A First Marine Vibrio Biocatalyst to Produce Ethanol from Alginate, which is a Rich Polysaccharide in Brown Macroalgal Biomass.

The use of un-utilized feedstock and seawater for material and/or energy production using marine microbial catalysts is one potential option toward contributing to the development of a more sustainable society. Ethanol production from alginate, which is an oxidized polysaccharide present in brown seaweed, is extremely difficult due to the imbalance of reducing power in the microbial cells. Production of ethanol by such means has so far been unsuccessful using marine microbial biocatalysts. To produce ethanol from alginate, an alternative pathway consisting of a pyruvate decarboxylase gene (pdc) and an alcohol dehydrogenase II gene (adhII) derived from Zymomonas mobilis strain ZM4 was implemented into a metabolically engineered bacterium, Vibrio halioticoli, which is a representative marine alginate decomposer. No ethanol from alginate was produced in the wild-type V. halioticoli; however, the engineered V. halioticoli harboring the pdc and adhII operon (Pet operon), designated to the V. halioticoli (Pet), was able to produce 880 mg/L ethanol in maximum from 1.5% alginate for 72 h. The Pet operon also worked on the other marine alginolytic vibrios for ethanol production from alginate. This is the first case of ethanol production from alginate using marine bacterial biocatalysts under seawater-based media.

Comparative Physiology and Genomics of Hydrogen-Producing Vibrios.

The Hyf-type formate hydrogen lyase (FHL) complex was first proposed based on sequence comparisons in Escherichia coli in 1997 (Andrews et al. in Microbiology 143:3633-3647, 1997). The hydrogenase in the Hyf-type FHL was estimated to be a proton-translocating energy-conserving [NiFe]-hydrogenase. Although the structure of FHL is similar to that of complex I, silent gene expression in E. coli has caused delays in unveiling the genetic and biochemical features of the FHL. The entire set of genes required for Hyf-type FHL synthesis has also been found in the genome sequences of Vibrio tritonius in 2015 (Matsumura et al. in Int J Hydrog Energy 40:9137-9146, 2015), which produces more hydrogen (H2) than E. coli. Here we investigate the physiological characteristics, genome comparisons, and gene expressions to elucidate the genetic backgrounds of Hyf-type FHL, and how Hyf-type FHL correlates with the higher H2 production of V. tritonius. Physiological comparisons among the seven H2-producing vibrios reveal that V. porteresiae and V. tritonius, grouped in the Porteresiae clade, show greater capacity for H2 production than the other species. The structures of FHL-Hyp gene clusters were closely related in both Porteresiae species, but differed from those of the other species with the presence of hupE, a possible nickel permease gene. Interestingly, deeper genome comparisons revealed the co-presence of nickel ABC transporter genes (nik) with the Hyf-type FHL gene only on the genome of the Porteresiae clade species. Therefore, active primary Ni transport might be one of the key factors characterizing higher H2 production in V. tritonius. Furthermore, the expression of FHL gene cluster was significantly up-regulated in V. tritonius cells stimulated with formate, indicating that formate is likely to be a control factor for the gene expression of V. tritonius FHL in a similar way to the formate regulon encoding the E. coli FHL.