What is a rhizosphere? Well rhizospheres refer to soil, in hydro, we are talking about a rhizoplane. A rhizosphere can be easily washed away with rain, while the rhizoplane remains even after vigorous flooding. It simply refers to the microlife and environment that surrounds the roots surface. Bacteria and fungi that live within the cells of the root are not considered a part of the rhizoplane, but are instead called endophytes.
If you are wondering what some of these microbes do, here is a short list of the more important ones.
Bacillus - The unifying characteristic of Bacillus bacteria is that they are Gram-positive, form endospores, and grow in the presence of O2. The trivial name assigned to them is aerobic sporeformers.
Bacillus subtilis - (
Bacillus uniflagellatus,
Bacillus globigii,
Bacillus natto) Bacillus subtilis cells are rod-shaped bacteria that are naturally found in soil and vegetation.
Bacillus subtilis grow in the mesophilic temperature range. The optimal temperature is 25-35 degrees Celsius. Stress and starvation are common in this environment, therefore,
Bacillus subtilis has evolved a set of strategies that allow survival under these harsh conditions.
Bacillus subtilis bacteria are non-pathogenic. They can contaminate food, however, they seldom result in food poisoning. They are used on plants as a fungicide. They are also used on agricultural seeds, such as vegetable and soybean seeds, as a fungicide. The bacteria, colonized on root systems, compete with disease causing fungal organisms.
Bacillus subtilis use as a fungicide fortunately does not affect human.
Paenibacillus polymyxa - Nitrogen fixer and plant growth-promoting rhizobacterium with a broad host range.
Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms.
Certain bacteria are capable of fixing nitrogen. In this process, nitrogen gas (N2) is converted to ammonium (NH4+), a form of nitrogen that is biologically available to plants. The reaction is catalyzed by the enzyme nitrogenase.
Because nitrogenase is inactivated by oxygen, the reaction must occur in a low oxygen environment. (
So we don't get much of this in a DWC)
Bacillus amyloliquefaciens - strains of B. amyloliquefaciens bacteria, which occur in association with certain plants, are known to synthesize several different antibiotic substances, including bacillaene, macrolactin, and difficidin. Among NRPS antibiotics,
Bacillus amyloliquefaciens was found to produce surfactin, iturin A, fengycin A and fengycin B. By modifying cell surface properties, surfactin and iturin were reported to positively influence cell spreading, swarming and biofilm formation and thus may globally favour plant root colonization. Furthermore, iturins and fengycins display strong antifugal activity and are inhibitory for the growth of a wide range of plant pathogens.
Another recently established role for lipopeptides from beneficial
Bacillus isolates is the stimulation of the plant immune system. Surfactins and, to a lesser extent, fengycins can induce a priming state in host plant which allows an accelerated activation of defense responses upon pathogen
or insect attack, leading to an enhanced resistance to the attacker encountered.
Trichoderma harzianum is a fungus that is also used as a fungicide. It is used for foliar application, seed treatment and soil treatment for suppression of various disease causing fungal pathogens. Trichoderma readily colonizes plant roots and some strains are rhizosphere competent i.e. able to grow on roots as they develop. Trichoderma spp. also attack, parasitize and otherwise gain nutrition from other fungi. They have evolved numerous mechanisms for both attack of other fungi and for enhancing plant and root growth. Different strains of Trichoderma control almost every pathogenic fungus for which control has been sought.
Glomus intradices - In numerous scientific studies
G. intraradices has been shown to increase phosphorus uptake in multiple plants as well as improve soil aggregation due to hyphae. In hydro the hyphae greatly increase the surface area of the roots. Helps in displacement of harmful microbes by depriving them of housing and food.
As I stated, the plant will actually sense the presence of microbes, and send some food down to the roots for them. This can appear as a very slight coating of slime.
Plant-derived compounds are responsible for providing the additional carbon that allows the rhizosphere to host a large variety of organisms. These compounds fall into five categories: exudates, secretions, mucilages, mucigel, and lysates.
Exudates include surplus sugars, amino acids, and aeromatics that diffuse out of cells to the intercellular space and surrounding soil.
Secretions are byproducts of metabolic activity.
Mucilages are cells sloughed off the root cap as the root grows.
Mucigel is a slime coating the surface of a root that increases the connectivity between plant roots and the surrounding soil.
Lysates from within the cell become available to the surrounding microbial community when an epidermal root cell dies and is broken open
Sources
Microbe-wiki
mass nature
Bacteria textbook
Cell Factories