George Hagstrom, Princeton University

Abstract

Bacteria remineralize dissolved organic carbon in the ocean, thereby influencing the cycles of carbon and other nutrients. To utilize organic substrates, bacteria must perform extracellular degradation of polymers, exploiting either dissolved substrates in the bulk environment or using motility to colonize particles and gels. Here we develop a theoretical model to study the co-evolution of motility and degradation traits in marine heterotrophic bacteria. With this model, we link evolutionarily stable trait combinations to properties of the organic matter pool. We find that the diversity of organic matter, i.e. the number of distinct compounds present in the organic polymer pool is a critical parameter controlling the ecology and kinetics of organic matter consumption. As diversity progressively increases, there is an evolutionary bifurcation of the population from small, non-motile bacteria which degrade polymers primarily in the bulk, to large, motile bacteria which colonize and particulate matter and gels. We show that when monomers are modeled as a leaky public good, there should be coexistence between non-motile and motile bacteria, with the latter responsible for degradation. We hypothesize that the ecological role of motility is for bacteria to colonize persistent gel particles, and that this also explains the correlations between motility and other copiotrophic traits in marine bacteria.
To further explore the influence of the gel-decomposition paradigm on organic matter, we use polymer physics to study the kinetics of gel formation and dispersion as a function of polymer length, and make predictions about the age-size spectrum of dissolved organic polymers.