Potent Allelochemicals Research Articles

Secondary Metabolites from Australian Lichens Ramalina celastri and Stereocaulon ramulosum Affect Growth and Metabolism of Photobiont Asterochloris erici through Allelopathy.

In the present work, the phytotoxic effects of secondary metabolites extracted from lichen Ramalina celastri (usnic acid) and lichen Stereocaulon ramulosum (a naturally occurring mixture of atranorin and perlatolic acid, approx. 3:1) on cultures of the aposymbiotically grown lichen photobiont Asterochloris erici were evaluated. Algae were cultivated on the surface of glass microfiber disks with applied crystals of lichen extracts for 14 days. The toxicity of each extract was tested at the two selected doses in quantities of 0.01 mg/disk and 0.1 mg/disk. Cytotoxicity of lichen extracts was assessed using selected physiological parameters, such as growth (biomass production) of photobiont cultures, content of soluble proteins, chlorophyll a fluorescence, chlorophyll a integrity, contents of chlorophylls and total carotenoids, hydrogen peroxide, superoxide anion, TBARS, ascorbic acid (AsA), reduced (GSH) and oxidized (GSSG) glutathione, and composition of selected organic acids of the Krebs cycle. The application of both tested metabolic extracts decreased the growth of photobiont cells in a dose-dependent manner; however, a mixture of atranorin and perlatolic acid was more effective when compared to usnic acid at the same dose tested. A higher degree of cytotoxicity of extracts from lichen S. ramulosum when compared to identical doses of extracts from lichen R. celastri was also confirmed by a more pronounced decrease in chlorophyll a fluorescence and chlorophyll a integrity, decreased content of chlorophylls and total carotenoids, increased production of hydrogen peroxide and superoxide anion, peroxidation of membrane lipids (assessed as TBARS), and a strong decrease in non-enzymatic antioxidants such as AsA, GSH, and GSSG. The cytotoxicity of lichen compounds was confirmed by a strong alteration in the composition of selected organic acids included in the Krebs cycle. The increased ratio between pyruvic acid and citric acid was a very sensitive parameter of phytotoxicity of lichen secondary metabolites to the algal partner of symbiosis. Secondary metabolites of lichens are potent allelochemicals and play significant roles in maintaining the balance between mycobionts and photobionts, forming lichen thallus.

Effect of Aboveground Aqueous Extracts of Anthemis Cotula on the Growth and Development of Certain Important Crops

Anthemis cotula (Asteraceae) is a dominant invader in the agroecosystems in Kashmir Himalaya, India. However, the extent of its allelopathic effects on different crops Is particularly unknown. The present study evaluated the allelopathic potential of aboveground extract (0.5 to 4% concentration) of A. cotula on the growth performance of four important crops (viz. Avena fatua, Oryza sativa, Triticum aestivum and Vigna radiata) in a laboratory bioassay. In addition, pH, electrical conductivity (EC) and total phenolic content (TPC) of the extracts were also measured. In this study, the extracts (0.5% to 4% concentration) of aboveground part of A. cotula significantly (P<0.05) affected the growth characteristics of the test plants in a dose-dependent manner. Radicle length, coleoptile / plumule length, dry weight, and chlorophyll content of the test plants were found to be reduced by 30–77% with the increasing extract concentration. Moreover, the responses observed were species-specific, where O. sativa was found to be most sensitive whereas T. aestivum reflected relatively less reduction in different growth parameters in response to the aboveground extracts. Besides pH, EC and TPC contents also showed significant (P<0.05) variation with the increasing concentration of the extract. Correlation analysis revealed a positive relationship between pH and the plant growth responses whereas negative relationships were observed between EC and TPC with growth parameters, particularly dry weight and chlorophyll content of different plant species. Based on the observations of the study, it can be inferred that A. cotula has substantial phytotoxic effects on the agricultural crops, especially O. sativa and A. fatua, by the leachation of potent allelochemicals.

Allelopathy and its application as a weed management tool: A review.

Weeds are a serious threat to crop production as they interfere with the crop growth and development and result in significant crop losses. Weeds actually cause yield loss higher than any other pest in crop production. As a result, synthetic herbicides have been widely used for weed management. Heavy usage of synthetic herbicides, however, has resulted in public concerns over the impact of herbicides on human health and the environment. Due to various environmental and health issues associated with synthetic herbicides, researchers have been exploring alternative environmentally friendly means of controlling weed. Among them, incorporating allelopathy as a tool in an integrated weed management plan could meaningfully bring down herbicide application. Allelopathy is a biological phenomenon of chemical interaction between plants, and this phenomenon has great potential to be used as an effective and environmentally friendly tool for weed management in field crops. In field crops, allelopathy can be applied through intercropping, crop rotation, cover crops, mulching and allelopathic water extracts to manage weeds. Accumulating evidence indicates that some plant species possess potent allelochemicals that have great potential to be the ecofriendly natural herbicides. This review is intended to provide an overview of several allelopathic species that release some form of the potent allelochemical with the potential of being used in conventional or organic agriculture. Further, the review also highlights potential ways allelopathy could be utilized in conventional or organic agriculture and identify future research needs and prospects. It is anticipated that the phenomenon of allelopathy will be further explored as a weed management tool, and it can be a part of a sustainable, ecological, and integrated weed management system.

Allelopathic effects of Mangifera indica leaves on the growth performance of Brassica rapa ‘Chinensis’ (Pak choi)

Mangoes are a popular home garden crop in Brunei Darussalam, and their leaf litter is often used as compost, which is an environmentally friendly approach to manage organic waste. This study was conducted in 2018 at Universiti Brunei Darussalam to investigate the allelopathic effects of mango leaves on the growth of a leafy vegetable, Pak choi. Powdered and aqueous extract of mango leaves at various mango leaf concentrations (0–30%) were prepared and mixed with potted soils. Seedling survival, number of leaves and relative growth rates (based on stem diameter and shoot height) of Pak choi was not significantly affected by mango leaf treatment types and concentrations. Irrespective of mango leaf treatment types (powder vs. aqueous extract), biomass allocations to leaves (LMR) and roots (RMR), but not stems (SMR), were significantly inhibited at 10% and 20% mango leaf concentrations, respectively. However, stimulation in LMR and RMR of Pak choi was significantly observed in the opposite mango leaf concentrations. The aqueous mango leaf extract significantly stimulated specific leaf area (SLA) of Pak choi, suggesting that it probably contain less potent allelochemicals compared to the leaf powder, which inhibited SLA by almost 100%. Number of leaves and relative growth rates of Pak choi were not significantly affected by mango leaves. The findings suggest that aqueous mango leaf extracts at 20% could possibly be used in a sustainable organic leafy vegetable farming due to its growth stimulation in LMR and SLA. However, further research on other leafy vegetables and in field conditions are necessary due to the inhibitory allelopathic potentials of mango leaves.

Enhanced Biocontrol of Cucumber Fusarium Wilt by Combined Application of New Antagonistic Bacteria Bacillus amyloliquefaciens B2 and Phenolic Acid-Degrading Fungus Pleurotus ostreatus P5.

Continuous monoculture of cucumber (Cucumis sativus L.) typically leads to the frequent incidence of Fusarium wilt caused by Fusarium oxysporum f. sp. cucumerinum (FOC). As potent allelochemicals, phenolic acids are believed to be associated with soilborne diseases. This study aimed to investigate the effect of single or co-inoculation of antagonistic bacteria Bacillus amyloliquefaciens B2 and phenolic acid-degrading fungus Pleurotus ostreatus P5 on the suppression of cucumber Fusarium wilt. The strain B2 was identified as B. amyloliquefaciens based on biochemical, physiological, and 16S rDNA and gyrB gene sequence analyses. Strain B2 showed indole-3-acetic acid (IAA) and siderophore production and phosphate solubilization in in vitro assays. Scanning electron microscope (SEM) imaging showed the ability of strain B2 to adhere to the root surface of cucumber. P. ostreatus P5 could effectively degrade mixed phenolic acids as its sole source of carbon and energy for growth in liquid medium. In a pot experiment, four treatments were established as follows: (1) CK, uninoculated control; (2) B2, inoculation of strain B2; (3) P5, inoculation of strain P5; and (4) B2 + P5, co-inoculation of strain B2 and strain P5. At the end of the 60-day pot experiment, the B2, P5, and B2 + P5 treatments significantly reduced disease incidence by 48.1, 22.2, and 63.0%, respectively, compared to the CK treatment (p < 0.05). All three inoculation treatments significantly increased the growth of cucumber seedlings and suppressed the FOC population compared to the control (p < 0.05). High-performance liquid chromatography (HPLC) analysis showed that total phenolic acids were decreased by 18.9, 35.9, and 63.2% in the B2, P5, and B2 + P5 treatments, respectively. The results from this study suggest that combined application of B. amyloliquefaciens B2 and P. ostreatus P5 could be a promising strategy for suppressing Fusarium wilt and improving plant growth of cucumber seedlings under continuous cropping conditions.

Nature of phytotoxic interference of alien weed ‘Calyptocarpus vialis’ against some crop plants

Calyptocarpus vialis (syn. Synedrella vialis; Asteraceae), a native of the tropical Americas, has acquired an invasive status in the eastern Asia and Africa and, of late, in India. It is an annual herbaceous weed that forms a dominant ground cover due to its prostrate expansion and interferes with the growth of other plant species. However, the reasons for this interference are largely unknown. Therefore, we examined the allelopathic interference of C. vialis via leachation and residue degradation on the emergence, growth, and development of three crop species (Brassica nigra, Triticum aestivum, and Avena sativa). In a laboratory bioassay, the leachates (0.5-4%) of C. vialis exhibited a dose-dependent inhibitory effect on various growth parameters of the test plants. Similarly, under screenhouse, C. vialis-amended soil (1-4%) affected the growth of test species in a dose-dependent manner. Further, the phytotoxicity of the residues of C. vialis was examined using rhizospheric soil (RS) and residue-amended soil (RAS). It was observed that RAS exerted the maximum allelopathic effect on the test species accompanied by significant changes in pH, electrical conductivity, and total water-soluble phenolic content, as compared with the control soil (CS) and RS. Liquid chromatography and mass spectroscopy analyses confirmed the presence of eleven allelochemicals as the major phytotoxins. The study demonstrated that C. vialis exerts strong phytotoxic effects on other plants through the release of potent allelochemicals, both via leachation and residue degradation.

Flavonoid levels rather than soil nutrients is linked with Fusarium community in the soybean [Glycine max(L.) Merr.] rhizosphere under consecutive monoculture

As potent allelochemicals, flavonoids are believed to be associated with the development of soil-borne diseases. Fusarium species infection is responsible for soybean [Glycine max(L.) Merr.] root rot. However, it is uncertain if and how flavonoids influence rhizosphere Fusarium communities under consecutive soybean monoculture. Quantitative real-time PCR and pyrosequencing were used to study the Fusarium community. Spectrophotometric techniques and ultra-performance liquid chromatography (UPLC) were used to quantify flavonoids and daidzein and genistein levels, respectively. In field soil, the sizes of Fusarium community were higher from 6 to 13 years monoculture than from 1 to 3 years. The abundance of Fusarium community was significantly positively correlated with rhizosphere concentrations of the flavonoids and isoflavonoids daidzein and genistein levels. Consecutive monocultured soil selectively inhibited or stimulated certain Fusarium species, F. oxysporum remaining the most abundant. The application of daidzein and a mixture of daidzein and genistein to soil affected the Fusarium community depending on the incubation time and ranging from inhibition to promotion over time. Consecutive soybean monoculture results in shifts in the composition and size of rhizosphere Fusarium community. The Fusarium community was strongly influenced by total flavonoids, in particular daidzein and genistein, rather than soil properties.

Potent phenolic allelochemicals from Celosia argentea var. cristata L leaf extract based on bioactive fractions

Potent phenolic allelochemicals from Celosia argentea var. cristata L leaf extract based on bioactive fractions

Allelopathic Effect of Ficus benjamina Leaf Extract, Litter, and Mulch on Germination and Growth of Sunflower

Abstract Allelopathy is an important biological process, which has direct or indirect effects on the germination and growth potentials of plants. Awareness about the allelopathic properties of plants which prevail in agricultural systems can help growers to amend crop cultivation patterns accordingly. In this study, we evaluated the allelopathic effects of Ficus benjamina on germination and early seedling growth of four hybrids of sunflower (Oliver, Parsun-3, SFH-80 and NK-S-278). Ethanolic and hot-water aqueous extracts from leaves, while litter and mulches of the test allelopathic plant significantly reduced germination, radicle and hypocotyle growth of sunflower. Germination percentage was drastically reduced in all the four sunflower hybrids by ethanolic, hot-water and litter extracts; however, compared to control, mulching assay significantly increased germination in hybrids Oliver (76%), Parsun-3 (42%), SFH-80 (78%) and NK-S-278 (30%) at 2, 4, 8 and 12g extract concentration, respectively. Hypocotyle and radicle length of test hybrids were significantly reduced in each assay type. Among tested assays, ethanolic extracts revealed more drastic effects on the studied parameters than hot-water, litter, and mulching. Sunflower hybrid NK-S-278 was more severely affected, while Parsun-3 exhibited resistance to the allelopathic stress. Inhibitory effects were more prominent with increasing concentration of the extracts. The order of the phytotoxic effects of tested bioassays was ethanolic extract˃ hot-water˃ litter˃ mulching. The study suggested that Ficus leaves may possess potent allelochemicals with growth inhibitory effects on sunflower seedlings. It is suggested that further study might be required to check the allelopathic effect of Ficus benjamina on germination and growth of these sunflower hybrids in field conditions.

Utilizing the Allelopathic Potential of Brassica Species for Sustainable Crop Production: A Review

Sustainable crop production under changing climate is crucial to feed the increasing population of the world. Efforts are underway to discover novel strategies to ensure global food security. Allelopathy is one such phenomenon that can help in this regard. It is a direct or indirect and positive or negative effect of plant species on other plant species and microorganisms, through the release of secondary metabolites known as allelochemicals. Brassica species are well known for their allelopathic potential as most of them endogenously produce potent allelochemicals such as glucosinolates, allyl isothiocyanates, and brassinosteroids. These allelochemicals are highly phytotoxic to target species when released at high concentrations and, therefore, affect their growth and development. This review illustrates the potential role of Brassica allelopathy for crop production in modern agriculture. Allelopathic potential of Brassica species can be utilized for weed management by using them as cover crops, companion crops, and intercrops, for mulching and residue incorporation, or simply by including them in crop rotations. Similarly, the expression of allelochemicals from these species have great value in the management of crop pests and diseases, and abiotic stresses. Most of these allelochemicals can also act as crop growth promoters when released or applied at low concentrations. Although the use of chemical herbicides, pesticides, and synthetic growth regulators is currently inevitable for crop production, the use of ecological options like allelopathy may help in achieving global food security sustainably. Exploring the potential of Brassica allelopathy could be promising in achieving higher productivity without compromising the environmental safety.

In vitro micropropagation and allelopathic effect of lantana (Lantana camara L.)

The invasive plant, lantana (Lantana camara L.), is well known as a traditional medicinal plant and it may become important in the development of modern drugs. Lantana has long been touted as containing potent allelochemicals and in vitro-produced tissues may be appropriate sources for the production and isolation of bioactive compounds. In this research, effective techniques for shoot multiplication and root and callus induction were developed and the allelopathic efficiency of in vitro leaf and callus was examined. The optimized medium for shoot multiplication was Murashige and Skoog (MS) medium supplemented with 12.0–20.0 μM thidiazuron. For rooting, high root numbers were obtained on MS medium containing 5.0 or 10.0 μM 1-naphthalene acetic acid (NAA). In addition, the highest relative growth rate of callus was achieved when lantana leaf was cultured on NB medium (MS medium with 21.5 μM NAA and 22.5 μM N6-benzyladenine). For allelopathic effects, the results suggested high potential activity of lantana leaf and callus that was able to variably inhibit the seed germination and seedling growth of all four test species. Leaf and callus extract had no significant effect on the germination of Brassica campestris var. chinensis. Callus extract showed superior ability to suppress germination for Ipomoea aquatica Forsk. and Zea mays L. but inferior inhibition ability for Sorghum bicolor L. These results suggested that the extract of lantana in vitro leaf and callus will be an interesting natural source for further study to develop natural herbicides.

Cyanobacterial Allelochemicals But Not Cyanobacterial Cells Markedly Reduce Microbial Community Diversity.

The freshwater cyanobacterium Phormidium sp. LEGE 05292 produces allelochemicals, including the cyclic depsipeptides portoamides, that influence the growth of heterotrophic bacteria, cyanobacteria, and eukaryotic algae. Using 16S rRNA gene amplicon metagenomics, we show here that, under laboratory conditions, the mixture of metabolites exuded by Phormidium sp. LEGE 05292 markedly reduces the diversity of a natural planktonic microbial community. Exposure of the same community to the portoamides alone resulted in a similar outcome. In both cases, after 16 days, alpha-diversity estimates for the allelochemical-exposed communities were less than half of those for the control communities. Photosynthetic organisms, but also different heterotrophic-bacteria taxa were found to be negatively impacted by the allelochemicals. Intriguingly, when Phormidium sp. LEGE 05292 was co-cultured with the microbial community, the latter remained stable and closer to non-exposed than to allelochemical-exposed communities. Overall, our observations indicate that although under optimal growth conditions Phormidium sp. LEGE 05292 is able to synthesize potent allelochemicals that severely impact different microorganisms, its allelopathic effect is not pronounced when in contact with a complex microbial community. Therefore, under ecologically relevant conditions, the allelopathic behavior of this cyanobacterium may be regulated by nutrient availability or by interactions with the surrounding microbiota.

Insights into the Mechanism of Proliferation on the Special Microbes Mediated by Phenolic Acids in the Radix pseudostellariae Rhizosphere under Continuous Monoculture Regimes.

As potent allelochemicals, phenolic acids are believed to be associated with replanting disease and cause microflora shift and structural disorder in the rhizosphere soil of continuously monocultured Radix pseudostellariae. The transcriptome sequencing was used to reveal the mechanisms underlying the differential response of pathogenic bacterium Kosakonia sacchari and beneficial bacterium Bacillus pumilus on their interactions with phenolic acids, the main allelochemicals in root exudates of R. pseudostellariae in the monoculture system. The microbes were inoculated in the pots containing soil and the medicinal plant in this study. The results showed that the addition of beneficial B. pumilus to the 2-year planted soil significantly decreased the activity of soil urease, catalase, sucrase, and cellulase and increased the activity of chitinase compared with those in the 2nd-year monocropping rhizosphere soil without any treatment. However, opposite results were obtained when K. sacchari was added. Transcriptome analysis showed that vanillin enhanced glycolysis/gluconeogenesis, fatty acid biosynthesis, pentose phosphate, bacterial chemotaxis, flagellar assembly, and phosphotransferase system pathway in K. sacchari. However, protocatechuic acid, a metabolite produced by K. sacchari from vanillin, had negative effects on the citrate cycle and biosynthesis of novobiocin, phenylalanine, tyrosine, and tryptophan in B. pumilus. Concurrently, the protocatechuic acid decreased the biofilm formation of B. pumilus. These results unveiled the mechanisms how phenolic acids differentially mediate the shifts of microbial flora in rhizosphere soil, leading to the proliferation of pathogenic bacteria (i.e., K. sacchari) and the attenuation of beneficial bacteria (i.e., B. pumilus) under the monocropping system of R. pseudostellariae.

Effect of benzoic acid on soil microbial communities associated with soilborne peanut diseases

As potent allelochemicals, phenolic acids are believed to be associated with soilborne diseases, and can influence plant-microbe interactions. Benzoic acid (BA) is one major phenolic acid found in peanut (Arachis hypogaea) root exudates. The objectives of this study were to estimate the BA degradation in the soil and its effects on soil bacterial and fungal communities and to detect the specific taxa responding to BA amendment. BA degradation was investigated by monitoring the BA retained in the soil using high-performance liquid chromatography (HPLC) and the CO2 production rate using gas chromatography (GC). The abundance and diversity of the bacterial and fungal communities were investigated by quantitative real-time PCR (qPCR) and Illumina MiSeq sequencing. The results showed that the BA concentration decreased significantly with an increased rate of CO2 production during the first 36h after amendment, implying that the BA in the soil was quickly metabolized by the microbes. Quantitative PCR analysis further detected a significant increase in soil bacterial and fungal abundances in response to BA addition, but a reduced bacteria-to-fungi ratio. As a result of BA amendment, the relative abundance of Fusarium, Bionectria and Trichoderma was markedly increased, whereas Metarhizium was reduced. Moreover, BA (0.1mmolL−1) promoted the mycelial growth, sporulation capacity and conidial germination of the peanut root rot pathogen Fusarium sp. in vitro. Among bacteria, the relative abundance of Betaproteobacteria was significantly increased in response to BA treatment, whereas the relative abundances of AD3 and Actinobacteria were reduced. A deeper taxonomic analysis of the Betaproteobacteria taxa showed a great increase in the abundance of the genus Burkholderia, from 0.71% to 10.12%, in response to BA amendment. Constructing clone libraries with the partial 16S rRNA genes of Burkholderia further demonstrated that BA amendment had modified the Burkholderia species composition. Our results highlight that the effects of BA in the soil are reflected by changes in populations of soil microbes and suggest that the response of specific microbes such as Fusarium and Burkholderia to BA might be associated with the development of soilborne diseases in monocultures.

Substituent effects of cis-cinnamic acid analogues as plant growh inhibitors.

1-O-cis-Cinnamoyl-β-D-glucopyranose is one of the most potent allelochemicals that has been isolated from Spiraea thunbergii Sieb by Hiradate et al. It derives its strong inhibitory activity from cis-cinnamic acid (cis-CA), which is crucial for phytotoxicity. By preparing and assaying a series of cis-CA analogues, it was previously found that the key features of cis-CA for lettuce root growth inhibition are a phenyl ring, cis-configuration of the alkene moiety, and carboxylic acid. On the basis of a structure-activity relationship study, the substituent effects on the aromatic ring of cis-CA were examined by systematic synthesis and the lettuce root growth inhibition assay of a series of cis-CA analogues having substituents on the aromatic ring. While ortho- and para-substituted analogues exhibited low potency in most cases, meta-substitution was not critical for potency, and analogues having a hydrophobic and sterically small substituent were more likely to be potent. Finally, several cis-CA analogues were found to be more potent root growth inhibitors than cis-CA.

Key structural features of cis-cinnamic acid as an allelochemical.

1-O-cis-cinnamoyl-β-D-glucopyranose is one of the most potent allelochemicals isolated from Spiraea thunbergii Sieb. It is suggested that it derives its strong inhibitory activity from cis-cinnamic acid, which is crucial for phytotoxicity. It was synthesized to confirm its structure and bioactivity, and also a series of cis-cinnamic acid analogues were prepared to elucidate the key features of cis-cinnamic acid for lettuce root growth inhibition. The cis-cyclopropyl analogue showed potent inhibitory activity while the saturated and alkyne analogues proved to be inactive, demonstrating the importance of the cis-double bond. Moreover, the aromatic ring could not be replaced with a saturated ring. However, the 1,3-dienylcyclohexene analogue showed strong activity. These results suggest that the geometry of the C-C double bond between the carboxyl group and the aromatic ring is essential for potent inhibitory activity. In addition, using several light sources, the photostability of the cinnamic acid derivatives and the role of the C-C double bond were also investigated.

Screening of 170 Peruvian plant species for allelopathic activity by using the Sandwich Method

Peru is one of the 20 botanically extremely diverse countries in the world, with &gt;17 000 flowering plants, of which 30% are endemic. So far, no systematic research has been conducted on the screening of the allelopathic plants. In this study, the allelopathic activity of 170 species from 61 families of Peruvian plants that were collected from the three main regions of Peru – the Costa (Pacific coastline), the Sierra (Andean mountains), and the Selva (Amazonian rainforest) – was evaluated. The allelopathic activity was determined by the Sandwich Method, which can evaluate the activity of leaf leachates. The species that were found to be highly inhibitory in this screening, under the criterion of &gt;90% inhibition of the radicle of lettuce (Lactuca sativa) seedlings, were Aristeguietia ballii and Diplostephium foliosissimum (Asteraceae) and Spondias mombin (Anacardiaceae). All of these species are native plants from Peru. This study gives a strong clue regarding the potential of isolating potent allelochemicals from these plants in the future.

Macroalgal terpenes function as allelopathic agents against reef corals

During recent decades, many tropical reefs have transitioned from coral to macroalgal dominance. These community shifts increase the frequency of algal-coral interactions and may suppress coral recovery following both anthropogenic and natural disturbance. However, the extent to which macroalgae damage corals directly, the mechanisms involved, and the species specificity of algal-coral interactions remain uncertain. Here, we conducted field experiments demonstrating that numerous macroalgae directly damage corals by transfer of hydrophobic allelochemicals present on algal surfaces. These hydrophobic compounds caused bleaching, decreased photosynthesis, and occasionally death of corals in 79% of the 24 interactions assayed (three corals and eight algae). Coral damage generally was limited to sites of algal contact, but algae were unaffected by contact with corals. Artificial mimics for shading and abrasion produced no impact on corals, and effects of hydrophobic surface extracts from macroalgae paralleled effects of whole algae; both findings suggest that local effects are generated by allelochemical rather than physical mechanisms. Rankings of macroalgae from most to least allelopathic were similar across the three coral genera tested. However, corals varied markedly in susceptibility to allelopathic algae, with globally declining corals such as Acropora more strongly affected. Bioassay-guided fractionation of extracts from two allelopathic algae led to identification of two loliolide derivatives from the red alga Galaxaura filamentosa and two acetylated diterpenes from the green alga Chlorodesmis fastigiata as potent allelochemicals. Our results highlight a newly demonstrated but potentially widespread competitive mechanism to help explain the lack of coral recovery on many present-day reefs.

Isolation of activity and partial characterization of large non-proteinaceous lytic allelochemicals produced by the marine dinoflagellate Alexandrium tamarense

Certain strains of the toxigenic dinoflagellate Alexandrium tamarense produce potent allelochemicals with lytic activity against a wide variety of marine microorganisms. Our efforts to characterize these allelochemicals from a lytic strain focused on the less polar components because of their higher lytic activity. Fractionation and partial purification after solid phase extraction (SPE) were achieved via alternative chromatographic methods, namely HPLC separation on C8 and HILIC phases. Through MALDI-TOF mass spectrometry we compared the mass differences in SPE, C8 HPLC, and HILIC fractions between a lytic and non-lytic strain of A. tamarense. Several large species with masses between 7 kDa and 15 kDa were found in the HILIC lytic fraction by MALDI-TOF MS. Tryptic digestion and tryptic digestion-coupled size-exclusion chromatography (SEC) suggested that the lytic compounds are large non-proteinaceous molecules (<23.3 kDa, trypsin). Although there is no direct proof that the large molecules found in the lytic HILIC fraction are responsible for the lytic activity of this fraction, the mass range deduced from SEC strongly supports this hypothesis. Total sugar content analysis showed that the lytic HILIC fraction contained two-fold more sugar than the non-lytic one. Nevertheless, the low percentage of saccharide per dry mass equivalent (0.18 ± 0.01%) indicates that sugar residues are likely not a major component of the lytic compounds. We concluded that at least one group of lytic allelochemicals produced by A. tamarense comprise a suite of large non-proteinaceous and probably non-polysaccharide compounds between 7 kDa and 15 kDa.

Allelochemicals produced by Caribbean macroalgae and cyanobacteria have species-specific effects on reef coral microorganisms

Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited >50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo- and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality.