Cyanobacterial Allelochemicals But Not Cyanobacterial Cells Markedly Reduce Microbial Community Diversity

Allelopathic Interactions Between the Green-Tide-Forming Ulva prolifera and the Golden-Tide-Forming Sargassum horneri Under Controlled Laboratory Conditions

Harmful algal blooms (HABs) represent a significant global marine ecological disaster. In the Yellow Sea, green and golden tides often occur simultaneously or sequentially, suggesting that interspecific competition involves not only spatial and resource competition but also allelopathy. This study investigated the allelopathic interactions between Ulva prolifera and Sargassum horneri using physiological and biochemical parameters, including relative growth rate (RGR), cell ultrastructure, chlorophyll fluorescence, enzyme activity, and metabolomics analysis. The results showed that S. horneri filtrate significantly inhibited U. prolifera growth, while U. prolifera filtrate had no significant effect on S. horneri. Both algal filtrates caused cellular damage and affected photosynthesis, enzyme activities, and metabolism. However, their allelopathic responses differed: U. prolifera may rely on internal compensatory mechanisms, while S. horneri may depend on defense strategies. These findings provide insights into the dynamics of green and golden tides and support the scientific control of HABs through allelopathy.

Nitrogen liberated via allelopathy can promote harmful algal blooms

Allelopathy is a biological mechanism that can promote harmful algal blooms (HAB) via the inhibition of sympatric phytoplankton. While nutrient loading can also promote HABs, the ability of allelopathy to stimulate HABs via the regeneration of nutrients has yet to be explored. To examine the impacts of allelopathically liberated N on HAB species, a series of experiments were performed using multiple allelopathic HAB species including the dinoflagellates Alexandrium catenella and Margalefidinium polykrikoides, and the pelagophyte, Aureoumbra lagunensis. These HAB species were paired with the cosmopolitan dinoflagellate, Akashiwo sanguinea, that was labeled with 15NO3- or 15NH4+, allowing the release and transfer of N to be traced as a time course during allelopathic interactions. During all experiments, the allelopathic inhibition of Akashiwo was accompanied by increases in cell densities, growth rates, and the δ15N content of the HAB species, evidencing the transfer of N liberated from Akashiwo. The cellular transfer of 15N and release of dissolved N was higher when Akashiwo was grown with 15NO3- compared to 15NH4+ suggesting a differential subcellular-compartmentalization of N sources. Regardless of the type of N, HAB species obtained 60 - 100% of their cellular N from lysed Akashiwo cells and there was an enrichment of the δ15N content of the dissolved NH4+ pool post-lysis of Akashiwo. Collectively, the results demonstrate that beyond facilitating species succession, allelopathy can supply HABs with N and, therefore, is likely important for promoting and sustaining HABs. Given that allelopathy is known to be a dose-dependent process, allelopathy may induce a positive feedback loop, whereby competitors are lysed, N is liberated, HABs are intensified and, in turn, become more strongly allelopathic.

Harmful and beneficial properties of cyanotoxins: Two sides of the same coin

Cyanotoxins are by definition "harmful agents" produced by cyanobacteria. Their toxicity has been extensively studied and reviewed over the years. Cyanotoxins have been commonly classified, based on their poisonous effects on mammals, into three main classes, neurotoxins, hepatotoxins and dermatotoxins, and, considering their chemical features, mainly identified as peptides, alkaloids and lipopolysaccharides. Here we propose a broader subdivision of cyanotoxins into eight distinct classes, taking into account their molecular structures, biosynthesis and modes of action: alkaloids, non-ribosomal peptides, polyketides, non-protein amino acids, indole alkaloids, organophosphates, lipopeptides and lipoglycans. For each class, the structures and primary mechanisms of toxicity of the main representative cyanotoxins are reported. Despite their powerful biological activities, only recently scientists have considered the biotechnological potential of cyanotoxins, and their applications both in medical and in industrial settings, even if only a few of these have reached the biotech market. In this perspective, we discuss the potential uses of cyanotoxins as anticancer, antimicrobial, and biocidal agents, as common applications for cytotoxic compounds. Furthermore, taking into account their mechanisms of action, we describe peculiar potential bioactivities for several cyanotoxin classes, such as local anaesthetics, antithrombotics, neuroplasticity promoters, immunomodulating and antifouling agents. In this review, we aim to stimulate research on the potential beneficial roles of cyanotoxins, which require interdisciplinary cooperation to facilitate the discovery of innovative biotechnologies.

Reforestation substantially changed the soil antibiotic resistome and its relationships with metal resistance genes, mobile genetic elements, and pathogens

Revealing the effects of reforestation on soil antibiotic resistome is essential for assessing ecosystem health, yet related studies remain scarce. Here, to determine the responses of the soil antibiotic resistome to reforestation, 30 pairs of cropland and forest soil samples were collected from southwestern China, a region with high environmental heterogeneity. All the forests had been derived from croplands more than one decade ago. The diversity and abundance of soil antibiotic resistance genes (ARGs), metal resistance genes (MRGs), mobile genetic elements (MGEs), and pathogens were determined by metagenomic sequencing and real-time PCR. The results showed that reforestation significantly increased soil microbial abundance and the contents of Cu, total carbon, total nitrogen, total organic carbon, and ammonium nitrogen. Nevertheless, it decreased the contents of soil Zn, Ba, nitrate nitrogen, and available phosphorus. The main soil ARGs identified in this region were vancomycin, multidrug, and bacitracin resistance genes. Reforestation significantly increased the soil ARG abundance by 62.58%, while it decreased the ARG richness by 16.50%. Reforestation exerted no significant effects on the abundance of heavy metal resistance genes and pathogens, but it doubled the abundance of MGEs. Additionally, reforestation substantially decreased the co-occurrence frequencies of ARGs with MRGs and pathogens. In contrast, the correlation between ARGs and MGEs was greatly enhanced by reforestation. Similarly, the correlations between soil ARG abundance and environmental factors were also strengthened by reforestation. These findings suggest that reforestation can substantially affect the soil antibiotic resistome and exerts overall positive effects on soil health by decreasing ARG richness, providing critical information for assessing the effects of "grain for green" project on soil health.

Effect of allelochemicals sustained-release microspheres on the ingestion, incorporation, and digestion abilities of Daphnia magna Straus

Allelochemicals sustained-release microspheres (ACs-SMs) exhibited great inhibition effect on algae, however, few studies have focused on ACs-SMs toxicity on invertebrate. In this study, the effects of single high-concentration ACs (15 mg/L, SH-ACs), repeated low-concentration ACs (3 × 5 mg/L, RL-ACs) and ACs-SMs containing 15 mg/L ACs exposure on the ingestion, incorporation, and digestion of Daphnia magna Straus (DS) were investigated by stable isotope ¹⁵N labeling method. Meanwhile, the diversity and abundance of microflora in DS guts were determined by 16S rRNA genes and cloning methods. The results showed that SH-ACs exposure caused 50% and 33.3% death rates for newborn and adult DS, while RL-ACs exposure caused 10% death rate for newborn DS and no obvious effect on the activity of adult DS. And ACs-SMs exposure did not diminish the motility of both newborn and adult DS, indicating the lower acute toxicity of ACs-SMs. Furthermore, SH-ACs inhibited the ingestion (-6.45%), incorporation (-47.1%) and digestion (-53.8%) abilities of DS and reduced the microbial abundance (-27.7%) in DS guts. Compared with SH-ACs, RL-ACs showed relatively low impact on the ingestion (-3.23%), incorporation (-5.89%) and digestion (-23.9%) abilities of DS. Interestingly, ACs-SMs enhanced the ingestion (+9.68%), incorporation (+52.9%) and digestion (+51.3%) abilities of DS and increased the microbial abundance (+10.7%) in DS guts. Overall ACs and ACs-SMs reduced the diversity of microflora in DS guts. In conclusion, ACs-SMs can release ACs sustainably and prolong the sustained release time, which not only effectively reduce the toxicity of ACs, but also had positive effects on DS.

Comparative metabolomic analysis of exudates of microcystin-producing and microcystin-free Microcystis aeruginosa strains

Cyanobacterial harmful algal blooms (cHABs) dominated by Microcystis aeruginosa threaten the ecological integrity and beneficial uses of lakes globally. In addition to producing hepatotoxic microcystins (MC), M. aeruginosa exudates (MaE) contain various compounds with demonstrated toxicity to aquatic biota. Previously, we found that the ecotoxicity of MaE differed between MC-producing and MC-free strains at exponential (E-phase) and stationary (S-phase) growth phases. However, the components in these exudates and their specific harmful effects were unclear. In this study, we performed untargeted metabolomics based on liquid chromatography-mass spectrometry to reveal the constituents in MaE of a MC-producing and a MC-free strain at both E-phase and S-phase. A total of 409 metabolites were identified and quantified based on their relative abundance. These compounds included lipids, organoheterocyclic compounds, organic acid, benzenoids and organic oxygen compounds. Multivariate analysis revealed that strains and growth phases significantly influenced the metabolite profile. The MC-producing strain had greater total metabolites abundance than the MC-free strain at S-phase, whereas the MC-free strain released higher concentrations of benzenoids, lipids, organic oxygen, organic nitrogen and organoheterocyclic compounds than the MC-producing strain at E-phase. Total metabolites had higher abundance in S-phase than in E- phase in both strains. Analysis of differential metabolites (DMs) and pathways suggest that lipids metabolism and biosynthesis of secondary metabolites were more tightly coupled to growth phases than to strains. Abundance of some toxic lipids and benzenoids DMs were significantly higher in the MC-free strain than the MC-producing one. This study builds on the understanding of MaE chemicals and their biotoxicity, and adds to evidence that non-MC-producing strains of cyanobacteria may also pose a threat to ecosystem health.

Bacterial communities in a subarctic stream network: Spatial and seasonal patterns of benthic biofilm and bacterioplankton

Water-column bacterial communities are assembled by different mechanisms at different stream network positions, with headwater communities being controlled by mass effects (advection of bacteria from terrestrial soils) while downstream communities are mainly driven by environmental sorting. Conversely, benthic biofilms are colonized largely by the same set of taxa across the entire network. However, direct comparisons of biofilm and bacterioplankton communities along whole stream networks are rare. We used 16S rRNA gene amplicon sequencing to explore the spatiotemporal variability of benthic biofilm (2 weeks old vs. mature biofilm) and water-column communities at different network positions of a subarctic stream from early summer to late autumn. Amplicon sequence variant (ASV) richness of mature biofilm was about 2.5 times higher than that of early biofilm, yet the pattern of seasonality was the same, with the highest richness in midsummer. Biofilm bacterial richness was unrelated to network position whereas bacterioplankton diversity was negatively related to water residence time and distance from the source. This pattern of decreasing diversity along the network was strongest around midsummer and diminished greatly as water level increased towards autumn. Biofilm communities were phylogenetically clustered at all network positions while bacterioplankton assemblages were phylogenetically clustered only at the most downstream site. Both early and mature biofilm communities already differed significantly between upstream (1st order) and midstream (2nd order) sections. Network position was also related to variation in bacterioplankton communities, with upstream sites harbouring substantially more unique taxa (44% of all upstream taxa) than midstream (20%) or downstream (8%) sites. Some of the taxa that were dominant in downstream sections were already present in the upmost headwaters, and even in riparian soils, where they were very rare (relative abundance <0.01%). These patterns in species diversity and taxonomic and phylogenetic community composition of the riverine bacterial metacommunity were particularly strong for water-column communities, whereas both early and mature biofilm exhibited weaker spatial patterns. Our study demonstrated the benefits of studying bacterioplankton and biofilm communities simultaneously to allow testing of ecological hypotheses about biodiversity patterns in freshwater bacteria.

Using different cultivation strategies and methods for the production of microalgal biomass as a raw material for the generation of bioproducts

Microalgal biomass and its fine chemical production from microalgae have pioneered algal bioprocess technology with few limitations such as lab-to-industry. However, laboratory-scale transitions and industrial applications are hindered by a plethora of limitations comprising expensive in culturing methods. Therefore, to emphasize the profitable benefits, the algal culturing techniques appropriately employed for large-scale microalgal biomass yield necessitates intricate assessment to emphasize the profitable benefits. The present review holistically compiles the culturing strategies for improving microalgal biomass production based on appropriate factors like designing better bioreactor designs. On the other hand, synthetic biology approaches for abridging the effective industrial transition success explored recently. Prospects in synthetic biology for enhanced microalgal biomass production based on cultivation strategies and various mechanistic modes approach to enrich cost-effective and viable output are discussed. The State-of-the-art culturing techniques encompassing enhancement of photosynthetic activity, designing bioreactor design, and potential augmenting protocols for biomass yield employing indoor cultivation in both (Open and or/closed) methods are enumerated. Further, limitations hindering the microalgal bioproducts development are critically evaluated for improving culturing techniques for microalgal cell factories, subsequently escalating the cost-benefit ratio in bioproducts synthesis from microalgae. The comprehensive analysis could provide a rational and deeper detailed insight for microalgal entrepreneurs through alternative culturing technology viz., synthetic biology and genome engineering in an Industrial perspective arena.

Microalgae and Cyanobacteria Strains as Producers of Lipids with Antibacterial and Antibiofilm Activity

Lipids are one of the primary metabolites of microalgae and cyanobacteria, which enrich their utility in the pharmaceutical, feed, cosmetic, and chemistry sectors. This work describes the isolation, structural elucidation, and the antibiotic and antibiofilm activities of diverse lipids produced by different microalgae and cyanobacteria strains from two European collections (ACOI and LEGE-CC). Three microalgae strains and one cyanobacteria strain were selected for their antibacterial and/or antibiofilm activity after the screening of about 600 strains carried out under the NoMorFilm European project. The total organic extracts were firstly fractionated using solid phase extraction methods, and the minimum inhibitory concentration and minimal biofilm inhibitory concentration against an array of human pathogens were determined. The isolation was carried out by bioassay-guided HPLC-DAD purification, and the structure of the isolated molecules responsible for the observed activities was determined by HPLC-HRESIMS and NMR methods. Sulfoquinovosyldiacylglycerol, monogalactosylmonoacylglycerol, sulfoquinovosylmonoacylglycerol, α-linolenic acid, hexadeca-4,7,10,13-tetraenoic acid (HDTA), palmitoleic acid, and lysophosphatidylcholine were found among the different active sub-fractions selected. In conclusion, cyanobacteria and microalgae produce a great variety of lipids with antibiotic and antibiofilm activity against the most important pathogens causing severe infections in humans. The use of these lipids in clinical treatments alone or in combination with antibiotics may provide an alternative to the current treatments.

Culture Growth of the Cyanobacterium Phormidium sp. in Various Salinity and Light Regimes and Their Influence on Its Phycocyanin and Other Pigments Content

A strain of the filamentous non N-fixing cyanobacterium Phormidium sp. isolated from the Messolonghi (W. Greece) saltworks, was cultured in the laboratory at six different combinations of salinity (20-40-60 ppt) and illumination (low-2000 lux and high-8000 lux). At salinities of 60 and 40 ppt and in high illumination (XL-8000 lux), the growth rate (μmax) presented the highest values (0.491 and 0.401, respectively) compared to the corresponding at 20 ppt (0.203). In general and at all salinities, the higher illumination (XL) gave the highest growth rates and shorter duplication time (tg) in comparison to the lower illumination (L). On the contrary, phycocyanin, phycoerythrin and allophycocyanin production was extremely increased in the lower illumination (L) in all salinities, from ~14 fold at 40 and 60 ppt to 269 fold at 20 ppt of those corresponding to higher illumination (XL). Similar analogies were also recorded for the other two billiproteins. Chlorophyll-a content was also higher in lower illumination at all salinities in contrast to total carotenoids that did not exhibit such a pattern. The high growth rate and high phycocyanin content along with the rapid sedimentation of its cultured biomass can set this marine Phormidium species as a promising candidate for mass culture.

The current state of knowledge on taxonomy, modulating factors, ecological roles, and mode of action of phytoplankton allelochemicals

Allelopathy is widespread in marine, brackish, and freshwater habitats. Literature data indicate that allelopathy could offer a competitive advantage for some phytoplankton species by reducing the growth of competitors. It is also believed that allelopathy may affect species succession. Thus, allelopathy may play a role in the development of blooms. Over the past few decades, the world's coastal waters have experienced increases in the numbers of cyanobacterial and microalgal blooming events. Understanding how allelopathy is implicated with other biological and environmental factors as a bloom-development mechanism is an important topic for future research. This review focuses on a taxonomic overview of allelopathic cyanobacteria and microalgae, the biological and environmental factors that affect allelochemical production, their role in ecological dynamics, and their physiological modes of action, as well as potential industrial applications of allelopathic compounds.

Increased algicidal activity of Aeromonas veronii in response to Microcystis aeruginosa: interspecies crosstalk and secondary metabolites synergism

Toxic Microcystis spp. blooms constitute a serious threat to water quality worldwide. Aeromonas veronii was isolated from Microcystis sp. colonies collected in Lake Kinneret. Spent Aeromonas media inhibits the growth of Microcystis aeruginosa MGK isolated from Lake Kinneret. The inhibition was much stronger when Aeromonas growth medium contained spent media from MGK suggesting that Aeromonas recognized its presence and produced secondary metabolites that inhibit Microcystis growth. Fractionations of the crude extract and analyses of the active fractions identified several secondary metabolites including lumichrome in Aeromonas media. Application of lumichrome at concentrations as low as 4 nM severely inhibited Microcystis growth. Inactivation of aviH in the lumichrome biosynthetic pathway altered the lumichrome level in Aeromonas and the extent of MGK growth inhibition. Conversely, the initial lag in Aeromonas growth was significantly longer when provided with Microcystis spent media but Aeromonas was able to resume normal growth. The longer was pre-exposure to Microcystis spent media the shorter was the lag phase in Aeromonas growth indicating the presence of, and acclimation to, secondary MGK metabolite(s) the nature of which was not revealed. Our study may help to control toxic Microcystis blooms taking advantage of chemical languages used in the interspecies communication.

Urbanization and Waterborne Pathogen Emergence in Low-Income Countries: Where and How to Conduct Surveys?

A major forthcoming sanitary issue concerns the apparition and spreading of drug-resistant microorganisms, potentially threatening millions of humans. In low-income countries, polluted urban runoff and open sewage channels are major sources of microbes. These microbes join natural microbial communities in aquatic ecosystems already impacted by various chemicals, including antibiotics. These composite microbial communities must adapt to survive in such hostile conditions, sometimes promoting the selection of antibiotic-resistant microbial strains by gene transfer. The low probability of exchanges between planktonic microorganisms within the water column may be significantly improved if their contact was facilitated by particular meeting places. This could be specifically the case within biofilms that develop on the surface of the myriads of floating macroplastics increasingly polluting urban tropical surface waters. Moreover, as uncultivable bacterial strains could be involved, analyses of the microbial communities in their whole have to be performed. This means that new-omic technologies must be routinely implemented in low- and middle-income countries to detect the appearance of resistance genes in microbial ecosystems, especially when considering the new ‘plastic context.’ We summarize the related current knowledge in this short review paper to anticipate new strategies for monitoring and surveying microbial communities.

Growth and toxicity of Halomicronema metazoicum (Cyanoprokaryota, Cyanophyta) at different conditions of light, salinity and temperature

Cyanobacteria may live in the water column and in the benthos of aquatic environments, or be symbionts of other organisms, as in the case of Phormidium-like cyanobacteria, known to influence the ecology of freshwater and marine ecosystems. A strain of Phormidium-like cyanobacteria has been recently isolated as a free-living epiphyte of leaves of Posidonia oceanica (L.) Delile in the Mediterranean sea and its biology and ecology are herein investigated. It was identified as Halomicronema metazoicum, previously known uniquely as a symbiont of marine sponges. We cultivated it in a range of light irradiances, temperatures and salinities, to establish the most suitable conditions for the production of allelopathic and toxic compounds. The bioactivity of its spent culture medium was measured by means of standard toxicity tests performed on two model organisms. Our results indicate that at least two bioactive compounds are produced, at low and high irradiance levels and at two temperatures. The main compounds influencing the survival of model organisms are produced at the highest temperature and high or intermediate irradiance levels. The present research contributes to the understanding of critical toxigenic relationships among cyanobacteria and invertebrates, possibly influencing the ecology of such a complex environment as P. oceanica Future isolation, identification and production of bioactive compounds will permit their exploitation for biotechnologies in the field of ecological conservation and medical applications.

Drivers and ecological consequences of dominance in periurban phytoplankton communities using networks approaches

Evaluating the causes and consequences of dominance by a limited number of taxa in phytoplankton communities is of huge importance in the current context of increasing anthropogenic pressures on natural ecosystems. This is of particular concern in densely populated urban areas where usages and impacts of human populations on water ecosystems are strongly interconnected. Microbial biodiversity is commonly used as a bioindicator of environmental quality and ecosystem functioning, but there are few studies at the regional scale that integrate the drivers of dominance in phytoplankton communities and their consequences on the structure and functioning of these communities. Here, we studied the causes and consequences of phytoplankton dominance in 50 environmentally contrasted waterbodies, sampled over four summer campaigns in the highly-populated Île-de-France region (IDF). Phytoplankton dominance was observed in 32-52% of the communities and most cases were attributed to Chlorophyta (35.5-40.6% of cases) and Cyanobacteria (30.3-36.5%). The best predictors of dominance were identified using multinomial logistic regression and included waterbody features (surface, depth and connection to the hydrological network) and water column characteristics (total N, TN:TP ratio, water temperature and stratification). The consequences of dominance were dependent on the identity of the dominant organisms and included modifications of biological attributes (richness, cohesion) and functioning (biomass, RUE) of phytoplankton communities. We constructed co-occurrence networks using high resolution phytoplankton biomass and demonstrated that networks under dominance by Chlorophyta and Cyanobacteria exhibited significantly different structure compared with networks without dominance. Furthermore, dominance by Cyanobacteria was associated with more profound network modifications (e.g. cohesion, size, density, efficiency and proportion of negative links), suggesting a stronger disruption of the structure and functioning of phytoplankton communities in the conditions in which this group dominates. Finally, we provide a synthesis on the relationships between environmental drivers, dominance status, community attributes and network structure.

Natural Products from Cyanobacteria: Focus on Beneficial Activities

Cyanobacteria are photosynthetic microorganisms that colonize diverse environments worldwide, ranging from ocean to freshwaters, soils, and extreme environments. Their adaptation capacities and the diversity of natural products that they synthesize, support cyanobacterial success in colonization of their respective ecological niches. Although cyanobacteria are well-known for their toxin production and their relative deleterious consequences, they also produce a large variety of molecules that exhibit beneficial properties with high potential in various fields (e.g., a synthetic analog of dolastatin 10 is used against Hodgkin's lymphoma). The present review focuses on the beneficial activities of cyanobacterial molecules described so far. Based on an analysis of 670 papers, it appears that more than 90 genera of cyanobacteria have been observed to produce compounds with potentially beneficial activities in which most of them belong to the orders Oscillatoriales, Nostocales, Chroococcales, and Synechococcales. The rest of the cyanobacterial orders (i.e., Pleurocapsales, Chroococcidiopsales, and Gloeobacterales) remain poorly explored in terms of their molecular diversity and relative bioactivity. The diverse cyanobacterial metabolites possessing beneficial bioactivities belong to 10 different chemical classes (alkaloids, depsipeptides, lipopeptides, macrolides/lactones, peptides, terpenes, polysaccharides, lipids, polyketides, and others) that exhibit 14 major kinds of bioactivity. However, no direct relationship between the chemical class and the respective bioactivity of these molecules has been demonstrated. We further selected and specifically described 47 molecule families according to their respective bioactivities and their potential uses in pharmacology, cosmetology, agriculture, or other specific fields of interest. With this up-to-date review, we attempt to present new perspectives for the rational discovery of novel cyanobacterial metabolites with beneficial bioactivity.

Biodegradation of the Allelopathic Chemical Pterostilbene by a Sphingobium sp. Strain from the Peanut Rhizosphere

Many plants produce allelopathic chemicals, such as stilbenes, to inhibit pathogenic fungi. The degradation of allelopathic compounds by bacteria associated with the plants would limit their effectiveness, but little is known about the extent of biodegradation or the bacteria involved. Screening of tissues and rhizosphere of peanut (Arachis hypogaea) plants revealed substantial enrichment of bacteria able to grow on resveratrol and pterostilbene, the most common stilbenes produced by the plants. Investigation of the catabolic pathway in Sphingobium sp. strain JS1018, isolated from the rhizosphere, indicated that the initial cleavage of pterostilbene was catalyzed by a carotenoid cleavage oxygenase (CCO), which led to the transient accumulation of 4-hydroxybenzaldehyde and 3,5-dimethoxybenzaldehyde. 4-Hydroxybenzaldehyde was subsequently used for the growth of the isolate, while 3,5-dimethoxybenzaldehyde was further converted to a dead-end metabolite with a molecular weight of 414 (C₂₄H₃₁O₆). The gene that encodes the initial oxygenase was identified in the genome of strain JS1018, and its function was confirmed by heterologous expression in Escherichia coli This study reveals the biodegradation pathway of pterostilbene by plant-associated bacteria. The prevalence of such bacteria in the rhizosphere and plant tissues suggests a potential role of bacterial interference in plant allelopathy.IMPORTANCE Pterostilbene, an analog of resveratrol, is a stilbene allelochemical produced by plants to inhibit microbial infection. As a potent antioxidant, pterostilbene acts more effectively than resveratrol as an antifungal agent. Bacterial degradation of this plant natural product would affect the allelopathic efficacy and fate of pterostilbene and thus its ecological role. This study explores the isolation and abundance of bacteria that degrade resveratrol and pterostilbene in peanut tissues and rhizosphere, the catabolic pathway for pterostilbene, and the molecular basis for the initial cleavage of pterostilbene. If plant allelopathy is an important process in agriculture and management of invasive plants, the ecological role of bacteria that degrade the allelopathic chemicals must be equally important.

A Multi-Bioassay Integrated Approach to Assess the Antifouling Potential of the Cyanobacterial Metabolites Portoamides

The cyclic peptides portoamides produced by the cyanobacterium Phormidium sp. LEGE 05292 were previously isolated and their ability to condition microcommunities by allelopathic effect was described. These interesting bioactive properties are, however, still underexplored as their biotechnological applications may be vast. This study aims to investigate the antifouling potential of portoamides, given that a challenge in the search for new environmentally friendly antifouling products is to find non-toxic natural alternatives with the ability to prevent colonization of different biofouling species, from bacteria to macroinvertebrates. A multi-bioassay approach was applied to assess portoamides antifouling properties, marine ecotoxicity and molecular mode of action. Results showed high effectiveness in the prevention of mussel larvae settlement (EC50 = 3.16 µM), and also bioactivity towards growth and biofilm disruption of marine biofouling bacterial strains, while not showing toxicity towards both target and non-target species. Antifouling molecular targets in mussel larvae include energy metabolism modifications (failure in proton-transporting ATPases activity), structural alterations of the gills and protein and gene regulatory mechanisms. Overall, portoamides reveal a broad-spectrum bioactivity towards diverse biofouling species, including a non-toxic and reversible effect towards mussel larvae, showing potential to be incorporated as an active ingredient in antifouling coatings.

Distribution of Toxigenic Halomicronema spp. in Adjacent Environments on the Island of Ischia: Comparison of Strains from Thermal Waters and Free Living in Posidonia Oceanica Meadows

Organisms adaptable to extreme conditions share the ability to establish protective biofilms or secrete defence toxins. The extracellular substances that are secreted may contain monosaccharides and other toxic compounds, but environmental conditions influence biofilm characteristics. Microorganisms that are present in the same environment achieve similar compositions, regardless of their phylogenetic relationships. Alternatively, cyanobacteria phylogenetically related may live in different environments, but we ignore if their physiological answers may be similar. To test this hypothesis, two strains of cyanobacteria that were both ascribed to the genus Halomicronema were isolated. H. metazoicum was isolated in marine waters off the island of Ischia (Bay of Naples, Italy), free living on leaves of Posidonia oceanica. Halomicronema sp. was isolated in adjacent thermal waters. Thus, two congeneric species adapted to different environments but diffused in the same area were polyphasically characterized by microscopy, molecular, and toxicity analyses. A variable pattern of toxicity was exhibited, in accordance with the constraints imposed by the host environments. Cyanobacteria adapted to extreme environments of thermal waters face a few competitors and exhibit a low toxicity; in contrast, congeneric strains that have adapted to stable and complex environments as seagrass meadows compete with several organisms for space and resources, and they produce toxic compounds that are constitutively secreted in the surrounding waters.

Selective Grazing by a Tropical Copepod (Notodiaptomus iheringi) Facilitates Microcystis Dominance

Top-down grazer control of cyanobacteria is a controversial topic due to conflicting reports of success and failure as well as a bias toward studies in temperate climates with large generalist grazers like Daphnia. In the tropical lowland lakes of Brazil, calanoid copepods of the Notodiaptomus complex dominate zooplankton and co-exist in high abundance with permanent blooms of toxic cyanobacteria, raising questions for grazer effects on bloom dynamics (i.e., top-down control vs. facilitation of cyanobacterial dominance). Accordingly, the effect of copepod grazing on the relative abundance of Microcystis co-cultured with a eukaryotic phytoplankton (Cryptomonas) was evaluated in a series of 6-day laboratory experiments. Grazer effects were tested in incubations where the growth of each phytoplankton in the presence or absence of the copepod Notodiaptomus iheringi was monitored in 1 L co-cultures, starting with a 6-fold initial dominance of Cryptomonas by biomass. Compared to the no grazer controls, N. iheringi reduced the growth of both phytoplankton, but Cryptomonas growth was reduced to negative values while Microcystis growth continued positively despite grazers. Hence, in a matter of 6 days selective grazing by N. iheringi increased the biomass of Microcystis relative to Cryptomonas by an order of magnitude compared to controls, and thus, facilitated the dominance of this cyanobacterium. To account for the potential effect of allelopathy, we performed a secondary experiment comparing the abundance and growth rate of Microcystis and Cryptomonas in single and mixed co-cultures in the absence of grazers. The growth rate of Microcystis was unaffected by the presence or relative abundance of Cryptomonas, and vice versa, indicating no allelopathic effects. Our results suggest that selectively grazing zooplankton may facilitate cyanobacteria blooms by grazing on their eukaryotic phytoplankton competitors in nature. Given that selective grazers predominate zooplankton biomass in warmer waters, grazer facilitation of blooms may be a common but poorly understood regulator of plankton dynamics in a warmer and more eutrophic world.

Allelopathic and Bloom-Forming Picocyanobacteria in a Changing World

Picocyanobacteria are extremely important organisms in the world's oceans and freshwater ecosystems. They play an essential role in primary production and their domination in phytoplankton biomass is common in both oligotrophic and eutrophic waters. Their role is expected to become even more relevant with the effect of climate change. However, this group of photoautotrophic organisms still remains insufficiently recognized. Only a few works have focused in detail on the occurrence of massive blooms of picocyanobacteria, their toxicity and allelopathic activity. Filling the gap in our knowledge about the mechanisms involved in the proliferation of these organisms could provide a better understanding of aquatic environments. In this review, we gathered and described recent information about allelopathic activity of picocyanobacteria and occurrence of their massive blooms in many aquatic ecosystems. We also examined the relationships between climate change and representative picocyanobacterial genera from freshwater, brackish and marine ecosystems. This work emphasizes the importance of studying the smallest picoplanktonic fractions of cyanobacteria.