Dear Colleagues,

Microorganisms inhabit almost all locations on Earth, including the deep subsurface. Although the prerequisites for life, such as the presence of water, nutrients and energy sources, and space, can be found from the deep subsurface, it also presents challenges for the microbial life. The lack of oxygen and light, the depth related increasing pressure and heat, and the scarcity of substrates set demands on the microbial metabolic activities in the deep subsurface, and may force the microorganisms to collaborate for synergistic benefits. So far, the subsurface microbial communities have been shown to possess a great variety of different metabolic properties, ranging from chemolithoautotrophy to fermentation and hydrocarbon degradation. At the same time, the deep subsurface microorganisms may have smaller genomes compared to surface microorganisms. Many novel microbial taxa, such as the Woesearchaea or Parcubacteria, that lack laboratory-grown representatives, have been shown by metagenomic sequencing methods to have genomes lacking even the fundamental genes for a self-sufficient lifestyle. This indicates that these groups are either parasites or symbionts. This may be a general feature for oligotrophic environments, as these microbacteria and -archaea are abundant in the deep subsurface, and this lifestyle may be especially useful in deep environments by bringing a greater metabolic repertoire for the host organism. Nevertheless, the deep subsurface has also been shown to sustain heterotrophic prokaryotic and eukaryotic microorganisms, those able to recycle organic matter for the benefit of the rest of the community. It was recently shown that certain fungi and sulphate reducing bacteria form synergistic relationships in the deep subsurface where the SRB benefit from the metabolic activities of the fungi, and vice versa. In addition, the relationship between autotrophic and heterotrophic microorganisms in deep subsurface environments and the effects of secreted metabolites in these environments are still largely unknown.

For this special issue we invite research papers, short communications and reviews covering microbial metabolisms and/or microbial interactions and networks in deep subsurface environments. We hope to learn more about how e.g. carbon and nitrogen flow between different microbial groups and how one microbe’s waste may be another microbe’s food. Can ancient carbon or carbonate in fracture fillings in rocks be used as carbon source? How do viruses or fungi contribute to the nutrient cycling in the deep subsurface? Does the high microbial diversity seen in many oligotrophic deep subsurface environment also mean a high, albeit dormant, metabolic diversity that can be put to use when environmental parameters change, e.g. due to tectonic activities? How does the phosphorus, sulphur or iron cycles work in the deep subsurface? The deep biosphere still presents many open questions on issues, which ultimately may have influence on global concerns, such as climate change scenarios, and may also present new theories for the possibility of life on other planetary bodies.

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• Deep biosphere
• Metabolic networks
• Geobiology
• Nutrient cycling
• Autotroph
• Lithotroph
• Anaerobic community
• Heterotroph

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