Bacteria are ubiquitous and diverse; they are integral biotic component of every ecosystem- let it be aquatic, marine or terrestrial. Depending upon availability of nutrients, favorable/tolerable environmental conditions and specific metabolic properties every species of bacterial community occupy specific space, what is called as niche. Root nodule, root surface, soil matrix, clay particles, endophytic tissue, root epidermis, stem tumors etc. can be niches of soil bacteria. Niches together make a habitat which can be an animal body, rhizosphere, and soil or water body. A species within a niche is engaged in interactions with external environment constantly for various purposes. These individual niche interactions are actually a basis of Ecology. This hierarchy is similar to World-Country-States-People (Ecology-Ecosystem-habitat-bacterial niche).
Simplicity and complexity of bacterial niche:
Niche is very special space occupied by a bacterium and no other bacterium even of a identical species and alike metabolic property can share it. Every niche has its defined boundary and specific measurements like temperature and pH. As the complexity of the interactions within a species and outside the species increases so increases the complexity of the niches and we reveal more diverse ecological phenomena. For example, Antarctic soil harbors bacteria that can tolerate freezing temperatures of less than 00C. So the Antarctic soil having this temperature range contain cryophilic bacteria of particular species which can perform metabolic activities within this temperature range is said to be niche. Beyond this niche other type of species can be found with different metabolic activities, thus representing another niche in that type of soil. Bacteria from a particular niche interact with bacteria from other niches or other biotic and abiotic components forming web or network of interaction making it very dynamic and diverse. Complexity of soil niches is further complexed by stress conditions of environment, such as salinity, drought, wetness, and scarcity of nutrients, contamination by heavy metals or hydrocarbons. These stress factors induce structural and functional changes in individual bacterium with which it carries out time dependent adaptation to the surrounding environment. Stress dependent structural and functional changes consist of exopolysaccharide or capsule synthesis, loss of motility or dispersal or colonization, formation of adhesion proteins, sporulation or going dormant for unlimited period. Bacterium thus itself is responsible for maintaining hospitability of its niche or accepting ‘changed/altered’ niche in response to dynamicity of the surroundings. That is why bacteria in general are found in variety of habitats, sustain in polluted soils and also degrade pollutants to be used as carbon/energy source, show thermotolerance and last-but-not-the-least suffer lowest rate of extinction. It is thus always happens during laboratory sampling of soil of particular region; just 1 gm of soil sample may yield (isolates) more than one type of bacteria representative of various physicochemical and functional properties. For example, a researcher aiming to isolate Gram negative antibiotic producing strain, may also find growth of Gram positive bacteria, IAA producing, nitrogen fixers or denitrifiers or phosphate solubilizers on culture medium used for isolation. In other words, these isolates from 1gm soil symbolize diverse niches present in that soil. Next time, soil sampling at the same site by exactly similar manner may yield totally different array of bacteria thus presenting us superficial simplicity but grave complexity of bacterial niches. To exemplify this statement in a best way: the niches of symbiotic and non-symbiotic nitrogen fixing bacteria would be always different. Moreover, the niches of individual bacteria of genera belonging to symbiotic and non-symbiotic group would also be different.
Sharing a niche and niche interactions:
If the niche is the so personal space of a bacterium, then a question arises: How does a bacterium break niche boundary and interact with other bacteria of same or different species and surroundings? Here it is mandatory to clarify that bacteria cannot remain confined to one niche of particular habitat; instead they are quite dynamic with respect to nutrients, space and shelter as we have seen in their abilities for adaptation. The environment which provides them these essential things would be their ideal niche. Thus rhizosphere region is more habitable than bulk soil for bacteria and they will try their best to establish in plant rhizosphere rather than bulk soil. Rhizosphere establishment would not be possible until they interact positively or harmoniously with bacteria of same species and metabolic requirements and negatively with native fauna and deleterious bacteria. Bacteria on the way of root colonization and establishment would have to compete for limited nutrients and space with ‘other’ bacteria. These interactions may seem to us as either competition or synergism. If one observes the plant rhizosphere and rhizoplane region microscopically, bacteria of different species with more or less similar metabolic properties could be seen living harmoniously; in other words rhizosphere region represent bacterial groups sharing niche for each other’s’ advantage. Those who cannot compete or sustain during competitive interaction would be automatically excluded from niche or sometimes from the habitat. Rhizosphere competence is the best example to justify niche interactions. Their competence is determined by their ability to overcome effect of native bacteria by various mechanisms like siderophore production to sequester iron during iron limiting conditions or ability to multiply and colonize host root system aggressively in order to establish on the rhizosphere. It would be sensible to state that rhizosphere competent bacteria can occupy maximum or all the niches available in or on the rhizosphere region in favor of host plant; excluding harmful bacteria and ignoring the presence of non-rhizosphere or neutral bacteria for highly successful establishment.
Importance of niche and concerned interactions of soil bacteria:
Plant growth promoting rhizobacteria (PGPR) which can successfully compete for food and space; are only capable of establishment in the rhizosphere of host plant. Here food (nutrient influx) and space (root region), both belong to host plant, hence plants are also said to be involved in direct selection of their bacterial companions, who after establishment provide them with plant growth hormones, sequester micronutrients and most important protect them from phytopathogenic fungi, bacteria and nematodes native to the soil. Those bacteria which are not able to interact beyond their niche environment are to be certainly excluded out. This exclusion is actually an important suppressive trait of useful biocontrol bacteria and has been used for the inoculations of various crop plants to increase growth, yield and control plant diseases. So it can be stated as competitive niche interactions are the basis of plant growth promotion and biocontrol.
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