Research | College of Biological Sciences

Geomicrobiology of Mid-Latitude Glaciers

Glaciers recharge alpine streams and snow and ice microbes contribute to the global carbon cycle. Snow microbiota are also a key component driving glacial retreat: Blooms of snow algae darken snow and glacial surfaces, reducing the amount of sunlight reflected by ice (albedo), leading to increased melting. Due to a lack of data, the role of glacier meltwater in alpine stream recharge remains poorly quantified and climate models do not incorporate the effects of snow algae communities on the global carbon cycle or biological albedo reduction. We aim to 1) characterize the contribution of glacial melt to downstream ecosystems and 2) parameterize biological albedo reduction so it can be incorporated into Earth system models.

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Photosynthesis across space and time

The ability to harvest light and fuel cellular processes through phototrophy is arguably one the most important biological innovations in Earth history. Yet, the early evolution of photosynthesis remains an outstanding question. The fitness and ecological success of the first phototrophs are difficult to interpret because geochemical proxies can be unreliable or low fidelity and characterized deeply branching isolates are rare. My lab aims to improve phylogenetic reconstructions of the evolution of photosynthesis and answer outstanding questions centering on the physiology of the earliest phototrophs, the nature of the first light-harvesting reaction center (which reaction center came first), and how oxygenic phototrophs evolved to use two reaction centers in concert to split water. To address these open questions, we characterize phototrophs across physicochemical gradients through lab, field, and sequencing approaches.

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Metabolically versatile Cyanobacteria

Cyanobacteria are only prokaryotes that carry out oxygenic photosynthesis and therefore played a crucial role in the evolution of Earth’s atmosphere as well as diversification of microbial and higher life forms. In sunlit environments where low oxygen concentrations and sulfide persist, some Cyanobacteria can use sulfide as the electron donor to photosystem I in the absence of oxygen generation, enabling photosystem II-independent photoassimilation of CO2. Examples of well-characterized Cyanobacteria capable of anoxygenic photosynthesis are rare and the process is not well understood. Utilizing a model cyanobacterium isolated from an environment where low levels of oxygen and sulfide are present, we are characterizing the genetic, regulatory and biochemical underpinnings of anoxygenic photosynthetic activity. This work is carried out in collaboration with a number of excellent scientists - Dr. Jenn Macalady, Dr. Miriam Weber and Dr. Christian Lott at the Hydra Institute, Dr. Dirk de Beer and Dr. Judith Klatt at the Max Planck Institute for Marine Microbiology.

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Freshwater Biodiversity

Lakes and reservoirs play key roles in the global carbon cycle: they are important carbon sinks and also a significant source of the greenhouse gas emissions including methane. Enrichment of nutrients (eutrophication) and rising global temperatures favor the proliferation of bloom-forming cyanobacteria which alter carbon and nutrient cycling in freshwater ecosystems. We are combining water-column and sediment studies to examine the timing and impact of eutrophication on greenhouse gas emissions and global carbon cycling.

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Trace metal availability and metalloenzymes

Early Earth was oxygen-poor, resulting in different bioavailability of redox sensitive trace metals than observed today. For instance, iron reacts with H2S in sulfide-rich water, forming insoluble sulfides that remove molybdenum from the water column. Throughout much of Earth's history, surface oceans experienced at least localized euxinic conditions. In contrast, iron is scarce in the modern oxygen-rich ocean whereas molybdenum, present as molybdate, is highly soluble and readily bioavailable. Because ocean redox conditions and redox-sensitive metal availability are linked, the evolution of metalloenzymes may follow a trajectory that mimics prevailing redox conditions. We are using model organisms to determine the bioavailability of metals that are complexed with sulfides and in more complex minerals to examine the links between redox conditions and the evolution of metalloenzymes.

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Competitive interactions and microbial community assembly

The co-occurrence of cyanobacteria and anoxygenic phototrophs is common in euxinic lakes, cyanobacterial mats, hot springs, and hypersaline lagoons where sufficient fluxes of reduced compounds are available to support anoxygenic photosynthesis. However, the geochemical and biological factors controlling competition among co-existing phototrophs in natural environments is poorly understood. Using pure cultures and in situ studies, we are identifying the physical, geochemical, and physiological factors that affect competition among chlorophototrophs and consequences of this competition.