Exploitation of algal-bacterial associations in a two-stage biohydrogen and biogas generation process

Abstract

Background: The growing concern regarding the use of agricultural land for the production of biomass for food/feed or energy is dictating the search for alternative biomass sources. Photosynthetic microorganisms grown on marginal or deserted land present a promising alternative to the cultivation of energy plants and thereby may dampen the 'food or fuel' dispute. Microalgae offer diverse utilization routes.

Results: A two-stage energetic utilization, using a natural mixed population of algae (Chlamydomonas sp. and Scenedesmus sp.) and mutualistic bacteria (primarily Rhizobium sp.), was tested for coupled biohydrogen and biogas production. The microalgal-bacterial biomass generated hydrogen without sulfur deprivation. Algal hydrogen production in the mixed population started earlier but lasted for a shorter period relative to the benchmark approach. The residual biomass after hydrogen production was used for biogas generation and was compared with the biogas production from maize silage. The gas evolved from the microbial biomass was enriched in methane, but the specific gas production was lower than that of maize silage. Sustainable biogas production from the microbial biomass proceeded without noticeable difficulties in continuously stirred fed-batch laboratory-size reactors for an extended period of time. Co-fermentation of the microbial biomass and maize silage improved the biogas production: The metagenomic results indicated that pronounced changes took place in the domain Bacteria, primarily due to the introduction of a considerable bacterial biomass into the system with the substrate; this effect was partially compensated in the case of co-fermentation. The bacteria living in syntrophy with the algae apparently persisted in the anaerobic reactor and predominated in the bacterial population. The Archaea community remained virtually unaffected by the changes in the substrate biomass composition.

Conclusion: Through elimination of cost- and labor-demanding sulfur deprivation, sustainable biohydrogen production can be carried out by using microalgae and their mutualistic bacterial partners. The beneficial effect of the mutualistic mixed bacteria in O2 quenching is that the spent algal-bacterial biomass can be further exploited for biogas production. Anaerobic fermentation of the microbial biomass depends on the composition of the biogas-producing microbial community. Co-fermentation of the mixed microbial biomass with maize silage improved the biogas productivity.

Keywords: Algal bacterial co-culture; Biogas; Biohydrogen; Metagenomics; Microalgae.

Figure 1

H ₂ accumulation (A) and O ₂ content (B) in the headspaces of the various cultures in time. Orange circles: mixed algal-bacterial co-culture (AB + S); green squares: algal-bacterial mixture with added E. coli ΔhypF (AE + S); blue triangles: sulfur-deprived bacterium-free co-culture of Chlamydomonas sp. and Scenedesmus sp. (A-S); red diamonds: bacterium-free co-culture of Chlamydomonas sp. and Scenedesmus sp. without sulfur deprivation (A + S).

Figure 2

Specific CH ₄ production from the various biomasses.

Figure 3

Weekly measured VOAs/TAC ratios. The area between the dashed red lines indicates the optimum range.

Figure 4

Weekly measured NH ₄ ⁺ concentrations. The dashed red line indicates the highest value recommended by the various studies.

Figure 5

Changes in N content during the AD of various substrates. Green: AB + S, orange: co-fermentation, blue: maize silage.

Figure 6

Microbial compositions of the substrates: (A) Maize silage, (B) AB + S. The communities at domain, phylum, class, and genus levels are indicated.

Figure 7

Changes in the domain Bacteria of the microbial community at phylum level. (A) Maize silage, (B) co-fermentation, and (C) algal-bacterial biomass.

Figure 8

Changes in the domain Bacteria of the microbial community at the order level. (A) Maize silage, (B) co-fermentation, and (C) algal-bacterial biomass.

Figure 9

Eukaryotic sequences in the reactors. Green: AB + S, orange: co-fermentation, blue: maize silage.

Figure 10

Distribution of the domain Archaea in the microbial community at the order level. (A) Maize silage, (B) co-fermentation, and (C) algal-bacterial biomass.