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The deep-sea environment, despite being rich in mineral nutrients and near-saturation levels of oxygen, because of its darkness is not conducive to photosynthesis, the process that sustains life on earth. If a liigh abundance and diversity of life yet persist in this ecosystem, it is because of the'transport of organic matter formed by photosynthesis several hundreds to thousands of meters above, which sinks to it. The exception is the hydrolhermal vents. Organic carbon of photosynthetic primary producers in the euphoric zone, as a waste product of such organisms, rains down to the deep sea in what is described as the 'biological pump' that transports such organic matter to the depths (30). A substantial portion of the atmospheric CO sub(2) is sequestrated into the deep sea by this process. The deep sea is the largest sink for organic carbon on the earth (84). The universal process of recycling such organic matter depends on microorganisms, which use part of the carbon and minerals for their biomass and release the rest as carbon dioxide and minerals through chemoheterotrophic processes. Bacteria and fungi are the major groups that are capable of accomplishing this. Several studies have shown intense bacterial activities in deep-sea sediments (93). Total bacterial numbers and their biomass from several sites in the world oceans have been estimated (20). In contrasl, almost no information exists in relation to fungi in the deep sea. This is in contrast to terrestrial ecosystems, where fungi are recognized to be major players in decomposition and remineralization of organic matter (54, 81). Studies from forest and prairie soils have shown that fungi have lower rates of remineralization but higher C assimilation efficiency than bacteria and thus may be more significant in terms of carbon sequestration (5, 8). In light of this, the diversity, abundance, and role of fungi in deep-sea sediments may form an important link in the global C biogeochemistry. This review focuses on issues related to collection and isolation of deep-sea fungi, direct detection in deep-sea sediments, diversity and biomass, growth and physiology, adaptations, and their biotechnological applications.
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