The roles of rhizospheric bacteria in case of terrestrial plants are well studied. They establish 'give together' condition and show any of mutualism, commensalism, amensalism or pathogenic relationship. They are considered as the hotspot of microbial activity often helping in multifaceted ways in nutrient mineralization & aquisition, plant growth promotion, developing 'immunity' against harmful microorganisms. Such activities may be mediated by IAA, siderophores, ACC deaminase, bacterial antibiotics produced by them. By there is huge knowledge gap in case of aquatic macrophytic rhizobacteria. Although the role of many macrophytes have been attempted in phytoremediation of heavy metals and other toxicants as well as in nutrient (nitrogen & phosphorus) removal and wastewater treatment but the real role of the rhizobacteria still remains neglected. Indeed they play the pivotal roles interacting with the macrophytes. The actual mechanisms of action and bacteria-macrophyte interaction need to be unveiled which would be helpful in understanding the phytoremediation and the nutrient reclamation/removal process more clearly.
All the RG colleagues are requested to contribute in discussing the issue with thoughts, experiences and resources. I look forward to an enriching and though-provoking brainstorming on the RG platform.
A mechanism that could merit to be examined and highlighted is the diffusion of oxygen from the roots into the rhizosphere, creating a oxygenated and a more anaerobic zone further away from the roots. This can create nitrifiers to be active near the root, transforming NH4 to (the more uptakeable ?) N03.
I suppose that microorganisms multiply or bear spores in soil or on the surface of the soil in the river and multiply again in the sea water after entering the estuary because it is known that microbes of red tide bloom are stimulated and multiply in the sea water containing the salt.
in the specific case of heavy metals, the macrophytes play a fundamental role in the process of complexing these metals, thus absorbing them without showing its toxic effect. The rhizobacteria in the other hand contribute in the bioremediation by changing the state of oxidation of the metals to less toxic species and by achieving the precipitation and immobilization in the soi.
On bacterial nitrogen fixation and symbiosis with aquatic macrophytes:
The water fern Azolla is closely associated with the cyanobacterium Anabaena azollae which captures elemental nitrogen N2 from the air to provide to the plant. In duckweed Lemna aequinoctialis the bacterial endophyte Rhizobium lemnae sp. was found, which supplies bound elemental nitrogen to the plant. (Kittiwongwattana & Thawai, 2014) The process of nitrogen fixation by bacteria only takes place in a strict oxygen free area.
Duckweeds have preference for ammonia (NH4 +) over nitrite (NO2-) (Lüönd, 1980) (Porath & Pollock, 1982) This is in contrast to land land plants, who prefer nitrite-nitrogen over ammonia. The uptake of ammonia requires water, which is not a limiting factor in aquatic macrophytes. So in my opinion nitrifiers are of less importance to macrophyte growth than nitrogen fixating microorganisms. Correct me please if you think I'm wrong.
This is a very interesting subject to me, as we live in a world governed by micro-organisms but yet have so much to learn about them.
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