Nikolaos Remmas

Greenhouse cultivation generates large quantities of plant residues, whose untreated disposal may pose a risk of phytopathogen proliferation and pesticide dispersal into agricultural soils. In this work, composting of greenhouse tomato plant and fruit residues was assessed as a sustainable bioeconomy treatment approach. The composting process progressed through two active thermophilic phases and a three-month maturation period, over which organic matter declined from 84.6% to 68.9% and soluble phenolics from 16.91 to 2.71 mg/g d.w., while ash content increased from 15.4% to 31.1% owing to organic matter loss. Mineralization raised electrical conductivity (EC) from 5.37 to 7.87 mS/cm and stabilized pH at 9.0, generating a mineral-rich, saline–alkaline environment enriched in Ca, K and Mg. High-throughput amplicon sequencing revealed pronounced bacterial succession, with the relative abundance of the lignocellulose-degrading Nocardiopsis decreasing from 37.93% to 2.46%, whereas halotolerant and alkalitolerant genera, i.e., Halomonas (10.10%), Marinimicrobium (6.01%), Salinicoccus (6.28%), Truepera (3.27%) and Salinimicrobium (1.84%), were enriched during maturation. Enterobacteriaceae and Enterococcus relative abundances also decreased, indicating effective sanitization. Regarding the 21 pesticides detected by LC-MS/MS in the initial mixture, the total pesticide load decreased from 2929 to 1129 ng/g d.w. Extracts of mature compost at 5–10% w/v appeared to enhance germination of Solanum lycopersicum and Lepidium sativum seeds, whereas inhibition of the germination was observed at 20% w/v or above, which may be attributed to salinity rather than residual phytotoxicity. Moreover, application of compost extract resulted in the suppression of Fusarium oxysporum f. sp. radicis-lycopersici in a dose-dependent manner. Of 55 bacterial strains isolated from the mature compost, five antagonized F. oxysporum f. sp. radicis-lycopersici and seven antagonized Phytophthora nicotianae, with isolate KGA-18, related to Ornithinimicrobium murale, inhibiting both phytopathogens. These in vitro antagonistic candidates were halotolerant and predominantly alkalitolerant or alkaliphilic. Composting of greenhouse tomato residues thus yields a detoxified, nutrient-rich amendment harboring candidate biocontrol-competent halotolerant and alkalitolerant bacteria, supporting more sustainable nutrient management and plant protection in greenhouse agriculture.

The transition toward a sustainable agri-food system, aligned with agricultural and environmental policy objectives, has increased interest in aromatic plants as non-synthetic pesticide alternatives. This study focused on evaluating the antifungal potential of five specific aromatic plant species, particularly Lavandula dentata, Origanum vulgare, Thymus vulgaris, Salvia officinalis and Rosmarinus officinalis, against the phytopathogenic soil-borne fungi Fusarium oxysporum f.sp. radicis-lycopersici and Verticillium dahliae. During screening, L. dentata and T. vulgaris extracts exhibited strong in vitro fungicidal activity. Bioactive compounds previously detected in both lavender and thyme were identified in their extracts using a triple quadrupole/linear ion trap mass spectrometer. Assessment of in vitro phytoprotective action of L. dentata extract in solid and liquid growth media demonstrated inhibitory effects against F. oxysporum f.sp. radicis-lycopersici at concentrations above 1% v/v, with inhibitory effects of L. dentata extract being observed at concentrations equal to or above 2% v/v. T. vulgaris extract inhibited V. dahliae growth on solid media at concentrations at 1% v/v or above, while inhibitory effects were observed in broth media containing 2% v/v thyme extract. Seed germination tests of both L. dentata and T. vulgaris revealed a concentration-dependent reduction in their germination index (GI) at concentrations equal or above 2% v/v, apart from the effect of lavender extract on cress, where inhibition occurred at dose application above 5% v/v. In planta experiments demonstrated the complete phytoprotective action of lavender extract against F. oxysporum f.sp. radicis-lycopersici, while a marginal improvement in plant survival was observed during application of T. vulgaris extract.

Considering that landfilling still remains among the most commonly used methods for the confrontation of solid wastes, effective methods should be applied to treat the leachate generated, due to its recalcitrant nature. In this work, a full-scale system consisting of two SBRs operating in parallel (350 m³ each) and two activated carbon (AC) columns operating in series (3 m³ each) was retrofitted by introducing a coagulation/flocculation/sedimentation (C/F/S) unit of 7.8 m³ and an ultrafiltration (UF) membrane of 100 m² to effectively treat landfill leachate. The raw leachate was characterized by high COD and NH₄⁺-N concentration, i.e., 3095 ± 706 mg/L and 1054 ± 141 mg/L respectively, a BOD/COD ratio of 0.22, and high concentrations of certain heavy metals. Leachate processing in this retrofitted multistage treatment system resulted in total COD removal efficiency of 89.84%, with biological treatment, C/F, UF, and AC contributing 46.31%, 4.68%, 15.98%, and 22.87% to the overall organic content removal. The retrofitted scheme achieved an overall NH₄⁺-N and TKN removal of 92.03% and 91.75% respectively, attributed mostly to the activity of an effective nitrifying community. Color number (CN) was reduced by 26.96%, 10.29%, 15.94%, and 5.39% after the activated sludge, the C/F, the UF, and the AC adsorption process respectively, corresponding to a 58.91% overall decrease. Regarding heavy metal removal, all elements examined, apart from Ni, i.e., effluent As, Cd, Co, Cr, Cu, Hg, Mg, Mn, and Pb, were below the legislative limits set by the national authorities for restricted or unrestricted irrigation. Lastly, total operating expenses (OPEX) were estimated as equal to 72,687 €/year or 6.64 €/m³.

Honey’s antibacterial activity has been recently linked to the inhibitory effects of honey microbiota against a range of foodborne and human pathogens. In the current study, the microbial community structure of honey samples exerting pronounced antimicrobial activity was examined. The honey samples were obtained from different geographical locations in Greece and had diverse pollen origin (fir, cotton, fir–oak, and Arbutus unedo honeys). Identification of honey microbiota was performed by high-throughput amplicon sequencing analysis, detecting 335 distinct taxa in the analyzed samples. Regarding ecological indices, the fir and cotton honeys possessed greater diversity than the fir–oak and Arbutus unedo ones. Lactobacillus kunkeei (basionym of Apilactobacillus kun-keei) was the predominant taxon in the fir honey examined. Lactobacillus spp. appeared to be favored in honey from fir-originated pollen and nectar since lactobacilli were more pronounced in fir compared to fir–oak honey. Pseudomonas, Streptococcus, Lysobacter and Meiothermus were the predominant taxa in cotton honey, whereas Lonsdalea, the causing agent of acute oak decline, and Zymobacter, an osmotolerant facultative anaerobic fermenter, were the dominant taxa in fir–oak honey. Moreover, methylotrophic bacteria represented 1.3–3% of the total relative abundance, independently of the geographical and pollen origin, indicating that methylotrophy plays an important role in honeybee ecology and functionality. A total of 14 taxa were identified in all examined honey samples, including bacilli/anoxybacilli, paracocci, lysobacters, pseudomonads, and sphingomonads. It is concluded that microbial constituents of the honey samples examined were native gut microbiota of melliferous bees and microbiota of their flowering plants, including both beneficial bacteria, such as potential probiotic strains, and animal and plant pathogens, e.g., Staphylococcus spp. and Lonsdalea spp. Further experimentation will elucidate aspects of potential application of microbial bioindicators in identifying the authenticity of honey and honeybee-derived products.

Sanitary landfilling is still considered worldwide as one of the most common methods applied for the management of the municipal solid waste. As a consequence, vast amounts of landfill leachate are generated annually, which are characterized by variability in physicochemical composition, owing to the stabilization process that occurs over the years. However, sustainable management of landfill leachate is a challenging issue, due to diverse chemical composition and high concentration in heavy metals and xenobiotics. Despite the fact that several studies have been reported on the biotreatment of landfill leachate, only in recent years has the microbial composition in such systems have been examined. In the present review, the key role of the microbial ecology involved in depurification and detoxification of landfill leachate in activated sludge and anaerobic systems is interpreted and ecological considerations influencing landfill leachate treatment are stated. Apart from the assessment of landfill toxicity on certain model organisms, this work provides an extensive overview on microbial communities performing key biological processes during landfill leachate treatment, including nitrification-denitrification, anammox and anaerobic digestion. Moreover, microbial aspects affecting nutrient removal efficiency in such biosystems are discussed.

The thriving agro-industry sector accounts for an essential part of the global gross domestic product, as the need for food and feed production is rising. However, the industrial processing of agricultural products requires the use of water at all stages, which consequently leads to the production of vast amounts of effluents with diverse characteristics, which contain a significantly elevated organic content. This fact reinforces the need for action to control and minimize the environmental impact of the produced wastewater, and activated sludge systems constitute a highly reliable solution for its treatment. The current review offers novel insights on the efficiency of aerobic biosystems in the treatment of agro-industrial wastewaters and their ecology, with an additional focus on the biotechnological potential of the activated sludge of such wastewater treatment plants.

Landfill leachate, due to its recalcitrant nature and toxicity, poses a serious environmental threat, which requires the implementation of effective treatment processes. In this work, a full-scale treatment system consisting of two Sequencing Batch Reactors (SBRs) was used for the processing of landfill leachate of intermediate to mature age (BOD/COD ratio of 0.16). Biosystem operation resulted in BOD₅, COD and TKN removal efficiencies of 81%, 39% and 76%, respectively, whereas the low residual NO₃^--N concentration in the effluent (4.01 ± 0.10 mg/L) was indicative of the efficient denitrification process. Assessment of hydrolytic potential of activated sludge revealed high endocellular and extracellular lipase activities, which reached values up to 206 and 141 U/g protein respectively, possibly as the consequence of plastics degradation during maturation process. Implementation of Illumina sequencing indicated the predominance of Alphaproteobacteria, accompanied by members of Bacteroidetes, Betaproteobacteria and Chloroflexi. Paracoccus was the predominant genus identified, followed by representatives of the genera Bellilinea, Flavobacterium, Thauera and Truepera. Nitrosomonas was the major ammonia-oxidizing bacterium (AOB), while nitrite oxidation was mainly achieved by the uncultured nitrite-oxidizing bacterium (NOB) Candidatus Nitrotoga.

Biotreatment of triclosan is mainly performed in conventional activated sludge systems, which, however, are not capable of completely removing this antibacterial agent. As a consequence, triclosan ends up in surface and groundwater, constituting an environmental threat, due to its toxicity to aquatic life. However, little is known regarding the diversity and mechanism of action of microbiota capable of degrading triclosan. In this work, an immobilized cell bioreactor was setup to treat triclosan-rich wastewater. Bioreactor operation resulted in high triclosan removal efficiency, even greater than 99.5%. Nitrogen assimilation was mainly occurred in immobilized biomass, although nitrification was inhibited. Based on Illumina sequencing, Bradyrhizobiaceae, followed by Ferruginibacter, Thermomonas, Lysobacter and Gordonia, were the dominant genera in the bioreactor, representing 38.40 ± 0.62% of the total reads. However, a broad number of taxa (15 genera), mainly members of Xanthomonadaceae, Bradyrhizobiaceae and Chitinophagaceae, showed relative abundances between 1% and 3%. Liquid Chromatography coupled to Quadrupole Time-Of-Flight Mass Spectrometry (LC-QTOF-MS) resulted in the identification of catabolic routes of triclosan in the immobilized cell bioreactor. Seven intermediates of triclosan were detected, with 2,4-dichlorophenol, 4-chlorocatechol and 2-chlorohydroquinone being the key breakdown products of triclosan. Thus, the immobilized cell bioreactor accommodated a diverse bacterial community capable of degrading triclosan. GRAPHICAL ABSTRACT

In this work, the metabolic uncoupler para-nitrophenol (pNP) was applied to suppress excess sludge production and to investigate its effects on the system’s performance and activated sludge community structure. The COD removal efficiency decreased from 99.0% to 89.5% prior to and after pNP addition, respectively. Application of pNP transiently reduced NH₄⁺-N, NO₃⁻-N and NO₂⁻-N removal efficiencies, suggesting partial inhibition of both nitrifying and denitrifying activity. However, no changes in the relative abundance of the nitrifying bacteria occurred. Phosphorus removal efficiency was sharply reduced after pNP addition, as the consequence of hydrolysis of stored cell reserves. Tetrasphaera, a key polyphosphate accumulating organism, was also affected by the addition of pNP, a fact that highly influenced system’s ability to remove phosphorus. A drastic drop in Soluble Microbial Products (SMP) and Extracellular Polymeric Substances (EPS) was also detected shortly after the introduction of the uncoupler. On the other hand, MBR’s physicochemical parameters were restored to initial values a week after the addition of pNP. Moreover, remarkable changes in beta-diversity were noted after pNP addition. An increase of Bacteroidetes, Gammaproteobacteria and Firmicutes over Actinobacteria and Alphaproteobacteria was also observed after pNP addition.

The evaluation of effluent wastewater quality mainly relies on the assessment of conventional bacterial indicators, such as fecal coliforms and enterococci; however, little is known about opportunistic pathogens, which can resist chlorination and may be transmitted in aquatic environments. In contrast to conventional microbiological methods, high-throughput molecular techniques can provide an accurate evaluation of effluent quality, although a limited number of studies have been performed in this direction. In this work, high-throughput amplicon sequencing was employed to assess the effectiveness of chlorination as a disinfection method for secondary effluents. Common inhabitants of the intestinal tract, such as Bacteroides, Arcobacter and Clostridium, and activated sludge denitrifiers capable of forming biofilms, such as Acidovorax, Pseudomonas and Thauera, were identified in the chlorinated effluent. Chloroflexi with dechlorination capability and the bacteria involved in enhanced biological phosphorus removal, i.e., Candidatus Accumulibacter and Candidatus Competibacter, were also found to resist chlorination. No detection of Escherichia indicates the lack of fecal coliform contamination. Mycobacterium spp. were absent in the chlorinated effluent, whereas toxin-producing cyanobacteria of the genera Anabaena and Microcystis were identified in low abundances. Chlorination significantly affected the filamentous bacteria Nocardioides and Gordonia, whereas Zoogloea proliferated in the disinfected effluent. Moreover, perchlorate/chlorate- and organochlorine-reducing bacteria resisted chlorination.