11:45 AM - 12:00 PM
[AAS12-23] Contribution of plant-associated microorganisms as global sinks of atmospheric hydrogen
Keywords:biogeochemistry, microbial ecology, tropospheric H2 cycle, vegetation
We first investigated the presence of hhyL gene in various plant species. The hhyL gene, which encodes for the large subunit of the novel group of hydrogenase, has been generally used as a functional biomarker to evaluate the distribution, taxonomic diversity, and abundance of high-affinity H2-oxidizing bacteria. In total, 42 hhyL gene sequences were successfully detected in all tested herbaceous plants, indicating a wide distribution of high-affinity H2-oxidizing bacteria in plants. It is noteworthy that the abundance levels of hhyL gene detected in plants were comparable to those detected in soil. High-affinity H2-oxidizing bacteria were isolated from inside herbaceous plant tissues. Among 145 isolates, 7 Streptomyces strains were shown to possess hhyL gene. The H2 uptake activity was evaluated by gas chromatography. All the isolates reduced H2 concentration to less than 0.530 ppmv, demonstrating the ability to consume H2 at ambient level. Sterile plant seedlings were inoculated with selected isolates to verify their ability to penetrate and disseminate in plant tissues and scavenge atmospheric H2 in plant. After four weeks of seedling inoculation, an internalization of the bacteria in plant tissues was visualized by fluorescence in situ hybridization imaging. H2 oxidation rates measured in plant fractions ranged from 1079 to 3472 pmol g(dw)-1 h-1. These rates are comparable to the previously observed activity of atmospheric tritium uptake in other plants. Importantly, atmospheric H2 is not oxidized in aseptically grown plants, clearly showing that plant-associated bacteria was responsible for H2 loss. H2 uptake activity per bacterial cell was comparable between plant and soil, demonstrating that both environments are favorable for the microbial-mediated H2 uptake.
In conclusion, this study demonstrated the occurrence of plant-associated high-affinity H2-oxidizing bacteria and their ability to consume atmospheric H2 on plant surface or inside plant tissues. From a global perspective, herbaceous and woody plant biomass represent approximately 64 Pg, and 736 Pg, respectively. Considering that high-affinity H2-oxidizing bacteria may be present and active in these plants, the contribution of plant-associated bacteria deserves more attention to better understand the global cycling of atmospheric H2.