Engineering bacteria-seaweed symbioses for modulating the photosynthate content of Ulva (Chlorophyta): Significant for the feedstock of bioethanol production

Seaweed biomass cultivation predicates the quantity and quality of this biorefinery feedstock. Unfortunately, the seaweed growth rate and chemical content are hardly predictable and are affected by environmental factors, including epiphytic bacteria. We hypothesize that microbiome engineering can control the chemical composition of Ulva biomass. We show that the engineered Maribacter sp. and Roseovarius sp. consortium modulate Ulva mutabilis growth rate and photosynthate content of constituents relevant for bioethanol production. Although minimal growth was observed in the axenic cultures (0.04 mm day⁻¹), Ulva mutabilis in a tripartite community showed a growth rate of 3.79 mm day⁻¹ in the growth phase. Furthermore, the content of glucose and glycerol in Ulva of the engineered community increased by 77 ± 19% and 460 ± 207% whereas xylose and glucuronic acid decreased by 37 ± 14% and 46 ± 15% in comparison to axenic culture. Interestingly, bacterial addition affected the rhamnose/xylose/glucuronic acid ratio (1.96:1:1: vs 1.34:0.85:1 in xenic vs axenic culture), indicating the impact of bacteria on ulvan synthesis. In addition, tyrosine and histidine increased by 191 ± 61% and 40 ± 26%; however, valine, isoleucine, aspartate, threonine, serine, and phenylalanine decreased by 22 ± 19% - 42 ± 23%. Flux-balance analysis of Saccharomyces cerevisiae, Escherichia coli, and Clostridium acetobutylicum was used to estimate the bioethanol yield from hydrolyzed Ulva biomass, in a one-step or two-step fermentation process. Simulation using S. cerevisiae (RN1016) with xylose isomerase resulted in a bioethanol yield of 85.62 for xenic vs. 71.31 mg/g dry weight (DW) axenic cultures of Ulva. The increased growth rate and the relative amounts of photosynthates of U. mutabilis are modulated by the engineered microbiome. Moreover, it results in biomass with a higher potential for bioethanol fermentation in comparison to axenic cultures.