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MBGC Biogeochemical Model of Microbial Mats
(The First Photosynthetic Ecosystems)
MBGC Microbial BioGeochemistry ecosystem simulation model is a mathematical representation of a hypersaline microbial mat from Exportadora del Sal, Guerrero Negro, Baja California Sur, Mexico.
Microbial mats are interesting because they represent some of the most ancient ecosystems on earth. Fossil remains of microbial mats called stromatolites have been dated to 4 billion years in age.
The microbial mat from Guerrero Negro was modeled because it contains within it an entire complex food web that represents the full biogeochemical cycle of carbon from initial fixation through decomposition.
MBGC builds on the work of deWit et al. (1995) and simulates the carbon, oxygen, and sulfur cycles of several guilds of bacteria (Cyanobacteria (CYA); Colorless Sulfur Bacteria (CSB), Purple Sulfur Bacteria (PSB), Sulfate reducing bacteria (SRB), and Methanogens (MET).
Model components include: environmental inputs of photosynthetically active radiation (PAR), near infrared radiation (NIR), and temperature. Attenuation of PAR with each layer is modeled, and NIR attenuation depends on the abundance of bacteriochlorophyll a. Growth of each bacterial guild depends on their specific metabolic relationship of each physicochemical state factor combined with the size of the biomass pool and factors which account for 1) movement through the mat and 2) conversion efficiency. Growth of cyanobacteria occurs by either oxygenic or anoxygenic photosynthesis, depending on the available substrate (O2 or H2S). Purple sulfur bacteria growth occurs by anoxygenic photosynthesis using NIR and H2S or by chemosynthesis chemosynthesis, using O2 and H2S. Growth of colorless sulfur bacteria occurs via chemosynthesis, also using O2 and H2S. Sulfate reducing bacterial growth occurs via anaerobic chemosyntheis utilizing SO42- and creating H2S in the process. A Methanogen metabolism model is currently under construction as an addition to the MBGC model.
The model consists of multiple layers, and gas diffusion occurs between successive layers. The model was constructed in a Stellatm environment.
Slice of a hypersaline microbial mat, superimposed with a simple diagrammatic overlay describing the features simulated in the current and future MBGC model versions. Shown are those features in the current version of MBGC (Cyanobacteria (CYA), sulfate-reducing bacteria (SRB), purple sulfur bacteria (PSB); colorless sulfur bacteria (CSB) are circled and are in the current version of MBGC.) (Methanogens (MET), green non-sulfur bacteria (GNS), and organotrophs (ORG) will be completed in future.) The model simulates dissolved inorganic carbon (DIC), sulfide, and O2 cycling through a mat that experiences daily light fluctuations (PAR:photosynthetically active radiation, and NIR:near infra-red radiation).
Authors: Kelly Decker & Christopher Potter

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