HABITATS (2)

BY : K. Sai Manogna (MSIWM014)

SOIL HABITATS:

For microorganisms, soils are widespread and essential ecosystems that play crucial roles in providing plants with nutrients. If you dig a hole in the earth, you will find that the ground has a structure with various levels of evidence. These include the top organic horizon (O horizon), which includes freshly fallen litter on top by partially decomposed organic matter lower down, followed by horizon A, which comprises a range of minerals. The horizon B where humus, clays, and other materials transported live, and finally, the weathered parent material horizon C. With thin soils overlying calcareous regions of the world, such as the Yucatan, and dense soils occurring in some of the rich farmlands, such as those found in the Midwestern United States, the depth of these layers can differ drastically. Areas where heavy rainfall occurs, have nutrient-poor soils, such as the tropics, and over time the rains leach nutrients from the soils. The degree to which nutrients and microorganisms can travel can influence the permeability of the soil. Light does not penetrate beneath the first centimetre or two, eliminating phototrophy as a means of energy acquisition. The rhizosphere, the region around plant roots, is a habitat where abundant microbial populations occur.

Microbial Food Webs:

One of the greatest treasures of novel species is antibiotics and insights into how populations are organized in soil ecosystems on earth. On average, there are 10⁹ bacterial cells in one gram of soil representing up to 5000 [or even 10,000] bacterial species by some estimates. The nature of soil food web and its inhabitants are more complicated than you might think. Plants play a significant role in physically structuring the under-ground ecosystem through their roots and the impact of their aboveground canopy, depending on the aridity of the aboveground environment. The larger organisms in the soil, such as earthworms, mites, springtails, nematodes, protists, and other invertebrates, were the subject of much of the soil biota research. These species help ‘engineer’ the soil conditions through their absorption and excretion of soil sections. On the scale of the microenvironment, bacteria and fungi even serve as engineers.

Soil microbial abundance varies with the microenvironment’s physical and chemical characteristics, including the content of moisture, organic matter abundance, and the size of soil aggregates. Although seasonal changes contribute to the dynamics of microbial soil populations, discussions of soil microorganisms typically refer to the topsoil sampled during the growing season. The most abundant microorganisms in the soil follow this sequence, as determined by plate count methods:

Algae < Fungi < Actinomycetes = Anaerobic bacteria < Aerobic bacteria

Microorganisms are most common on the surface and decrease as the depth increases in colony-forming units. In the variety of physiological forms of bacteria in various soil environments, soil types, including different organic content and related microbial processes, are seen. There is a substantial genetic diversity of bacteria in soil, and many of the physiological classes still have to be cultivated in the laboratory. Thus, molecular techniques produce more knowledge than conventional plating exercises for the study of the soil culture.

Soils comprise almost all the main microbial groups: bacteria, viruses, fungi, and archaea. Progress has been made in delineating which groups are most prevalent using culture-independent approaches in soil societies. Thirty-two different libraries of sequences from different soils were studied. Their results are impressive: 32 different phyla were present in the tests, but nine phyla were dominant: Proteobacteria, Acidobacteria, Actinobacteria, Verrucomicrobia, Bacteroidetes, Chloroflexi, Planctomycetes, Gemmatimonadetes, and Firmicutes. Proteobacteria account for the highest percentage of soils (39 percent on average). The majority of the sequences were new, and the results of this study vary significantly from the cultivation studies found in previous decades.

Symbiotic nitrogen fixers and mycorrhizae, which provide 5-20 percent of grassland and savannah nitrogen and 80 percent of nitrogen in temperate and boreal forests, are two main classes of bacteria in soil have been extensively studied. Nitrogen and phosphorus derived from symbiotic microorganisms are dependent on at least 20,000 plant species. Plants require nitrogen, and they are unable to fix atmospheric nitrogen in a beneficial form without their symbiotic partners. Examples of essential nitrogen-fixing bacteria are Frankia, an actinomycete that is important in forest growth, and Rhizobium, a key player in the health of crop legumes. These trees may grow in more marginal areas where nitrogen is restricted by interactions between plants, such as the alder tree (Alnus) and Frankia. More than 25 different genera of trees and shrubs have been recorded for cultivation in association with Frankia. Chemoheterotrophs, free-living in the soil or associated with a broad range of legumes, including alfalfa, clover, lupines, and soybeans, are rhizobia, including Azorhizobium, Bradyrhizobium, and Rhizobium. In order to infect them, plants release chemical compounds to attract soil rhizobia. On root hairs, most of the nodules form, but some form on stems. Nodules can use 7-12 percent of the plant’s photosynthetic production when active, but the expense is well worth the return in the form of fixed nitrogen available to the plant.

Despite the progress in understanding soil food webs, our understanding of soil food webs’ mechanisms is hindered by significant challenges. Different feeding classes are generally aggregated because, in their feeding patterns, most soil species are very “flexible,” muddling the distinctions between trophic stages. Relatively unknown are the diets of tiny species. New molecular techniques such as fluorescent in situ hybridization are exciting instruments that can help to expose the dynamic relationships in the soil population of who eats whom. Another instrument that is used to expose feeding relationships and energy sources is stable-isotope analysis.