MICRO-CYCLE: Unravelling the role of microbial genomic traits in organic matter cycling and molecular composition along the river continuum

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Global biogeochemical cycles describe the transformation and transport of carbon and other nutrients between the major components of the Earth system. Rivers and streams represent a major component in this cycle, linking flows of carbon (C), in the form of organic matter (OM), and other nutrients (nitrogen, N; phosphorus, P) between the terrestrial environment and the atmosphere and oceans. Bacteria in rivers and streams, which can number in the millions of cells and thousands of species per millilitre of water (called bacterioplankton), use OM and the carbon, nitrogen and phosphorus it contains, as a source of food, both for growth and for respiration. The OM pool contains a similarly diverse range of compounds, with tens of thousands of molecules, with varying C, N, P and other chemical constituents held in a wide variety of different chemical structures. Bacteria have preferences for different forms of OM, linked to these differing structures and contents, but we know little about which species of bacteria exploit which OM molecules. This is important, as the OM from terrestrial ecosystems is changed in both concentration and chemical structure during transport from the headwaters of a river to downstream reaches, estuaries and the sea. The interactions between the bacterial community and OM play a critical role in determining how much C, N and P are released into the oceans and how much is respired as carbon dioxide or released as nitrogen gas to the atmosphere.The MICRO-CYCLE project seeks to better understand the role that bacterioplankton play in using and modifying OM as they both flow from the headwaters of rivers towards the sea (called the 'river continuum'). This new knowledge of the ecological processes that determine what types of bacteria are present in rivers, how this varies from headwaters to the sea, and what this means for how OM is used and transported along rivers, will help us create models that can better predict how rivers function, as well as how they might change in future.We will do this by filling the gaps of knowledge in:1. The mechanisms by which bacterioplankton communities are structured across the river continuum, from headwaters to the lower reaches of rivers. This will be achieved by spatially structured sampling across the River Thames and its sub-catchments representing different landscape and hydrological characteristics.2. Which species of bacteria (the bacterial community) play an active role in using OM and which species are just 'passing through' and not contributing functionally to OM cycling (and how this varies along the river continuum in response to environmental fluctuations).3. The role that the chemical composition of OM has in determining the structure and function of the bacterial community along the river continuum, and the role that the bacterial community has in changing the chemical composition of OM.4. The ecological and functional structure of bacteria along the river continuum, and the relationships between bacterial communities and OM, to build predictive models of how bacteria control the metabolism of rivers and the export of C and N to the atmosphere and C, N and P to the oceans.