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In session 19i: Bioenergetics in Geochemical Modeling today, Marc Alperin put forward the provocative idea that methane-cycling ANME archaea in consortia with sulfate-reducing bacteria are not oxidizing methane, as commonly presumed, but producing it.

Boetius et al., 2000 Nature

The hypothesis comes from modeling the consortia in a diffusion-reaction model. When archaea (red cells in the figure) are modeled as methane-oxidizers, predicted rates of methane oxidation and sulfate reduction are orders of magnitude short of what is actually observed. However, if the model allows the archaea to produce methane instead, predicted rates of sulfate reduction are close to observed rates. The new model predicts that sulfate-reducing bacteria (green in the figure) will even have higher energy yields. They do even better by sharing H2 with methanogens.

Talks that challenge dogma are always attention-getting and provoke new thinking. Now the onus is on everyone to go out and test the new model – and to determine how the methane is being oxidized if not by ‘reverse’ methanogenesis.

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Oxygen for anaerobes

The Houtermans Medal was presented today to Nathan Yee, assistant professor at Rutgers. His medal talk, delivered in Session 16a, was titled “The Genetics and Geochemistry of Microbe-Selenium Interactions“.

It’s ever more beneficial for biogeochemists to be fluent in the genetics of  geochemistry. Yee is. Describing his work in Enterobacter and E. coli he outlined the discovery of genes involved in selenate reduction, including a novel selenate reductase.

Although selenate itself might not play a major role in global biogeochemical cycles, it may provide clues about evolution and Earth history. Selenate reduction occurs only under anaerobic conditions, but selenate is the oxidized form of  selenium, and is difficult to form without O2. With selenate scarce to nonexistent prior to the Great Oxidation Event, respiration by selenate reduction probably didn’t exist. Yee also points out that the selenate reductase enzyme – like many related reductases involved in anaerobic respiration – requires molybdenum, which also is widely available only in an aerobic world. By allowing molybdenum to become widely available the Great Oxidation Event may, ironically, have unlocked a number of new possible pathways for anaerobic respiration.

One wonders how anaerobic metabolism might have been affected after the GOE, such as during the Mesoproterozoic when sulfidic oceans may have limited molybdenum availability. Along with inhibiting primary productivity, the molybdenum scarcity may have had a negative impact on some anaerobic respiration pathways, potentially influencing rates of organic carbon recycing, or driving the evolution of replacement enzymes.  Geochemistry exerts its power over enzyme function, which in turn influences geochemistry. Uncovering these feedbacks make it increasingly apparent that we must appreciate both genes and geochemistry to make sense of either.

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Oxygen Oases: A Breath of Life in an Oceanic Desert

Lured by the promise of posters and breakfast, I wandered down to the Ice Rink area to check out the posters this morning, and I’m glad I did, because I got to meet Stuart Daines, a postdoctoral researcher in Tim Lenton’s Earth Systems Modelling Group (http://researchpages.net/ESMG/). They have developed a model of “oxygen oases” before the Great Oxidation Event 2300 million years ago. Oxygen oases are hypothesized regions of the ocean where, due to high primary production by oxygenic phytoplankton, the ocean could have had a much higher concentration of oxygen than the ocean did on average. These oxygen oases are important for understanding the evolution of life on Earth because they were regions in which the modern ocean carbon-oxygen system probably evolved. In their box model, organic carbon is converted to CO2 and methane during methanogensis; this is starkly contrasted with our groups model, where organic matter is used to feed sulfate and ferric iron reduction. Contrasting these two systems – their methane-oxygen coastal system with our global redoxcline ocean – gives us a broader picture of the biogeochemistry of the early Earth. However, like our model, it raises more questions than it answers. For example, what impact would the Paleoproterozoic global glaciations (~2400 million years ago – Kirschvink et al., 2000) have on primary production? What was the dominant limiting factor on primary production at this time? Could the ocean even support a redoxcline globally for such a long time period? These questions will hopefully be answered not only through future modelling endeavors, but also through direct analysis of the rock record. Hopefully, some of these answers will be discussed at later sessions, such as 07c: Records of Ocean Anoxia and their Impact on Life.

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