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Interactions between helophytes and endophytic bacteria: their synergistic effects in bioremediation of contaminants

Syranidou Evdokia

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Summary

Emerging organic contaminants (EOCs) consist of a large and relatively new group of chemical compounds such as endocrine disrupting chemicals (EDCs) as well aspharmaceuticals and personal care products (PCPs). Despite their generally lowconcentrations in the environment they can cause toxic effects on biota while theyremain in the environment since they are continuously released. They are mainlyentering to the environment through the effluents of wastewater treatment plantsbecause these systems have not been developed to treat this kind of compounds. Metals are considered as the main toxic and genotoxic compounds present in hydrosoluble fractions. When they are released to soil or water bodies, they remain there since they cannot be degraded. They can only be transferred.Phytoremediation–based technologies are environmentally friendly alternatives forcleaning up soils or (ground)waters contaminated with metals and/or a variety oforganic pollutants. These treatment methods exploit plants, their associatedmicroorganisms and the developed interactions for contaminant removal orenhancement of plant stabilization and survival in such adverse environments.Constructed Wetlands (CWs) are low-cost wastewater treatment technologies that are part of phytoremediation applications. They are simplified systems but severalphysicochemical and biological processes take place in order to clean the water.In this thesis, an integrated approach exploiting the wetland plant Juncus acutus and its indigenous endophytic community was followed in order to investigate the capability of this meta-organism to clean water contaminated with metals, bisphenol-A, ciprofloxacin and/or sulfamethoxazole.After confirming experimentally the ability of the plant to treat efficiently bisphenol-A-contaminated water, the associated endophytic community of Juncus acutus wasisolated and characterized. Many strains expressed plant growth promotingcharacteristics and were found to possess increased tolerance to metals such as Zn, Ni, Pb and Cd. Moreover, several endophytic bacterial strains tolerated and even used bisphenol-A and/or antibiotics (ciprofloxacin and sulfamethoxazole) as the sole carbon source.Some strains combined many of the desired characteristics and they were further used in a bioaugmentation strategy in order to investigate their potential to improve the efficiency of the wetland helophyte Juncus acutus to deal with mixed pollutionconsisting of emerging organic contaminants (EOCs) and metals at two concentration levels. The beneficial effects of inoculation with tailored endophytic bacteria separately and as a mixture were more prominent in case of high contamination. Especially, the plants inoculated with an endophytic consortium removed higher percentages of organics and metals from the liquid phase in shorter times compared to the noninoculated plants without exhibiting significant oxidative stress. Moreover, the consortium inoculated plants’ phytoextraction capacity was enhanced in terms of observed metal concentrations in the plant compartments and also as total metal mass accumulated in the whole plant.A significant shift of the root endosphere communities was observed due to increased presence of contaminants while the inoculation effort did not have a significant impact. Metal concentration decreased the root bacterial diversity but the root composition of plants inoculated with the endophytic consortium was not affected by the increased metal concentrations. At all levels of contamination, the leaf endophytic communities were not affected either by contaminants or by the inoculation effect. Based on the experimental evidence from this work it can be inferred that bioaugmentation with indigenous endophytic bacteria is an appropriate strategy to be employed in systems such as constructed wetlands treating water with mixed contaminants. It appears that the developed synergistic relationships between plants and endophytic bacteria may point towards more efficient, resilient and robust phytoremediation applications.

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