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Multiple Nutrient Limitation in Microbial Ecosystems

First, classic models for microbial growth and endogenous metabolism are reviewed, with a focus on a generic cell model with two components. Growth is represented as the increase of one of these components (the structural scaffolding or `frame'). A novel feature of the generic model presented here is the explicit modelling of (partial) metabolic shutdown under conditions where maintenance requirements cannot be met.

Liebig's Law of the Minimum is then reformulated in terms of biomass composition dynamics. The doctrine of the single limiting nutrient is shown to be invalid generally.

It is proposed that ``being limiting'' should be defined in terms of `reserve surplus' variables. On the basis of this definition, it can be decided whether a nutrient, or combination of nutrients, is `limiting', both in transient and steady states. `Multiple limitation' is shown to have two distinct meanings on these definitions.

A `non-interactive' minimum model, based on a `hard' minimum operator, is introduced. Smooth `interactive' models may be formulated which have this minimum model as a limiting case. One such model is described. Numerical simulations show how the behaviour of this smooth model can approximate that of the minimum model: apparently hard non-linearities can arise in the smooth model, through time-scale separation.

Subsequently, the model is extended to deal with redox and light limitation. Microbes not only extract nutrients from their immediate surroundings, but they also oxidize electron donors, reduce electron acceptors, exude the `waste' products of endogenous redox metabolism, and, finally, effect light harvesting. These exchange fluxes are summed up in a generic model, which covers photoautotrophs as well as chemoheterotrophs. The focus is on endogenous metabolism and the cellular homeostasis of both reducing and phosphorylating equivalents. A novel result is the formulation of four `rules', akin to the Pasteur effect, which govern the compatibility of endogenous metabolism with various assimilatory processes.

A further extension of the model is the incorporation for a mechanism of `adaptive re-allocation'. To achieve minimization of surplus reserves (nutritional balancing), the microbes need to re-allocate molecular building blocks among the catalytic machineries that underly the various assimilatory pathways.

It is shown how a regulatory feedback mechanism which monitors the internal surpluses can achieve such re-allocation. At steady state, under stationary but arbitrary ambient saturation factors, the model trichome attains the diet functional response.

Another re-allocation topic is then taken up: the distribution of building blocks between assimilatory machinery and proliferative machinery. The former type of machinery is involved in nutrient uptake, whereas the latter type enables the trichome to grow. Given the availability of nutrients in the environment, how many building blocks should be allocated to the synthesis of assimilatory machinery, and how many to the synthesis of proliferative machinery? This question is answered for a particular model, which is a generalization of the Droop quota model.

The final two chapters of the thesis deal with ecosystems. First, the community matrix is reviewed in the context of Trophic Cascades Theory, which was later assimilated into the Bottom-Up:Top-Down Theory.

The concept of ratio-dependence has been put forward as an explanation of why trophic cascades must peter out, away from the locus of direct perturbation. It is shown that ratio-dependence achieves this in virtue of satisfying a more general condition on so-called `coupling strengths'.

The final chapter relates dimethylsulfide (DMS) emission from a microbial mat to the flux of dimethylsulfoniopropionate (DMSP) that is exuded into the interstitial space of the mat by phototrophs. DMSP may be either cleaved or demethylated. Only cleavage results in the production of DMS, which itself is further oxidized or escapes from the mat. The fate of DMSP depends on the functional group composition of the mat, the physiological characteristics of these groups, and the eco-physiological conditions oxic/anoxic and light/dark, which both vary in a diel cycle. These three factors are accounted for in a mathematical model of a microbial mat typical of the Wadden Islands of The Netherlands and Germany. Model simulations quantify increased DMS production under alkaline stress as well as additional DMSP loads.

next up previous contents
Next: Calendar Up: Summaries of PhD-thesis Previous: Summaries of PhD-thesis

Bob Kooi
Fri May 15 12:26:03 MET DST 1998