Understanding the interactions within a methanotroph-photoautotroph coculture through kinetic modeling

Significance 

Though underutilized, agricultural, municipal and industrial waste streams containing carbon are potential feedstocks for producing chemicals and fuels. Anaerobic digestion (AD), which breaks down organic wastes to produce biogas, is an efficient approach for managing waste streams and preventing severe eutrophication and leaching of minerals. Although biogas is a promising feedstock for producing commodity chemicals and energy, the limited implementation of anaerobic digester remains a big barrier to realizing global biogas potential.

While heat and power production is the common route for AD-generated biogas utilization, several limitations, such as the presence of contaminants in biogas and high capital and operational expenses, make it economically unviable. Recently, effective biotechnologies have been identified as a potential and economically viable solution for valorizing biogas produced from waste streams. Importantly, recent findings have demonstrated the effectiveness of using methanotroph-photoautotroph (M-P) cocultures for biogas conversion, enabled by the metabolic coupling of oxygenic photosynthesis and methane oxidation

Different M-P cocultures have been extensively studied in recent years, and their efficiency in biogas conversion while utilizing all the carbon contained in the methane and carbon dioxide has been demonstrated. However, the development of advanced biotechnology with the M-P coculture as biocatalyst requires robust kinetic models for predicting microbial growth and optimal design in different conditions. Unfortunately, there are no kinetics models for M-P coculture available due to the complex and largely unknown interactions within the coculture, as well as the difficulties of characterizing M-P cocultures.

Herein, PhD candidate Kiumars Badr, Professor Peter He and Professor Jin Wang from Auburn University developed a novel semi-structure kinetic model that can accurately predict the coculture growth under a wide range of conditions. Based on the explicit modeling of the growth stoichiometry of the individual organisms, the interspecies O2/CO2 metabolic coupling and the total mass balance, dynamics of substrate consumption and biomass production were estimated. The model accuracy was validated through wet-lab experiments using Methylomicrobium buryatense 5 GB1 and Arthrospira platensis species as model coculture. The work is published in the journal, Chemical Engineering Journal.

The authors demonstrated efficient coupling of the biomass growth with mass transfer between liquid and gas phases, capturing O2/CO2 in situ exchange produced between two species and implicit modeling of unknown interactions. The model accurately predicted both the individual consumption/production rates of oxygen and carbon dioxide and the growth dynamics of the M-P coculture under different growth conditions. Through integrating the semi-structure model and design experiments, the research team confirmed the existence of additional emergent metabolic interactions within the coculture. Their effects on the growth of the two species was also quantified.

Different factors, such as the composition of the feeding gas, light intensity and inoculum ratio of the coculture, were found to affect the coculture growth, though light availability was the main affecting factor. Since methanotroph in the coculture mainly depends on the oxygen generated by the photoautotroph to grow, it has been thought to be the partner benefitting from the coculture environment. This was not the case, however, according to the study findings, because the two species displayed enhanced growth rates than those of single cultures.

In summary, the study presents a very first and accurate semi-structured kinetic model for M-P cocultures. It not only captured the known cross-feeding mechanism within the mixed culture but also validated the existence of emergent interspecies interactions. Using less complicated model equations and parameters, the model structure was kept simple and robust. In a statement to Advances in Engineering, Professor Jin Wang explained that the new model provided the foundation for effective design and optimization of coculture-based photobioreactors needed for biogas conversion technologies, as well as guidance on the further studies to identify molecular cross-feeding mechanisms.

Reference

Badr, K., He, Q., & Wang, J. (2022). A novel semi-structured kinetic model of methanotroph-photoautotroph cocultures for biogas conversionChemical Engineering Journal, 431, 133461.

Go To Chemical Engineering Journal

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