Extensive use of fossil fuels has encouraged governments and research community to find alternative energy sources in order to reduce greenhouse gas (GHG) emissions and dependence on fossil fuels. One of these alternative energy sources is biogas, a mixture of gases, mainly CH4 and CO2, produced by the anaerobic fermentation of organic matter. One of the multiple substrates for the production of biogas are microalgae which are microorganisms that can transform sunlight and CO2 into biomass.

In this work, a new approach for the production of biogas from the anaerobic fermentation of microalgae was investigated. Here we propose to perform the anaerobic digestion at alkaline conditions (pH~10; 2.0 M Na+). This can have several advantages over traditional anaerobic digestion systems. One advantage is that due to the higher CO2 solubility at alkaline conditions, the medium in the anaerobic reactor can act as a CO2 scrubber resulting in biogas rich in methane.

To perform the anaerobic digestion at such alkaline conditions, sediments from alkaline lakes were used as inoculum. Several anaerobic reactors were set up to study the anaerobic digestion of Spirulina. These reactors were used to determine the optimal process parameters such as hydraulic retention time and organic loading rate. Several bottle necks were identified in this new approach, mainly related to ammonia inhibition and to poor granule formation. The produced biogas was rich in methane which made it suitable for the direct use in national gas grids or as fuel for vehicles.

Metagenome analysis was the applied method to taxonomically and functionally characterize the unique extremophilic microbial community present in the alkaline anaerobic reactors. The taxonomy profile of this particular anaerobic alkaline community was similar, at higher taxonomic levels, to known traditional anaerobic digester communities with Bacteria clearly dominating over Archaea. The main observed differences could be attributed to the type of bacteria and archaea found, mainly haloalkaliphilic. Uncultured haloalkaline bacteroidetes and other halotolerant bacteria such as Halanaerobium were among the most dominant bacteria, while among the methanogenic archaea a clear dominance of Methanocalculus was observed.

Genes related to the different strategies used by haloalkaline bacteria to cope with high pH were detected in the functional analysis. In the alkaline anaerobic reactor, the hydrogenotrophic methanogenesis pathway was the most prominent pathway while most genes of the acetoclastic pathway were practically absent.