Light-Enhanced Bacterial Growth
Light is an abundant resource in surface environments, but less than 0.5% of available photons are captured by Chl a for photosynthesis. What happens to the rest of them? Our recent work on rhodopsin abundance in the Chesapeake Bay showed that ion-pumping rhodopsins, simple light-activated pumps that generate transmembrane ion gradients, are most abundant in sites where bacterial heterotrophs are most active. In marine systems, rhodopsins tend to be associated with nutrient limitation, and help microbes to survive starvation. In estuaries like the Baltic Sea and Chesapeake Bay, they seem to be associated with availability of organic carbon. We still don’t know why they are so important in these environments, but it may be related to energizing membrane-associated functions like active transport.
We are also characterizing heterotrophic freshwater Actinobacteria that grow faster in the light than in the dark, even though they have no way to convert light energy to chemical energy. Instead, we think that they use light as a source of information, and that they upregulate organic carbon transport and processing in response to light. In sunlit environments, primary producers release organic carbon and nitrogen along with other growth factors during the day. The ability of Actinobacteria to coordinate the uptake and utilization of organic carbon with the time of maximum production of photosynthate could enable them to grow more efficiently in the daytime. This coordination with primary producers potentially gives Actinobacteria a competitive advantage over heterotrophs that either constitutively produce and maintain organic uptake machinery, which is energetically costly, or synthesize it only after detection of the substrate(s), which delays their response. Understanding how light both directly contributes to production of organic carbon and cues its transport and conversion to biomass is thus key to understanding biochemical mechanisms within the carbon cycle, the fluxes through it, and the variety of mechanisms by which light enhances growth.
In collaboration with Tom Hanson, we were recently awarded an NSF-EDGE grant to develop genetic tools to better understand the ecophysiology of freshwater Actinobacteria. Our model organisms for this effort are Rhodoluna lacicola strain MWH-Ta8 and Aurantimicrobium photophilum strain MWH-Mo1. Our goals are to develop tools for both targeted and random mutagenesis as well as methods for heterologous gene expression in one or both strains.
Microbial Communities of Concrete
Concrete is a very common, but harsh environment: dry, salty, and very alkaline (its pH can be as high as 12.5), and subject to daily and seasonal fluctuations in temperature, moisture content, and salinity. Despite this, a diverse community of microbes inhabits ordinary pavement concrete, and representatives of this community can be detected by 16S analysis and cultivated in the lab. We are using 16S rRNA gene analysis to characterize the microbial communities in ordinary concrete and concrete undergoing the alkali-silica reaction (ASR), a common cause of concrete deterioration. We hope to use this data to identify bio-indicators that can be used to assess the structural health of concrete prior to appearance of visible cracks.
Phosphate Starvation and its Consequences for Global Warming
Phosphorus is a limiting nutrient in many freshwater systems, and in Lake Matano, Indonesia, it cannot even be detected. In this iron-rich stratified lake, nanoparticulate Fe(III) minerals in the surface water adsorb phosphate, removing it from the water column. However, this lake has a robust population of microbes. We investigated phosphorus acquisition strategies by heterotrophic bacteria in Lake Matano using a combination of metagenomic analysis and cultivation. We isolated several heterotrophs, and showed that they can utilize dissolved phosphate, mineral-associated phosphate, phosphate esters, and phosphonates as phosphorus sources. We also showed that when these bacteria use methylphosphonate as a phosphorus source, methane is released as a byproduct, suggesting that some of the methane present in the surface water in Lake Matano may be produced by aerobes.
Stormwater Runoff Filtration
The Chesapeake Bay is a critical natural and economic resource in the Mid-Altantic. Because it is so close to both major agricultural regions and major urban areas, it has historically been polluted by nutrient runoff from agriculture and wastewater from cities, leading to algal blooms and a large annual hypoxic zone (“dead zone”). Decades-long efforts to remove nutrients and other pollutants from agricultural and urban runoff before they reach the Bay have improved water quality in the Chesapeake Bay (see the 2018 report from the Chesapeake Bay Foundation here): the water is clearer in places, phosphorus concentrations and sediment loads are lower, and native estuarine grasses are rebounding. Stormwater is now one of the main sources of pollution in the Chesapeake Bay. In collaboration with Paul Imhoff, we are testing hardwood-derived biochar as an amendment to stormwater filters. Biochar extends the retention time of stormwater in the system, allowing more time for removal of N and P by either sorption or biological processes, and we are investigating the mechanisms by which this happens.