A generalized optimization model of microbially driven aquatic biogeochemistry based on thermodynamic, kinetic, and stoichiometric ecological theory

SAR11 bacteria have a high affinity and multifunctional glycine betaine transporter

Marine bacterioplankton face stiff competition for limited nutrient resources. SAR11, a ubiquitous clade of very small and highly abundant Alphaproteobacteria, are known to devote much of their energy to synthesizing ATP-binding cassette periplasmic proteins that bind substrates. We hypothesized that their small size and relatively large periplasmic space might enable them to outcompete other bacterioplankton for nutrients. Using uptake experiments with ¹⁴ C-glycine betaine, we discovered that two strains of SAR11, Candidatus Pelagibacter sp. HTCC7211 and Cand. P. ubique HTCC1062, have extraordinarily high affinity for glycine betaine (GBT), with half-saturation (K _s ) values around 1 nM and specific affinity values between 8 and 14 L mg cell^-1  h^-1 . Competitive inhibition studies indicated that the GBT transporters in these strains are multifunctional, transporting multiple substrates in addition to GBT. Both strains could use most of the transported compounds for metabolism and ATP production. Our findings indicate that Pelagibacter cells are primarily responsible for the high affinity and multifunctional GBT uptake systems observed in seawater. Maximization of whole-cell affinities may enable these organisms to compete effectively for nutrients during periods when the gross transport capacity of the heterotrophic plankton community exceeds the supply, depressing ambient concentrations.

The Thermodynamics of Marine Biogeochemical Cycles: Lotka Revisited

Nearly 100 years ago, Alfred Lotka published two short but insightful papers describing how ecosystems may organize. Principally, Lotka argued that ecosystems will grow in size and that their cycles will spin faster via predation and nutrient recycling so as to capture all available energy, and that evolution and natural selection are the mechanisms by which this occurs and progresses. Lotka's ideas have often been associated with the maximum power principle, but they are more consistent with recent developments in nonequilibrium thermodynamics, which assert that complex systems will organize toward maximum entropy production (MEP). In this review, we explore Lotka's hypothesis within the context of the MEP principle, as well as how this principle can be used to improve marine biogeochemistry models. We need to develop the equivalent of a climate model, as opposed to a weather model, to understand marine biogeochemistry on longer timescales, and adoption of the MEP principle can help create such models.

Thermodynamic constraints on the utility of ecological stoichiometry for explaining global biogeochemical patterns

Carbon and nitrogen cycles are coupled through both stoichiometric requirements for microbial biomass and dissimilatory metabolic processes in which microbes catalyse reduction-oxidation reactions. Here, we integrate stoichiometric theory and thermodynamic principles to explain the commonly observed trade-off between high nitrate and high organic carbon concentrations, and the even stronger trade-off between high nitrate and high ammonium concentrations, across a wide range of aquatic ecosystems. Our results suggest these relationships are the emergent properties of both microbial biomass stoichiometry and the availability of terminal electron acceptors. Because elements with multiple oxidation states (i.e. nitrogen, manganese, iron and sulphur) serve as both nutrients and sources of chemical energy in reduced environments, both assimilative demand and dissimilatory uses determine their concentrations across broad spatial gradients. Conceptual and quantitative models that integrate rather than independently examine thermodynamic, stoichiometric and evolutionary controls on biogeochemical cycling are essential for understanding local to global biogeochemical patterns.