Subaerial biofilms

The presence of EPS in subaerial biofilms (Fig. 11.1) is also an important prerequisite for inter-organismal interactions and communication. The EPS layer enables other microorganisms to join the biofilm by direct adhesion and might also stimulate growth of accompanying bacteria (Fig. 11.4), as well as promote contact with photobionts (Gorbushina et al., 2005). 

Fig. 11.4. The development of the fungal network on the marble surface (Crimea, Ukraine). As demonstrated by scanning electron microscopy, these hyphae are accompanied by intensive bacterial growth on their surface (A) and thus probably are the pioneer organisms of subaerial biofilms. (B) oxalate crystal formation was observed only when the fungal colonies were cultivated subaerially or in sterile sand. Coverage of the colony with podzol soil blocked the biomineralization process (data not shown) probably by metabolic products of soil microorganisms.

Environmental conditions related to rock surface exposure have profound effects on biofilm development, as metabolic activity and growth are directly connected to the availability of water, energy sources and nutrients, as well as to conditions of temperature and irradiation. Another important factor for establishment of subaerial biofilms is the resistance of the supporting substrate to environmentally and biologically influenced disintegration and dissolution (wear-down). Rapidly weathering rock surfaces (e.g. porous sandstone in an intertidal coastal zone) show little or... 

Fig. 11.5. Many different organisms inhabit rock surfaces and by covering the grains and intergranular spaces form a subaerial biofilm. In the deeper layers phototrophic organisms are situated (A), while the surface is mainly inhabited by stress-tolerant dark-pigmented fungi (B).

Subaerial biofilms and geochemical change in the rocky substrate by organic metabolite secretion... 

The strong survival potential of the rock biofilm indicates that the permanent presence of water is not the most essential attribute for the evolution and spread of life (Reysenbach Cady, 2001 Costerton Stoodley, 2003). The existence of a microbial biofilm on the rock surface is more determined by the interactions of the organisms with the mineral substrate. Lack of water even over several years is tolerated by rock biofilms. However sporadic the supply might be in subaerial conditions, some water from rain, snow, ice, dew, or fog is always present in terrestrial environments. However, for chemoorganotroph life forms the mineral substrate harbours additional difficulties as it is, or almost immediately becomes, deficient in organic matter, as nutrients and energy resources reach this habitat mainly from the atmosphere as particulates and volatile matter. This difficulty is partially overcome by the presence of EPS, which significantly increase the residence time of air-borne particles on any rock surface. 

The Moon was created from the mantle-derived ejecta. Physically, over the aeons this may have played a useful sheltering role in protecting the planet from some meteorite impacts. Arguably more important, the presence of the Moon leads to the tides. These create the intertidal and near-subtidal habitat with rapidly varying geochemical settings, from wet submarine to dry subaerial, in which sediment is repeatedly flushed with fluid. Such cyclically varying habitats may have been vital in the early evolution of microbial biofilms and eventually microbial mats. 

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