Trophic Organization of Microbial Communities

The organization of the prokaryotic community is most clearly demonstrated by the cyano-bacterial mat. Sign (-) denotes here two components: cyanos as prime producers and bacteria as decomposers in regenerative cycle. In the mat, distinct layers are found: the upper illuminated level is occupied by cyanobacteria; below is the white layer of sulfur bacteria, followed by the purple layer of anoxy-genic phototrophs, and then the black layer of sulfide-producing bacteria. Still below are the layers of dead bacteria. The whole system has dimensions of 2-4 mm.

It is called in German 'Streiffarbsandwatt'. The architecture of cyano-bacterial mats is similar in hypersaline lagoons, soda lakes, thermal springs, etc. The structure of mat is maintained by exopolysaccharides produced by cyanobacteria, which are edificators (from 'edifice') for the community. The main factor is illumination and self-shadowing by the upper layers of cyanobacteria, which move to the optimal illumination. Minor differences in the composition of mats are caused, for instance, by the absence of purple bacteria in thermal habitats or strong development of planktonic forms in soda lakes. The 'Winogradsky column' illustrates stratified planktonic microbial community: cylinder with water from the site supplemented by mud with organic debris and gypsum at the bottom. Blooming microbes in the column produce alternating black, purple, and green layers. The column might sustain for years.

Trophic links in the microbial community are organized into the trophic network of a cascade of degradative reactions. The rule is that each step should be sufficient to support the species performing the transformation. Degradation begins with hydrolysis of biopolymers, the most resistant being structural components of the cell walls. Aerobic and anaerobic dissipotrophic bacteria in the cascade of reactions utilize low-molecular-weight compounds, dissipating from the sites of hydrolysis. The final result should be complete decomposition of organic matter, so-called 'mineralization'. In fact, decomposition is not complete and recalcitrant substances are formed in minor part, giving rise to humic substances, and dispersed organic matter. It should be noted that the physical environment strongly contributes to the trapping of undecomposed organic matter, preventing microbial activity.

This is a brief description of biogeochemical cycles catalyzed by bacteria. The main conclusion is that bacteria act as a cooperative community with the interlinked metabolic pathways of the main elements; only the cooperative community is autonomous due to the links between productive and regenerative cycles. Each step of catalysis is performed by a functional or trophic group ofspecialized bacteria. Links provide the trophic network. Cyclic pathways make such a community autonomous, depending mainly on the energy for photosynthesis. Cooperative community is an operational unit for the ecosystem at the landscape level. However, biogeochem-ical cycles are not entirely closed: there is formation of products, which escape recycle. The changes in the community composition known as succession are caused by the accumulation of products as well as exhaust of substrates. In the microbial community, it is the development from fast-growing copiotrophs to a climacteric community with well-balanced interactions. In fact, the microbial community exists all the time in a transitional state.

When we consider a larger temporal scale, the most important concept of biogeochemical succession arises.

It may be illustrated by the composition of the atmosphere, which is formed by microbial activity, since the main components of the atmosphere are metabolized by bacteria: CO2, O2, CH4, CO, H2, N2, NO*, NH3, and sulfur species. Due to the accumulation of the waste products - oxygen is the most evident example - biosphere becomes uncomfortable to the community, here the initial anoxic microbial community. Less evident but more important is accumulation of Corg in sedimentary rocks, which rolls cycles with the passage of time. As a result, the atmosphere moves from the neutral to the oxygenated state. It changed conditions on the Earth's surface. Biosphere overturned: anaerobic pockets remain under the shield of aerobic O2 consumers. Much the same occurred with the ocean (or hydrosphere), which is in equilibrium with the atmosphere. Three main steps seem to be identified: the first before approximately 2.4 billion years with the domination of iron cycle; beginning of the biosphere with O2 in the atmosphere and pronounced S-cycle; and present-day biosphere with O2-atmosphere where biogenic O2 substituted part of CO2. Pathways in the community were reoriented to the new environment. Consider that any sustainable system should be able to support its own existence by effective feedbacks otherwise it is not sustainable. S. Winogradsky in 1896 suggested the qualitative concept of cycle of life as a 'huge organism' (or the goal-oriented system) with microorganisms acting as the main catalysts. Later V. Vernadsky in The Biosphere (1926) introduced the quantitative approach, considering biogeochemical cycles as the main mechanisms. The Geospheric-Biospheric Program and Global Change concept represent the contemporary approach to the problem. The expediency of the links in the biosphere leads to its interpretation as 'Gaia'.

However, biosphere was always within the geographic envelope of the Earth. This means that there was always a mosaic of landscapes arranged in climatic zones. Landscapes give the possibility of lateral interaction and formation of geochemical barriers. The mosaic of landscapes furnishes refugia, places for survival of particular communities. Landscapes on geological timescale are dependent on the tectonic. Weathering-sedimentation pathway leads to equilibrium if no metamorphism and geological cycle occurs.

Since bacteria catalyzed main cycles and established the primary biogeochemical system, they form the dynamic environment, into which Protists, Metaphyta, and Metazoa were incorporated in the course of evolution. The system was developed by the substitution of prime producers by algae, kelps, and plants. The terrestrial system changed with the appearance, about 300 Ma ago, of vascular plants, which changed the atmospheric hydrological cycle by the involvement of deeper layers of ground water and producing a new illuminated surface within the leaves for derivates of cyanobacteria converted into chloroplasts. Plant cover significantly changed the terrestrial environment. However, microbes remain as the main catalysts in the system of biogeochemical cycles.

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