Today, the earth's cryosphere, especially habitats such as ice cores and subglacial lakes, is somewhat similar to the earth in the early days of the origin of life; The life process and characteristics of cryosphere microorganisms may provide some enlightenment for us to understand the origin of life and the low temperature adaptation limit of life. The cryosphere is considered to be an environment similar to Mars, Europa, Ganymede, Callisto and Callisto, and the clues of microorganisms in the cryosphere are expected to provide enlightenment for exploring extraterrestrial life.
General situation of research on cryosphere microorganisms
The study of cryosphere microorganisms began with human exploration and scientific investigation of polar regions and mountains. Up to now, it has been carried out for more than 0/00 years, and it has gone through three development stages: microbial morphology research, microbial physiology research and microbial genomics research. The research scope of cryosphere microorganisms covers the main elements of cryosphere-frozen soil, glaciers, snow and sea ice. The researchers found and separated different microbial groups from various elements in the cryosphere, including archaea, bacteria and fungi. See table 1 for the latest authoritative analysis data of the number and total amount of microbial cells in various habitats in the cryosphere.
Table 1 number of microbial cells in cryosphere habitat
Bacteria and fungi in frozen soil
Permafrost accounts for 25% of the earth's land area, and it is an important habitat for microorganisms in the earth's cryosphere, mainly distributed in Antarctica, Arctic and mountainous areas. For a long time, the study of frozen soil microorganisms has attracted much attention. A large number of studies show that there are various microorganisms in frozen soil with a density of about105-108 cells g-1. Because the depth of frozen soil is divided by age, bacteria of different ages can be separated from it. For example, the living microorganism of 1× 107 cell g- 1 was detected in the frozen soil in the northeast of northwest Asia, and 65438 was detected in the arctic frozen soil in1-3 million years.
In frozen soil, fungi mainly exist in the form of spores, and the number is much lower than that of prokaryotic microbial cells. Among them, the number of culturable yeasts in Arctic permafrost is103—104 cfu g-1,belonging to Cryptococcus, Rhodotorula, Saccharomyces and Sporozoa. Geotrichum, Cladosporium, Penicillium and Aspergillus of Ascomycetes are the most common filamentous fungi groups. In contrast, the number of culturable yeasts and filamentous fungi in Antarctic frozen soil is often very low, and Cryptococcus and Moraxella are the main fungal groups. Diatoms are the main group of fungi in sea ice. More than 550 species of diatoms have been identified in the Arctic region alone, including 446 species of pinnate diatoms and 0/22 species of columnar diatoms.
Bacteria and fungi in ice and snow
Because snow is easy to sample, the researchers conducted in-depth microbial analysis. The analysis of microorganisms in Antarctic, Arctic and alpine snow shows that the density of microbial cells in seasonal snow is 102- 105 cells mL- 1, but there are few new types of microorganisms. This is mainly because the microorganisms in snow come from soil indirectly, and the atmospheric transport determines the source and species of microorganisms in snow.
The cryosphere ice consists of glaciers, polar ice shelves and sea ice. At first, people thought that there were no microorganisms in polar ice, but a lot of analysis showed that microorganisms grew and multiplied in all kinds of ice, and their species and cell numbers decreased with the increase of ice depth. For example, the density of microbial cells in surface ice is 104- 108 cell ml- 1, while the density in glaciers is reduced to10/-103 cell ml- 1. Microorganisms in ice cores mainly come from land dust, marine aerosol and volcanic ash, and their abundance is related to the annual snowfall. The more snowfall, the more microbial cells in the ice core. 180 cfu ml- 1 bacterial cells were detected in the ice core of Gu Liya ice sheet in Asia, about 200 years ago. 0- 10 cfu ml- 1 bacterial cells were detected in the ice core of Taylor dome, Antarctica, and the age was about 1800 years. The Greenland Ice Sheet Project (GISP2) drilled an ice core with a depth of 3 042.80 m, and it was detected that the melt water of the ice core contained 6.1×107 cells mL- 1 microbial cells. Low-temperature ice caves are micro-habitats formed by local melting of ice on ice shelves and glaciers, and their microbial communities are unique-their relatively high abundance and activity have an important impact on glacier melting and carbon cycle on glaciers. Although the salinity of sea ice is high, the microbial cell density is102-105 cells mL- 1. Subglacial lakes that have been sealed under the Antarctic and Arctic ice sheets for hundreds of thousands of years are unique habitats in the cryosphere, and microbial cells with the density of102-105 cells ml-1have also been detected. In recent years, many kinds of fungi have been isolated from ice and snow. Among them, 3×103—/kloc-0 /×104cfu ml-1yeast cells were detected in glacier ice core of Svalbard Island, and many cold-loving and cold-loving yeasts were isolated from glacier ice in Alps.
Cryosphere virus
The cryosphere habitat has low nutritional level and short biological chain. Therefore, the virus has an important impact on the cryosphere ecosystem and material circulation. Viruses control the diversity and richness of bacteria and fungi by cracking host cells, and can release organic matter into the environment. Through horizontal gene transfer, the evolution and evolution of host bacteria are affected. In recent years, with the maturity of virus metagenome technology, the research on cryosphere virus has become a hot spot, especially in glaciers, ice shelves, lakes, soils and ice in the South Arctic Sea. Up to now, viruses with relatively high abundance and diversity have been detected in various habitats in Antarctica and the Arctic, including DNA viruses such as phage, circular single-stranded DNA virus, double-stranded DNA virus, algae virus and algae-infected virus phage, and RNA viruses such as picornavirus. DNA viruses belonging to the family Parvoviridae were identified in the Taylor Valley Ice Cave, Antarctica. Computer analysis estimates that there are about/10 000 viruses in Antarctic glacial lakes, which is much higher than about 800 viruses in North American lakes. The density of virus-like particles in Antarctic and Arctic sea ice reached105-108ml-1; The average abundance of virus-like particles in Antarctic sea ice core samples is10.9×105 ml-1,and the average virus/bacteria ratio is 5.3. The average ratio of virus to bacteria is an index to understand the abundance of cryosphere virus and its relationship with host. Although this data varies greatly in different habitats, the collected data show that the virus-bacteria ratio is very high in the ice and caves in the Antarctic and Arctic (Table 2).
Table 2 Virus-bacteria ratio of cryosphere habitat
In recent years, with the expansion of related research scope, the understanding of cryosphere virus has been deepened. By analyzing the genomes of related viruses in Gu Liya glacier ice core of Qinghai-Tibet Plateau 520 years ago and 654.38+500,000 years ago, 33 different virus genetic information were found, which can be divided into 4 known genera and 28 unknown genera. The results of gene prediction analysis showed that 18 virus species were closely related to the number of various bacteria in the ice core, indicating that the virus hosts in the ice core were diverse. The giant virus isolated from Siberian frozen soil 30 thousand years ago still has the activity of infecting the target host. A large number of viruses have also been found in the frozen soil of Alaska, mainly distributed in unfrozen water of frozen soil.
Microbial resources in cryosphere
New microorganisms in the cryosphere
The evolution of cryosphere microorganisms is influenced by the uniqueness of their environment, and the species and groups of microorganisms with strong adaptability gradually become the dominant groups in all cryosphere components. After thousands or even millions of years of evolution, a stable microbial community and ecosystem have been formed. In the past century, researchers have isolated a large number of new microbial species or groups from the cryosphere, including new archaea, bacteria and fungi. With the development of molecular biology technology, the isolation and identification of new species of cryosphere microorganisms are also accelerating, and more and more new species will be reported. In the future, the task we face is to isolate and preserve the new microorganisms in the frozen zone as much as possible before they disappear, so as to preserve microbial resources for further research and utilization.
The extreme survival conditions of cryosphere not only shape microbial groups, but also change the metabolic pathway of microorganisms; In the process of evolution, this change has continuously increased the environmental adaptability of microorganisms. The new metabolites of microorganisms provide the possibility for human beings to obtain new bioactive compounds, such as antibiotics. For example, in recent years, several new strains of Streptomyces have been isolated from the frozen soil of the Qinghai-Tibet Plateau, which have the activity of resisting MRSA (Method-resistant Staphylococcus aureus). The separation and analysis of these secondary metabolites of Streptomyces showed that some new compounds had the application prospect as antibiotics. At present, this research is in progress.
Cryophilic microorganisms in cryosphere
The low temperature environment of the cryosphere has chosen the cold adaptation evolution of microorganisms living in it-psychrophilic microorganisms and psychrophilic microorganisms that have been separated and studied from the cryosphere. Among them, the psychrophilic bacteria studied in depth are Colwellia psychrerythraea34H isolated from the sediments of the Arctic Ocean, Psychromonas ingrahamii isolated from the Arctic sea ice, Burton II DSM 6242 isolated from the Arctic lake ice, halophilic frozen staphylococci in the Arctic frozen soil, Arthur sp. Tad20 in the Antarctic soil and Arthur's psychrophilic bacteria F2, etc. Many cold-tolerant molds and yeasts have also been isolated from the cryosphere habitat.
The isolation, cultivation, research and utilization of psychrophilic microorganisms provide important strain resources for the development of cryosphere microbial resources. The study of psychrophilic microorganisms makes us deeply understand the various mechanisms of microbial adaptation to cold environment; It also provides us with a variety of low-temperature bacteria, low-temperature enzymes and low-temperature proteins. These strains, enzymes and proteins have been applied in various fields such as industry, agriculture, medical care and environmental protection, and have achieved great economic and social benefits. Some cold-adapted microorganisms also show good prospects in bioenergy. Like Chlamydomonas. ICE-L isolated from Antarctic ice accumulated more lipids at 0℃ and 5℃ than at 15℃, reaching 84 μ l L-1at 6℃. Antarctic cold-tolerant yeast Mrakia blollopsis SK-4 can efficiently convert lignocellulose into ethanol at 10℃. The combination of these psychrophilic microorganisms and cold adaptation related genes with biotechnology is promoting the birth of a new biotechnology industry.
Genetic resources of cryosphere
Although the cryosphere may contain a large number of microbial species with important application value, only a small proportion of microorganisms can be isolated and cultivated in the laboratory, which greatly limits the research and utilization of microorganisms in the cryosphere. Metagenomics technology provides us with a brand-new strategy: by comprehensively measuring the DNA sequence information of microorganisms in the environment and assembling the genomes and genes in environmental samples, we can not only reveal the systematic and metabolic diversity of microorganisms and their environmental adaptability, but also identify related functional genes, and then deeply analyze and express the gene functions in different sources to obtain the corresponding protein. For example, metagenomics research revealed that there were cold adaptation-related genes in Noefer's glacier ice, including genes related to antifreeze and polyunsaturated fatty acid synthesis.
Microbiological challenges caused by cryosphere degradation
Global warming is changing the cryosphere ecosystem.
Nowadays, one of the biggest challenges facing mankind is global warming. The cryosphere is the most sensitive layer to global change on the earth; Global warming leads to the rapid reduction of cryosphere, including glacier retreat and frozen soil melting. However, people still don't know what impact the retreat of cryosphere will have on microbial biodiversity and downstream ecosystems and their biodiversity. Frozen soil, which accounts for about14 of the earth's land area, is an important carbon pool of the earth. Global temperature rise will accelerate microbial transformation of organic carbon and nutrients frozen in frozen soil, leading to the release of greenhouse gases CO2, CH4 and N2O. The retreat of glaciers, which account for about 10% of the earth's land area, will directly lead to the release of inorganic substances and organic substances (including pollutants) stored in them for a long time. The release of various nutrients buried in ice will affect the downstream water system and terrestrial ecosystem and their biodiversity.
The degradation of cryosphere accelerates the disappearance of microbial habitat in cryosphere.
Long-term low temperature has selected cryosphere microorganisms, which makes cryosphere microorganisms unique. Most species of microorganisms in the cryosphere can only live in the cryosphere habitat, and the melting of the cryosphere will have a disastrous impact on microorganisms, and endemic species will disappear from now on. A study shows that about 6%- 1 1% of all biological groups responding to glacier retreat become losers, while 19%-26% become winners. Most of the losers are unique groups of glacier habitats, and some of them can only survive in the glacier ecosystem. The winners of the reaction are widespread or invasive groups, which can usually settle in the downstream habitat of glaciers. In frozen soil, there are microorganisms stored in it for thousands or even millions of years in a dormant state, and a large number of them are unknown species. The melting of frozen soil will lead to the change of its habitat, so that these unknown microbial species will disappear before they are known.
Cryosphere ablation releases unknown microorganisms
As mentioned above, there are many unknown microbial groups in the cryosphere, and these new unknown microorganisms will inevitably have a great impact on the downstream ecosystem and even human society. Whether there are new pathogenic microorganisms in the microorganisms sealed in the cryosphere for a long time, including pathogenic microorganisms of animals, plants and humans; Especially the cryosphere virus, what impact it will bring after its release is a severe challenge. Obviously, there is still a lack of research on this issue. The research on cryosphere virus shows that about 3. 15× 102 1 bacterial and archaea cells are released from the Arctic glacier ice into the downstream environment every year. According to the average virus/bacteria ratio in the glacier of 30∶ 1, 1023 viruses will be released from the Arctic glacier into the downstream environment every year. In this process, bacteria and viruses trapped in the cryosphere for tens of thousands to hundreds of thousands of years will be directly released into the environment, which will have potential harm to human survival. In 20 16, anthrax broke out in Siberia, resulting in the death of more than 2,000 reindeer and the hospitalization of 96 people. Related research shows that the epidemic was caused by the thawing of permafrost, which resulted in the thawing of a deer carcass infected with anthrax spores. In addition, related research revived a giant virus with a history of 30,000 years from Siberian permafrost, and found that this virus can still infect its target-single-celled amoeba. With the intensification of global warming, the process of releasing unknown microorganisms from the cryosphere will be accelerated, and more viruses will enter the downstream oceans and rivers with the melting water of polar and alpine glaciers. Such a huge amount of virus particles will spread and survive in the new ecosystem and may infect completely different hosts, which will have a great impact on the new host ecosystem.
prospect
Strengthen the research on microbial ecology of cryosphere under the background of global change. At present, our research on cryosphere microorganisms is mainly limited to laboratory research, especially the research on cryosphere microbial ecology. There are few studies on predicting the dynamics of microbial communities and the response of their functions to global changes by using mathematical ecological models, so strengthening the integration of multidisciplinary theories and experimental research is the direction that needs to be improved in the future. Climate change threatens the microbial diversity in the cryosphere, and it is urgent to understand the relationship between cryosphere biodiversity and climate and function. How will the function and behavior of cold-adapted microorganisms related to climate warming (such as glacier retreat and frozen soil melting) respond to climate change? Future research needs a multidisciplinary perspective, and comprehensively examines the interaction and feedback among chemical, physical, biological and environmental factors in time and space dimensions.
The study of cryosphere virus is another field that has been paid more and more attention. At present, the scientific research data about cryosphere virus community and its adaptation to extreme environment are still limited. Through the analysis of virus metagenome, the biological function of virus gene coding was identified, and the relationship between it and virus adaptability to extreme environment was understood, thus revealing the role of virus in cryosphere.
It is urgent to strengthen the research on cryosphere microbial culture in order to obtain and preserve as many culturable microbial resources as possible for subsequent research and utilization. It is also an important task in the future to deeply understand the potential pathogenic microorganisms of animals and plants in various cryosphere habitats, especially the old and undiscovered new pathogenic microorganisms sealed in them.
(Author: Chen Tuo, researcher and doctoral supervisor of Northwest Institute of Ecological Environment and Resources, China Academy of Sciences; Li Weng, Key Laboratory of Extreme Environmental Microbial Resources and Engineering in Gansu Province, Northwest Institute of Ecology and Environmental Resources, China Academy of Sciences. Contributed by Journal of China Academy of Sciences)