River Biogeochemistry

Microbial biogeochemical cycling in rivers

A fundamental question in microbial ecology is how change in environmental conditions influences structure and abundance of bacterial communities and their function and if there is a link between these two factors. The function of environment can be followed by monitoring rates of biogeochemical processes, e.g., nitrogen and carbon cycling, and this function can vary due to different physical and chemical conditions. Variation in environmental function can be due to physiological changes within a microbial community, changes in microbial abundance, or due to a shift in the microbial community.

Structure and function of microbial communities in biofilms.

Biofilms consist of bacteria, algae, and protozoans embedded in a matrix of extracellular polymeric substances (EPS) associated with a surface, e.g., the streambed. In addition, sediment particles are incorporated into stream biofilms and the majority of bacteria in streams live in biofilms. Biofilms are biochemical hotspots in small streams. Autotrophic microorganisms contribute strongly to stream primary production and heterotrophic biofilm bacteria play an important role in the turnover of dissolved organic molecules and in the breakdown of external carbon sources. They display higher sugar assimilation rates compared to planktonic bacteria, and enzyme activities in epilithic biofilms are much higher than in the water column. However, in contrast to the microbial loop in planktonic food webs, microbial food webs in biofilms are poorly understood.

In this project, we used experimental flow channels to simulate small creek ecosystems with planktonic and biofilm-associated microbial communities. The goals of our biofilm work are to 1) track carbon flow in the microbial food web of biofilms and to determine the amount of carbon channeled into higher trophic levels, 2) investigate the effect of primary production on the turnover of external carbon sources, 3) analyze the effect of sediment particles on the structure and function of stream biofilms, and 4) to assess the potential for biofilm microbial communities in nitrogen cycling. Important methods used in these studies included stable isotope analysis of lipid biomarkers, laser scanning microscopy, high pressure liquid chromatography (HPLC), microsensor measurements, and molecular techniques targeting total and ammonia-oxidizing prokaryotes.

Yenisei River project

Large increase in surface air temperatures in last few decades had a remarkable effect on the hydrological cycles and permafrost thaw leading to increased mobilization of organic carbon (OC) to Arctic rivers. Microbes are important players in the global carbon cycle; however, the role of microbes in OC mobilization and turnover in large Arctic rivers is still unknown.

The Yenisei River is the largest freshwater system flowing to the Arctic Ocean. It transports the largest amount of OC to the Kara Sea, exporting annually 4.1 to 4.9 x 10¹² g C in the form of dissolve organic carbon (DOC) and 0.17 x 10¹² g C in the form of particulate organic carbon (POC, Dittmar and Kattner, 2003). The Yenisei River originates in the Tuva republic of Mongolia by conflux of Bii Khem (also known as Big Yenisei) and Kha Khem (Small Yenisei) and ends in the Kara Sea of the Arctic Ocean. The length of the river from the conflux of Big and small Yenisei in Kyzyl, Tuva republic (Russia) is 3487 km. On its way to the Arctic Ocean it receives carbon input from various sources like permafrost, peat lands, taiga and tundra forests, industry and hydroelectric power plants.

The relative contribution of carbon from various vegetation and soil types is virtually unknown and is important to understand as the Yenisei receives huge inputs from permafrost and peatlands, which are potential sources of DOC in warming climate. Melting permafrost results in methane and CO₂ emissions, which are potential greenhouse gases. During permafrost thaw there are rapid changes in the microbial community structure and function and in pathways that lead to the emission of greenhouse gases, e.g., methane and carbon dioxide. As there is a close relationship between global carbon and nitrogen cycles, it is important to study the biogeochemical role of microorganisms in these cycles. To determine the effect of source and type of organic matter input on microbial community and functional along the flow path of the river, sampling will be conducted in two seasons: spring and summer. We will also investigate the activity of microbes involved in releasing OC by decomposition.