What kinds of organic molecules fuel bacterial respiration in freshwaters?

PhD student Jenny Bowen shows how we bring the stream into the lab and run it through bioreactors where bacteria act as analytical chemists to tell us what compounds they like turn into carbon dioxide.

One of my lab’s main goals is to understand key controls on the cycling of C between its organic and inorganic (CO2) forms in freshwaters. Freshwaters contribute at least 1.2 Pg of CO2 to the atmosphere from the conversion of DOM to CO2 (Fig. 1 on main research page). The understanding has been that DOM is converted to CO2 mainly by bacterial respiration. Because DOM that fuels bacterial respiration is derived primarily from degrading plant and soil matter (i.e., terrestrially-derived), there are many thousands of organic molecules within the DOM pool which vary in lability (ease of use) to bacteria. Further, biological degradation of DOM occurs on a continuum of timescales spanning seconds to years. Our current knowledge of how specific DOM molecules fuel bacterial respiration is insufficient to predict C cycling between organic and inorganic forms across the land to water continuum.

We use optical and mass spectrometry to show how the rate of DOM degradation is connected to the composition of the constituent molecules. For example, with biogeochemists Lou Kaplan and organic geochemist Pat Hatcher, we’ve linked DOM composition to bacterial degradation rates by identifying C associated with humic DOM (i.e., DOM derived from either terrigenous or microbial sources), and C associated with free or combined fluorescent amino acids (Cory & Kaplan 2012)(Sleighter et al. 2014). Our approach is to use flow-through bioreactors which employ stream microbes as “analytical chemists” to separate DOM into labile, semi-labile, and recalcitrant DOM. Using this approach we developed the first relationships between C classes, stream uptake lengths, and biological turnover times to quantify C lability over time or space in stream sediments.

We’re studying carbon cycling in mountain streams in Costa Rica.

Our findings put bounds on the residence time of different pools of organic C that flow through rivers and help to close the mass balance of the labile DOM pool supporting bacterial respiration of DOM to CO2 in stream sediments. For example, while only a small fraction of “humic C” within DOM is respired by bacteria as DOM is transported downstream, this C accounts for a large fraction of the total C in DOM. Thus, our work our work shows that humic C contributes to the labile DOM pool, overturning some previous ideas on what constitutes labile C.

We are expanding our work from temperate to tropical streams to test ideas on controls on the compounds within DOM that are relatively more labile vs. more recalcitrant. DOM exported to streams draining contrasting plant communities, bedrock geology, and soil mineralogy across biomes and climatic regions likely differ in prior processing in soils and in chemical composition.

PhD student Jenny Bowen (right) and our collaborator collect stream water.