Andrew D. Steen bio photo

Andrew D. Steen

I'm interested in carbon, microorganisms, and aquatic ecosystems.

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Overview of the Steen Lab

We’re in the Department of Earth and Planetary Sciences at the University of Tennessee in Knoxville. We’re trying to understand how organic carbon gets oxidized in aquatic environments. Since heterotrophic microorganisms play a big role in this, we spend a lot of time thinking about the extracellular enzymes that microorganisms use to break down complex organic carbon. Lately we’ve been working on novel abiotic mechanisms that may be responsible for organic carbon oxidation, too.

Environments

Deep marine sediments

In ocean sediments as deep as a kilometer below the seafloor, populations of microorganisms manage to stay alive - and apprently to thrive. Many of thse organisms appear to be heterotrophs, meaning that they get their energy by breaking down organic carbon. This fact has important geochemical implications: subsurface sediments are a major sink for carbon, so the activities of subsurface heterotrophs influences the amount of carbon that is ultimately sequestered in marine sediments. These microorganisms are weird, though: they seem to have ‘lifespans’ on the order of thousands of years, and most are only very distantly related to microbes that have grown in culture. The organic carbon they’re eating is very old, and contains very few bioavailable compounds.

The Steen Lab is trying to understand how those heterotrophic microorgnaisms access organic matter. One way we’re doing this is to study the extracellular enzymes that subsurface heterotrophs produce. We’re also investigating what happens to those enzymes once they are produced, and what sorts of abiotic processes may influence microbe-organic matter interactions in deep sediments.

Mountain Rivers, Lakes and Streams

Inland waters are important players in regional carbon budgets. Microbes in freshwaters use extracellular enzymes to access orgnaic matter - but despite the fact that the activities of a few of those enzymes (leucine aminopeptidase, beta glucosidase) have been studied extensively, the full range of extracellular enzymes present in freshwaters have not been identified. We are assaying a wide set of peptidases (protein-degrading enzyme) in diverse rivers, lakes and streams of the Appalachian mountains to understand what kinds of organic matter can and can’t be metabolized to carbon dioxide, and what controls variations in organic matter bioavailability among environments.