Amino Acids in the Root Exudates of Agave lechuguilla Torr. Favor the Recruitment and Enzymatic Activity of Nutrient-Improvement Rhizobacteria.

Agave lechuguilla is a widely distributed plant in arid ecosystems. It has been suggested that its microbiome is partially responsible for its great adaptability to the oligotrophic environments of the Chihuahuan Desert. To lead the recruitment of beneficial rhizobacteria, the root exudates are essential; however, the amino acids contained within these compounds had been largely overlooked. Thus, we investigated how the variations of amino acids in the rhizosphere at different growth stages of A. lechuguilla affect the rhizobacterial community composition, its functions, and activity of the beneficial bacteria. In this regard, it was found that arginine and tyrosine were related to the composition of the rhizobacterial community associated to A. lechuguilla, where the most abundant genera were from the phylum Proteobacteria and Bacteroidetes. Moreover, Firmicutes was largely represented by Bacillus in the phosphorus-mineralizing bacteria community, which may indicate its great distribution and versatility in the harsh environments of the Chihuahuan Desert. In contrast, we found a high proportion of Unknown taxa of nitrogen-fixing bacteria, reflecting the enormous diversity in the rhizosphere of these types of plants that remains to be explored. This work also reports the influence of micronutrients and the amino acids methionine and arginine over the increased activity of the nitrogen-fixing and phosphorus-mineralizing bacteria in the rhizosphere of lechuguillas. In addition, the results highlight the multiple beneficial functions present in the microbiome that could help the host to tolerate arid conditions and improve nutrient availability.

Hydrogen metabolic patterns driven by Clostridium-Streptococcus community shifts in a continuous stirred tank reactor.

The hydrogen (H2) production efficiency in dark fermentation systems is strongly dependent on the occurrence of metabolic pathways derived from the selection of microbial species that either consume molecular H2 or outcompete hydrogenogenic bacteria for the organic substrate. In this study, the effect of organic loading rate (OLR) on the H2 production performance, the metabolic pathways, and the microbial community composition in a continuous system was evaluated. Two bacterial genera, Clostridium and Streptococcus, were dominant in the microbial community depending on the OLR applied. At low OLR (14.7-44.1 gLactose/L-d), Clostridium sp. was dominant and directed the system towards the acetate-butyrate fermentation pathway, with a maximum H2 yield of 2.14 molH2/molHexose obtained at 29.4 gLactose/L-d. Under such conditions, the volumetric hydrogen production rate (VHPR) was between 3.2 and 11.6 LH2/L-d. In contrast, relatively high OLR (58.8 and 88.2 gLactose/L-d) favored the dominance of Streptococcus sp. as co-dominant microorganism leading to lactate production. Under these conditions, the formate production was also stimulated serving as a strategy to dispose the surplus of reduced molecules (e.g., NADH2+), which theoretically consumed up to 5.72 LH2/L-d. In such scenario, the VHPR was enhanced (13.7-14.5 LH2/L-d) but the H2 yield dropped to a minimum of 0.74 molH2/molHexose at OLR = 58.8 gLactose/L-d. Overall, this research brings clear evidence of the intrinsic occurrence of metabolic pathways detrimental for biohydrogen production, i.e., lactic acid fermentation and formate production, suggesting the use of low OLR as a strategy to control them.