RESUMO
Diverse microorganisms specifically inhabit extreme environments, such as hot springs and deep-sea hydrothermal vents. To test the hypothesis that the microbial community structure is predictable based on environmental factors characteristic of such extreme environments, we conducted correlation analyses of microbial taxa/functions and environmental factors using metagenomic and 61 types of physicochemical data of water samples from nine hot springs in the Kirishima area (Kyusyu, Japan), where hot springs with diverse chemical properties are distributed in a relatively narrow area. Our metagenomic analysis revealed that the samples can be classified into two major types dominated by either phylum Crenarchaeota or phylum Aquificae. The correlation analysis showed that Crenarchaeota dominated in nutrient-rich environments with high concentrations of ions and total carbons, whereas Aquificae dominated in nutrient-poor environments with low ion concentrations. These environmental factors were also important explanatory variables in the generalized linear models constructed to predict the abundances of Crenarchaeota or Aquificae. Functional enrichment analysis of genes also revealed that the separation of the two major types is primarily attributable to genes involved in autotrophic carbon fixation, sulfate metabolism and nitrate reduction. Our results suggested that Aquificae and Crenarchaeota play a vital role in the Kirishima hot spring water ecosystem through their metabolic pathways adapted to each environment. Our findings provide a basis to predict microbial community structures in hot springs from environmental parameters, and also provide clues for the exploration of biological resources in extreme environments.
RESUMO
The surface of Hadean Earth was mainly covered with three types of rocks-komatiite, KREEP basalt and anorthosite-which were remarkably different from those on the modern Earth. The water-rock interaction between these rocks and water provided a highly reducing environment and formed secondary minerals on the surface of the rocks that are important for producing metallo-enzymes for the emergence of primordial life. Previous studies suggested a correlation between the active site of metallo-enzymes and sulfide minerals based on the affinity of their structures, but they did not discuss the origin of metallic elements contained in these minerals which is critical to understanding where life began. We investigated secondary minerals formed through water-rock interactions of komatiite in a subaerial geyser system, then discussed the relationship between the active site of metallo-enzymes and secondary minerals. Instead of komatiite, we used serpentinite collected from the Hakuba Happo area, Nagano Prefecture in central-north Japan, which is thought to be a modern analog for the Hadean environment. We found several minor minerals, such as magnetite, chromite, pyrite and pentlandite in addition to serpentine minerals. Pentlandite has not been mentioned in previous studies as one of the candidates that could supply important metallic elements to build metallo-enzymes. It has been shown to be a catalyst for hydrogen generation possibly, because of structural similarity to the active site of hydrogenases. We consider the possibility that nickel-iron sulfide, pentlandite, could be important minerals for the origin of life. In addition, we estimated what kinds of minor minerals would be obtained from the water-rock interaction of these rocks using thermodynamic calculations. KREEP basalt contains a large amount of iron and it could be useful for producing metallo-enzymes, especially ferredoxins-electron transfer enzymes, which may have assisted in the emergence of life.
