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
Hydrogen (H2) is an important constituent of the atmosphere, with a typical mixing ratio of 0.530 parts per million by volume (ppmv). Rising H2 emissions under a future H2-based economy are concerned to increase the atmospheric burden of H2, resulting to the indirect influence of the lifetime of greenhouse gas CH4, an alteration of temperature and ozone loss in the stratosphere. Thus, mitigation of H2 emission is of critical importance for atmospheric chemistry. The most part (~80%) of tropospheric H2 is consumed by microorganisms in soil. A recent literature survey of H2 flux measurements unveiled that soil H2 uptake is responsible for the loss of 40 to 90 Tg yr-1. Recently, high-affinity H2-oxidizing bacteria possessing novel hydrogenase have been found as important contributors to the soil H2 uptake. Although previous experiments using molecular tritium reported the occurrence of significant H2 uptake activity in vegetation, there has been no report on the identification and diversity of the responsible microorganisms. This study aimed to verify the existence of plant-associated bacteria possessing the ability to consume atmospheric H2.
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.
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.