomethanes were correlated to the abundance of methionine, suggesting that the highly active microbial communities in the upwelling region were directly using this amino acid as a methylating agent, instead of synthesizing it from vitamin B₁₂. The relevance of methionine on halomethane synthesis in natural microbial communities was also demonstrated in a grow-out mesocosm experiment. In this experiment, the addition of methionine increased the synthesis of halomethanes by the native microbial communities several folds while vitamin B₁₂ enrichments did not show a response beyond the observed in the treatments enriched with inorganic nutrients (N and P) alone. Therefore, we concluded that the relevance of methionine on marine biogeochemical cycles such as the synthesis of biogenic gases seems more relevant than previously thought. To date, the role of methionine in the marine environment has not been appropriately addressed.

Broader Impacts

Despite the importance of marine biogenic halomethanes in atmospheric chemistry, relatively little scientific effort has been invested in studying the organisms and environmental conditions relevant to their production in the ocean. This project sought to characterize the relevance of marine heterotrophic bacteria in halomethanes biosynthesis and tried to elucidate their connection with the presence of methylating compounds in seawater. These data contributes to achieving a more accurate quantification of global halomethane sources and a better understanding of how halomethane synthesis patterns may shift together with anthropogenic climate change in the coming decades.

Last Modified: 07/28/2020
Modified by: Sergio A Sanudo-Wilhelmy