Molecular biology approach to anaerobic digestion of marine biomass

The current drive towards renewable energy has led to interest in the use of marine biomass, including seaweed. The presence of readily hydrolysable sugars, low amounts of cellulose and zero lignin enhances the suitability of seaweed for methane production, but this process has so far received little attention. As seaweeds have been shown to contain constituents with antimicrobial properties, understanding the microbial interactions in the system is crucial.
The aim of this study was to investigate the microbial community associated with seaweed anaerobic digestion in order to understand the intricate interaction between the microbial population and process functions. Selected seaweeds (Laminaria digitata, Saccharina lattissima and Fucus serratus) found commonly on the west coast of Scotland were subjected to 50-day anaerobic batch digestion using different inoculum sources. A number of molecular techniques including, denaturing gradient gel electrophoresis, quantitative polymerase chain reaction, cloning and sequencing were employed to study the microbial ecology of the seaweed fed reactors. Results show that marine sediment is a viable microbial source for efficient methane fermentation of L. digitata and S. latissima at seawater salinity level, and indicates that methane production from both seaweeds compares favourably with other types of biomass, including terrestrial crops.
Results obtained suggest that microbial numbers fluctuate during anaerobic digestion, potentially in response to substrates availability. Analysis of microbial community structure highlights temporal and spatial variations in microbial diversity within and across reactors, possibly as a result of process conditions and functions.
Identification of the dominant archaea and methanogens indicate that Methanomicrobiales and Methanosarcinales-related species could dominate sediment and sludge inoculated reactors, indicating the potential for utilisation of a diverse range of substrates. Results from the functional gene clone library, suggest that hydrogenotrophic, acetoclastic and methylotrophic methanogenesis could potentially be involved in methane production.
Overall, this study provides insights into the microbial ecology of seaweed anaerobic reactors and the microbial responses to changing conditions. Results highlight possible routes for optimisation of the anaerobic digestion process, which could help prevent system failure during large-scale seaweed anaerobic digestion.