Research projects

Active projects

 Impact of viral infections on the biogeochemistry of phototrophic sulfur oxidizers

Mahoney Lake, BC, Canada

Mahoney Lake water at 7 meters deep

Viral infections of bacteria are a major driver of biogeochemical cycles in the modern ocean. Through gene expression during infection and cell death, viruses affect essential Earth system processes, such as marine photosynthesis. Observations from modern viral ecology lead to the prediction that virus-bacteria interactions influenced biogeochemical cycling and significant global change transitions in the past. We hypothesize that deeply rooted virus-bacteria relationships modulated the Earth's sulfur and carbon cycles through geological time. 

Our preliminary data demonstrated the presence of viral genomes encoding genes with potential to modify the metabolism of photosynthetic sulfur bacteria in modern euxinic lakes. Because these bacteria (purple and green sulfur bacteria) were prolific primary producers in many intervals during the geological past, viral infections were likely agents in the establishment of the sulfur cycle on Earth.

Our first paper, which has important implications for interpreting the redox environments implied by geologic records, is now accepted for publication by Nature Communications Earth and Environment. This project in funded my NASA exobiology division.

Microbial Fe reduction under oxic conditions: when microfluidics meets geochemistry 

Iron (Fe) is one of the major elements of Earth’s crust, and microorganisms have been cycling it for at least 3.0 billion years between its oxidized ferric (Fe(III)) and reduced ferrous (Fe(II)) forms. Facultative Fe-reducing anaerobes such as Shewanella oneidensis MR-1 are expected to reduce Fe(III) in the absence of O2, leading to the production of Fe(II) in anoxic environments. In contrast, dissolved Fe(II) is rare in oxic environments because it is rapidly oxidized in presence of O2. Here using batch and microfluidic reactors integrated with planar O2 sensors, we provide geochemical evidence for microbially induced Fe(III)oxide (ferrihydrite) reduction in oxic aqueous environments and the maintenance of a persistent Fe(II) pool. 

Our discovery of oxic ferrihydrite reduction could fundamentally change the way we interpret the deposition of Banded Iron Formations (key proxies of the atmosphere oxygenation process 2.5 billion years ago) and significant impact on modern soils and sediments by triggering the release of adsorbed and co-precipitated contaminants into pore waters and, ultimately, into surface waters.

Are historical eutrophic Swiss lakes greenhouse gas sinks or sources?

Lakes are one of the largest natural sources of the greenhouse gas methane in the atmosphere. Switzerland has approximately 1500 lakes, many undergone a eutrophication phase in the mid-20th century that led to the burial of organic matter and expansion of bottom water anoxia. Such conditions are thought to stimulate microbial methane production, exacerbating global warming. However, methane can simultaneously be consumed by so-called methanotrophs, and the impact of eutrophication on this biological methane sink remains unexplored. 

Here I am exploring a novel and provocative hypothesis that past eutrophication events, now buried in the sedimentary record as nutrient-rich intervals, can "feed" microbial methane oxidation with the electron acceptors nitrate and iron. I am combining methods from geology and biology to investigate how past eutrophication events modulate the abundance and diversity of methanogens (methane producers) and methanotrophs (methane consumers) microorganisms; and to characterize bioauthigenic Fe mineral products by the microbial processes. 

The data produced here will be helpful to future studies targeting climate control alternatives through microbially controlled greenhouse gas sequestration.

Environmental constraints and ecology of early Earth: perspectives from the São Francisco craton

Using sulfur and carbon isotopes (d34S, D33S and d13C), iron speciation, and trace elements (in particular rare earth elements) as tools, I described in the ~2.7 billion-year-old Brazilian rocks: (1) a record of molecular oxygen generation at least 300 million years before the GOE; (2) signatures of sulfate reduction, and so the record of the build-up of sulfate/sulfide in the oceans, developing one of the oldest records of euxinic systems that implies an essential role for anoxygenic photosynthetic sulfur bacteria as primary producers; (3) an episode of sulfur mass-independent fractionation (S-MIF) expansion and retraction in response to ecological changes (limited methanotrophy leading to the formation of an atmospheric organic haze) at 400 million years before the GOE; and (4) a record of a complete transition from ferruginous to oxic water column conditions.