Work Package 10 (Blasius, Dittmar, Simon)

"Ecological modeling of molecule-organism interactions in marine habitats"

Broader background of the proposed research project

The competition of organisms for essential resources is one of the driving forces for structuring ecological communities and limiting biodiversity (Tilman 1982, Ryabov & Blasius 2011). Natural marine bacterial communities and their interrelation with a diverse mixture of small dissolved organic compounds is a prominent example. It is still not understood how the structural diversity of the marine geometabolome depends on the combined effect of a complex molecular reaction network, the metabolic activity of heterotrophic microbial communities, and the refractory structure of specific highly stable compounds. Classical resource competition theory does not capture the huge diversity of marine DOM (Grzybowski et al. 2009). Caused by the huge diversity of marine DOM the concentration of specific substrates is extremely small, yielding rare encounter rates between microorganisms and their specific organic resource molecules. This may give rise to a severe dilution effect (Schmidt & Ostfeld, 2001), so that the high diversity of organic compounds may reduce competitive forces at small concentrations and thus inhibit the competitive exclusion of organic substrates. Consequently, community structure may be better described by neutral species interactions, assuming a per-capita equivalence of all compounds (Hubbel 2001).

Outline for the proposed PhD research project

We will use modern concepts of theoretical ecology and complex network theory to model the interaction of organics substrates with communities of microorganisms of huge diversity. Thereby, we develop a new conceptual framework to understand how the structural diversity of marine DOM and microbial communities shape and mutually influence each other. We will model the interaction between microbial consumers and small dissolved molecules as a bipartite network. This molecular reaction network provides the base to model the dynamics of populations of microorganisms and of organic compounds. Using the models we will investigate: (i) emerging equilibrium distributions, their diversity and rank-abundance patterns, (ii) species turnover, comparing to predictions from neutral theory, (iii) the influence of the dilution effect to maintain DOM diversity; and (iv) the relation between the structure of the interaction network and the resulting dynamics. The mathematic modeling will be complemented in cooperation with WP 11 by analysis of data sets about the geometabolome in pelagic and benthic marine habitats, comprising DOM data obtained from ultrahigh-resolution mass spectrometry (WP4), structural analysis by targeted metabolite analysis (WP 3), proteogenomics (WP 2) and bacterioplankton communities (WP 8).