Diversity, Heat Adaptation And Relationships Among Culturable Fungi In The Eryuan Geothermal Ecosystem,Southwest China

Terrestrial hot spring is one type of extreme environment with a high temperature, low/high pH, high concentration of acid/alkali compounds and sparse vegetation. However, as a kind of stable environment, it supported rich extremophile communities and provided a unique opportunity to study the origin and evolution of organisms and their interactions with each other and the environment. To date, the bacteria and archaea in hot spring have been characterized extensively. In contrast, there is still no available report addressing the diversity of endophytic fungi and their ecological roles in geothermal ecosystems. In the present study, the diversity and heat adaptation of endophytic fungi in the six dominant plant species growing next to hot spring and fungi in the associated rhizospheric soil as well as fungi in hot spring were investigated, and the phylogenetic relationships among these fungi were studied, too. The results would provide a clear insight into the fungal diversity in the geothermal ecosystem and also may contribute to the understanding of the ecological roles and the evolution of fungi in the extreme environment.1. The endophytic fungal diversity of six dominant plant species (Hedyotis diffusa, Trifolium repens, Digitaria ischaemum. Silene tenuis, Cynodon dactylon, and Alternanthera philoxeroides) collected from the Eryuan geothermal ecosystem were investigated. A total of 627 culturable endophytic fungi were isolated from 960 tissue segments. The colonisation rate of endophytic fungi in the six plant species ranged from 43.75% to 75.63%, and the mean colonisation rate of endophytic fungi in the roots (76.04%) was significantly higher than that in the steams (46.04%) (X-2=128.14, P<0.001, chi-squared test). These results suggested that endophytic fungi colonized in the plants widely.2. Based on the morphological characteristics and the ITS sequence analysis, the isolates were identified to 30 taxa, including Penicillium, Fusarium, Aspergillus, and so on, of which Curvularia, Acrophialophora, Penicillium, Fusarium, Aspergillus, Dothiorella, Gloeosporium, and Phialophora were the dominant genera, and the relative frequencies were 14.35%,14.04%,11.80%,8.29%, 8.13%,7.66%,6.54%. and 6.54%, respectively. The Shannon Indices of the endophytic fungi from the six plant species ranged from 1.80 to 2.56. These results suggested that the endophytes from the Eryuan geothermal ecosystem have high diversity.3. A total of 962 fungi were isolated from the rhizospheric soils associated with the six plant species. The isolates were identified to 15 taxa, of which Curvularia, Gloeosporium, Acrophialophora, Penicillium, Gibberella, and Aspergillus were the dominant genera, and their relative frequencies were 21.83%,14.76%, 14.55%,13.83%,11.95%, and 11.54%, respectively. The Shannon Index and fungal CFUs of the fungi from the rhizospheric soils associated with the six plant species were 0.73-2.11 and 11.27-39.17 CFU/g, respectively. The fungal diversity showed a decreasing trend with the increasing soil temperature. The composition of fungi in rhizospheric soil showed a significant difference among different plant species, which indicated that the plant species may have an effect on the composition of fungi in rhizospheric soil.4. Only two fungal strains were isolated from the six hot spring water samples and they were identified as Penicillium sp. and Aspergillus fumigatus. The numbers of fungal colony forming units in the six water samples ranged from 0 to 0.33 CFU/ml. The reason for the lower number and diversity of fungi in the water samples may be because of the higher temperature, lower nutrients, and higher concentration of acidic/alkaline compounds in hot spring water; those factors are disadvantageous for fungal survival.5. The heat adaptation of dominant fungal genera from hot spring, rhizospheric soil, and plant was investigated. The results showed that the tested isolates could be divided into three groups based on their growth temperatures:0.74% were thermophilic fungi,59.78% were thermotolerant fungi, and 39.48% were mesophilic fungi. All of the thermophilic fungi were isolated from hot spring water, while 67.90% of the thermotolerant fungi came from rhizospheric soils. 29.01% of the thermotolerant fungi from plant roots, and only 3.09% of the thermotolerant fungi came from plant stems. The results suggested that temperature may be one of the most important environmental factors shaping the fungal heat adaptation in the geothermal ecosystem, as the number of heat-adapted/thermophilic fungi increased with increasing environmental temperatures.6. A total of 43 strains from 33 morphological taxa were further subjected to phylogenetic analysis. The result showed that more than three fourths of the isolates (n=35,81.40%) showed 97-99% similarity with their closest relatives. Furthermore, it was found that most of the strains from different origins showed a close relationship with each other. Almost all of the taxa found in soils co-existed in the plants, and the dominant genera in the rhizospheric soils and hot springs were also the dominant genera in the plants. The results suggested that the fungi in the geothermal ecosystem may have the same origin and some of them have adapted to the extreme conditions and become the dominant and endemic species in the area.