A physiological model for the marine cyanobacteria, Trichodesmium

Nitrogen fixation by the marine cyanobacterium, Trichodesmium, is believed to form a substantial component of the nitrogen budget of the Great Barrier Reef Lagoon. Here, we present a new, physiologically-based model to predict the distribution and growth of Trichodesmium. The model has been incorporated into a large-scale, process-based, three-dimensional hydrodynamic, sediment dynamic and biogeochemical model of the Great Barrier Reef Lagoon through eReefs, a major collaborative project that is developing near-real-time and forecasting models to inform management of this important environmental asset. The model simulates the growth and respiration of Trichodesmium colonies, along with uptake of nutrients, fixation of atmospheric nitrogen, changes in cellular buoyancy, grazing by zooplankton and death associated with lysis by cyanophages. To facilitate improved simulation of nutrient dynamics as well as changes in carbohydrate ballasting (which affects buoyancy), the model allows variable intracellular C:N:P:Chlorophyll a ratios. Chlorophyll a accumulation and Trichodesmium growth depend on the intracellular availability of nutrients and fixed carbon. Carbon accumulation is a function of the spectrally resolved light environment, so that changes in light quality as well as light intensity may affect growth. As Trichodesmium colonies accumulate carbon, their buoyancy decreases, allowing the vertical movement of Trichodesmium through the water column to be simulated. Particular attention is paid to simulating the nitrogen dynamics of Trichodesmium. Where sufficient ammonium is available in the water column, this is taken up preferentially, reflecting the lower energetic cost of this nitrogen source. When external ammonium concentrations are not sufficient to supply the cellular demand, Trichodesmium colonies take up nitrate. A novel model formulation is presented to simulate this preferential uptake dynamic without introducing an additional parameter. Only if the supply of dissolved inorganic nitrogen is insufficient do nitrogen fixation pathways become active in Trichodesmium cells. In this case, the growth rate of Trichodesmium is reduced in proportion to the energetic cost (in terms of ATP) of nitrogenase activity. With few exceptions, the values of parameters used in the model can be derived from direct measurements or theory. Important processes not included in the model are also discussed; these include iron limitation of nitrogen fixation and the dynamics of surface scum. Unfortunately, it is not possible at this stage to include iron limitation, as iron inputs to the Great Barrier Reef Lagoon are not monitored; hence, iron is not included in the biogeochemical model. The dynamics of surface accumulations of buoyant Trichodesmium are an interesting problem in terms of physics, chemistry and biological processes, and may be considered in a future version of the model.