Endophytic Microorganisms from Synanthropic Plants—A New Promising Tool for Bioremediation
Fast industrialization and urbanization are inseparable from the increasing pollution of the environment with organic compounds. Deterioration of the natural environment is a global problem that requires the development of new ecological and sustainable methods for removing pollution. Bioremediation technology, which uses the metabolic potential of microorganisms, is one of the most effective, economically viable and socially accepted method of cleaning up the environment. However, the progressive degradation of the natural environment indicates the deficient potential of microorganisms to effectively decompose hardly degradable pollutants and their mixtures. Regarding the search for new sources of microorganisms, showing high activity towards such contaminations as polychlorinated biphenyls, polyaromatic hydrocarbons, petroleum compounds, aromatic dyes, pesticides, etc., the interest of researchers in endophytic microorganisms has increased in recent years. It seems that synanthropic (ruderal) plants may be a particularly promising source of endophytes with high degradation activity. Dining the long course of the plant-endophyte relationship in a contaminated site, endophytes not only help in the mobilization of contaminants but also help in promoting plant growth, as well as in developing tolerance to various biotic and abiotic stress. Thus, these microorganisms can be a highly effective tool for modem bioremediation techniques (Gupta et al. 2020). In this chapter, the potential of endophytic microorganisms, mainly bacteria, in removing hydrocarbons from the environment is discussed. The potential of synanthropic plants as a source of microorganisms with high degradation activity and biosurfactant production will be reviewed.
Bioremediation—the right way for contamination removal
The state of pollution of the environment with organic compounds inclines to seek solutions that limit drastic and long-lasting changes in ecosystems, at the same time allowing for their effective utilization. One of the main goals of environmental biotechnology is to develop highly effective biological processes using naturally occurring ones’ catabolic potential of microorganisms to eliminate and detoxify pollutants. Bioremediation is one of the most effective strategies for cleaning up the environment.
For the first time, the concept of bioremediation appeared in the scientific literature in 1987. It describes a method of biological removal of various types of contamination chemical substances from the natural environment, mainly with the participation of microorganisms, to a level safe for the life of organisms. This technology is based on the use of microorganisms living in a polluted environment (natural bioremediation, biostimulation) and/or specially selected for microorganisms for the given contamination (bioaugmentation), which enzymes have the ability of transforming toxic organic compounds into simpler substances, harmless to the environment. Bioremediation techniques can be used for the removal of pollution with natural substances as well as those from the anthropogenic origin, including xenobiotics. Although bioremediation technology has many advantages, the main disadvantage is related to the long time of purification of the contaminated environment. To overcome this phenomenon, scientists are constantly seeking for new microorganisms, with extraordinary degradation activities. Over the last ten years, the attention of sourcing bioremediation agents has been directed to microorganisms living inside plant tissues, called endophytes.
Until twentieth-century, scientists believed that plants are free of microorganisms (Waghunde et al. 2017). Galippe (1887) was the first researcher who reported for the first time that microorganisms can leave inside vegetable plants, e.g., carrot, onion, potato, cabbage, radish, etc. (Waghunde et al. 2017). Nowadays, it is known that in natural conditions, plants growth is highly affected by the presence of different microorganisms. These microbes are formed both by environmental factors and plant metabolic activity itself. The most favorable area for microbes to live is near the roots of plants which is rich in roots’ secretion that can be utilized as a carbon, nitrogen and energy source. The area affected by root secretions is called the rhizosphere (Pisarska and Pietr 2014). Microorganisms inhabiting that zone are usually neutral for plants. However, some of them may exhibit non- antagonistic, favorable interactions with plants which may result in plant growth and promotion. Such interactions can be symbiotic, and even more. A special kind of symbiosis observed among microorganisms and plants is endosymbiosis. It occurs when cells of one organism (microorganism) are living inside other (plant). It is characterized by the fact that microorganisms do not show any harmful or disease symptoms for the plant host (Malfanova et al. 2013). Endophytic microorganisms are bacteria and fungi that are living in different plant tissues for a part, or all of then life. The most important feature of endophytes is causing no harm to the host. Usually, endophytes affect the host beneficially by inducing systemic resistance against phytopathogens and/or animals, promoting the plant’s growth or increasing resistances for environmental stresses. In return, the plant host provides nutrients and safe residency for bacteria (Weyens et al. 2010, Afzal et al. 2014, Zhang et al. 2014).
There are a few types of endophytic microorganisms: obligate, competent, passenger and facultative. Microbial colonization depends on many different factors like the growth stage and plant genotype, the type of plant tissues and soil environmental conditions (Singh et al. 2009).
A large number of studies have shown that each plant can be the host to one or more kinds of endophytes. Therefore, single plant can be a host for several different species of endophytes, which can be isolated from different plant tissues, like roots, stems, leaves or even fruits (Bulgari et al. 2012, Stepniewska and Kuzniar 2013, Faeglieh and Harighi 2018, Ekta et al. 2018, Maggini et al. 2019).
The relations between plants and bacteria are not the only since that kind of relations occurs between plants and fungi as well. They are described as a smooth change from mutualism through commensalism even to parasitism (Schulz and Boyle 2006, Aly et al. 2010).
This kind of symbiosis is documented even more than symbiosis with endophytic bacteria and occurs rather in difficult environments, where plants are exposed to lack of water, extreme temperature and pH (Rodriguez et al. 2008, Redman et al. 2011).
Presence of endophytic fungi inside tissues of plants usually has a positive effect, thus they have an ability to reduce water stress of the host plant, take a part into plant nutrition, and reduce some environmental stresses (Redman et al. 2002, Hanis-Valle et al. 2009, Rodriguez et al. 2009, Vos et al. 2014, Johnson et al. 2012, Aschehoug et al. 2014). In return for so many positive effects, the plant supplies the symbiotic fungus with secretions that are a source of carbon and energy. Thanks to their presence, plants are better protected against some pathogens (Aschehoug et al. 2014). However, the reports on the use of endophytic fungi in bioremediation and biodegradation are rather scarce.