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Evolutionary Adaptations of Plants and Microbes in Low Oxygen, Saline Environments

By Edited Sep 11, 2016 0 0

Eastern Skunk Cabbage
Credit: Caroline Hardman

For thousands of years plant species have been adapting to their surrounding environments in response to landscape alternations and environmental stressors. The success and failure of some plant communities can be linked to plant interactions with microbial organisms, or microbes.  Plant microbes are generally broken down into four categories which include bacteria, fungi, viruses, and protozoa. Like plant species, microbes have also needed to adapt for their own survival.

Generally, plants and microbes are reliant on an abundant supply of oxygen and water for continued persistence. Increased levels or presence of water, or waterlogging, can result in an anoxic environment where there is little available oxygen and increased levels of salt which can be fatal to plant and microbe communities.  The following are several unique physiological (cellular-level) adaptations made by plants and microbes in response to environments containing low oxygen and high levels of salt.

Primary microbial organism adaptations in response to reduced volumes of oxygen and higher volumes of salt include changes in their metabolic and cellular processes. To survive in an anoxic environment microbes either adapt or switch their metabolism to function using anaerobic respiration.  Based on Mitsch and Gosselink, 2007, many bacterial organisms are capable of switching from aerobic to anaerobic respiration, while others have adapted for survival in only anaerobic environments.  Under anaerobic respiration, microbes use oxygen “substitutes” during energy harvesting chemical processes, such as bacteria that respires by reducing sulfate (SO4-) to hydrogen sulfide (H2S)[2793].

Increased salt within an ecosystem can damage microbes through osmosis (essentially dehydrating the organisms’ cells) or toxicity. In response to increased salt levels, microbes have adapted two ways for increased survivability: 1.) Accumulation of less toxic ions (i.e. Potassium instead of other salts) to raise internal cell concentrations and reduce the diffusion of sodium into the cell, and 2.) accumulation of small, non-toxic organic compounds to increase overall concentration of solutes in the cell which will maintain osmotic pressure. 

Like microbes, plants also adapted to anoxic environments through use of anaerobic respiration.  Plants respire in anaerobic conditions through alcohol fermentation which almost always produces ethanol, and less frequently, fatty acids. Other physiological adaptations in plants within anoxic environments include pressurized gas flow and rhizosphere oxygenation. Pressurized gas flow is an adaptation that forces oxygen down to root structures in an anoxic soil environment.  The process occurs when the plant surface and gas molecules within internal air spaces are warmed by the sun to the point where diffusion out of plant lenticels is difficult.  While the warm gas molecules cannot easily move out, cooler molecules can move in, resulting in pressure in the plant that forces gas, including oxygen, down to the roots.  Rhizosphere oxygenation is the leaking of excess oxygen from a plant root system into the area immediately around the root surface, typically resulting in an oxidized rhizosphere.  “Oxygen diffusion from the roots is an important mechanism that moderates the toxic effects of soluble reduced ions such as manganese in anoxic soil and restores ion uptake and plant growth”[2793].

A couple vascular plant adaptations to saline environments include salt exclusion and salt excretion. Plants, such as the mangrove (Avicennia marina) have root structure adaptations which form barriers at the root periderm and exodermis that protect the root cortex from high salt concentrations [2793]. This exclusionary adaptation prevents the plant cells from becoming dehydrated, and the filtering of salts results in the sap being almost pure water, which is then excreted out of the leaves by pressure build up. Other plants that cannot completely exclude salt at the roots have developed specialized salt glands that excrete excess salt. The combination of salt exclusion and salt secretion protects the plant from high concentrations of salt and may keep a relatively balanced osmotic concentration in the plant cells.

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Bibliography

  1. Mitsch, William J., and James G. Gosselink. Wetlands. Hoboken: John Wiley & Sons, Inc., 2007.

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