Daily bursts of biogenic cyanogen bromide (BrCN) control biofilm formation around a marine benthic diatom

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

Emission of marine volatile organic compounds (VOCs) by phytoplankton- a review

Oceans cover over 71% of the Earth's surface and play crucial roles in regulating the global climate. In the marine boundary layer, the levels of volatile organic compounds (VOCs) have been shown to have positive relations with the marine algal biomass, indicating that the marine biological activities can be an important biogenic VOCs (BVOCs) source. The emitted BVOCs will enhance the formation of secondary organic aerosols, and perturb the radiative forcing, which ultimately affects the climate. To date, knowledge on the emission processes (i.e., synthesis processes and emission rates) of BVOCs from marine phytoplankton is still lacking compared to the more well-known BVOCs released from terrestrial plants. In this review, we focus on the BVOCs emissions from the marine phytoplankton. Based on the available literature from field and laboratory studies, we listed the types of BVOCs being emitted by different marine phytoplankton species, summarized the diversity of BVOCs related to phytoplankton taxonomy and physiology and abiotic factors affecting their emissions in various marine environments, and discussed the biosynthesis and ecological function of important marine VOCs such as DMS, terpenoids and VHCs from phytoplankton. Finally, we highlighted the existing gaps in the current knowledge and the needs of future study for better understanding the physiological and ecological roles of BVOCs emission from marine phytoplankton.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Volatile organic compounds in aquatic ecosystems - Detection, origin, significance and applications

Volatile organic compounds (VOCs) include a broad range of compounds. Their production influences a large number of processes, having direct and secondary effects on different fields, such as climate change, economy and ecology. Although our planet is primarily covered with water (~70% of the globe surface), the information on aquatic VOCs, compared to the data available for the terrestrial environments, is still limited. Regardless of the difficulty in collecting and analysing data, because of their extreme complexity, diversification and important spatial-temporal emission variation, it was demonstrated that aquatic organisms are able to produce a variety of bioactive compounds. This production happens in response to abiotic and biotic stresses, evidencing the fundamental role of these metabolites, both in terms of composition and amount, in providing important ecological information and possible non-invasive tools to monitor different biological systems. The study of these compounds is an important and productive task with possible and interesting impacts in future practical applications in different fields. This review aims to summarize the knowledge on the aquatic VOCs, the recent advances in understanding their diverse roles and ecological impacts, the generally used methodology for their sampling and analysis, and their enormous potential as non-invasive, non-destructive and financeable affordable real-time biomonitoring tool, both in natural habitats and in controlled industrial situations. Finally, the possible future technical applications, highlighting their economic and social potential, such as the possibility to use VOCs as valuable alternative source of chemicals and as biocontrol and bioregulation agents, are emphasized.

Microalgae show a range of responses to exometabolites of foreign species

Studies on microalgae interspecific interactions have so far focused either on nutrient competition or allelopathic effects due to excreted substances from Harmful Algal Bloom (HAB) species. Evidence from plants, bacteria and specific microalgae groups, point to a range of responses mediated by sensing or direct chemical impact of exometabolites from foreign species. Such processes remain under-investigated, especially in non-HAB microalgae, despite the importance of such knowledge in ecology and industrial applications. Here, we study the directional effect of exometabolites of 4 "foreign" species Heterosigma akashiwo, Phaeocystis sp., Tetraselmis sp. and Thalassiosira sp. to each of three "target" species across a total of 12 treatments. We disentangle these effects from nutrient competition by adding cell free medium of each "foreign" species into our treatment cultures. We measured the biomass response, to the foreign exometabolites, as cell number and photosynthetic biomass (Chla), whereas nutrient use was measured as residual phosphorus (PO4) and intracellular phosphorus (P). Exometabolites from filtrate of foreign species were putatively annotated by untargeted metabolomics analysis and were discussed in association to observed responses of target species. Among others, these metabolites included L-histidinal, Tiliacorine and dimethylsulfoniopropionate (DMSP). Our findings show that species show a range of responses with the most common being biomass suppression, and less frequent biomass enhancement and intracellular P storage. Filtrate from the green microalgae Tetraselmis caused the most pronounced negative effects suggesting that non-HAB species can also cause negative chemical interference. A candidate metabolite inducing this response is L-histidinal which was measured in high abundance uniquely in Tetraselmis and its L-histidine form derived from bacteria was previously confirmed as a microalgal algicidal. H. akashiwo also induced biomass suppression on other microalgae and a candidate metabolite for this response is Tiliacorine, a plant-derived alkaloid with confirmed cytotoxic activity.

Biochemical Characterization of a Novel Redox-Regulated Metacaspase in a Marine Diatom

Programmed cell death (PCD) in marine microalgae was suggested to be one of the mechanisms that facilitates bloom demise, yet its molecular components in phytoplankton are unknown. Phytoplankton are completely lacking any of the canonical components of PCD, such as caspases, but possess metacaspases. Metacaspases were shown to regulate PCD in plants and some protists, but their roles in algae and other organisms are still elusive. Here, we identified and biochemically characterized a type III metacaspase from the model diatom Phaeodactylum tricornutum, termed PtMCA-IIIc. Through expression of recombinant PtMCA-IIIc in E. coli, we revealed that PtMCA-IIIc exhibits a calcium-dependent protease activity, including auto-processing and cleavage after arginine. Similar metacaspase activity was detected in P. tricornutum cell extracts. PtMCA-IIIc overexpressing cells exhibited higher metacaspase activity, while CRISPR/Cas9-mediated knockout cells had decreased metacaspase activity compared to WT cells. Site-directed mutagenesis of cysteines that were predicted to form a disulfide bond decreased recombinant PtMCA-IIIc activity, suggesting its enhancement under oxidizing conditions. One of those cysteines was oxidized, detected in redox proteomics, specifically in response to lethal concentrations of hydrogen peroxide and a diatom derived aldehyde. Phylogenetic analysis revealed that this cysteine-pair is unique and widespread among diatom type III metacaspases. The characterization of a cell death associated protein in diatoms provides insights into the evolutionary origins of PCD and its ecological significance in algal bloom dynamics.

GPCR Genes as Activators of Surface Colonization Pathways in a Model Marine Diatom

Surface colonization allows diatoms, a dominant group of phytoplankton in oceans, to adapt to harsh marine environments while mediating biofoulings to human-made underwater facilities. The regulatory pathways underlying diatom surface colonization, which involves morphotype switching in some species, remain mostly unknown. Here, we describe the identification of 61 signaling genes, including G-protein-coupled receptors (GPCRs) and protein kinases, which are differentially regulated during surface colonization in the model diatom species, Phaeodactylum tricornutum. We show that the transformation of P. tricornutum with constructs expressing individual GPCR genes induces cells to adopt the surface colonization morphology. P. tricornutum cells transformed to express GPCR1A display 30% more resistance to UV light exposure than their non-biofouling wild-type counterparts, consistent with increased silicification of cell walls associated with the oval biofouling morphotype. Our results provide a mechanistic definition of morphological shifts during surface colonization and identify candidate target proteins for the screening of eco-friendly, anti-biofouling molecules.

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The model diatom Phaeodactylum tricornutum shifts morphology to form biofilms

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G-protein-coupled receptors (GPCRs) can modulate diatom surface colonization

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GPCR1A expression can induce biofouling morphotype and UV resistance

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Identified genes and pathways can serve as targets for anti-biofouling discoveries

New Methods, New Concepts: What Can Be Applied to Freshwater Periphyton?

Microbial interactions play an essential role in aquatic ecosystems and are of the great interest for both marine and freshwater ecologists. Recent development of new technologies and methods allowed to reveal many functional mechanisms and create new concepts. Yet, many fundamental aspects of microbial interactions have been almost exclusively studied for marine pelagic and benthic ecosystems. These studies resulted in a formulation of the Black Queen Hypothesis, a development of the phycosphere concept for pelagic communities, and a realization of microbial communication as a key mechanism for microbial interactions. In freshwater ecosystems, especially for periphyton communities, studies focus mainly on physiology, biodiversity, biological indication, and assessment, but the many aspects of microbial interactions are neglected to a large extent. Since periphyton plays a great role for aquatic nutrient cycling, provides the basis for water purification, and can be regarded as a hotspot of microbial biodiversity, we highlight that more in-depth studies on microbial interactions in periphyton are needed to improve our understanding on functioning of freshwater ecosystems. In this paper we first present an overview on recent concepts (e.g., the "Black Queen Hypothesis") derived from state-of-the-art OMICS methods including metagenomics, metatranscriptomics, and metabolomics. We then point to the avenues how these methods can be applied for future studies on biodiversity and the ecological role of freshwater periphyton, a yet largely neglected component of many freshwater ecosystems.

Trivalent Phosphorus and Phosphines as Components of Biochemistry in Anoxic Environments

Phosphorus is an essential element for all life on Earth, yet trivalent phosphorus (e.g., in phosphines) appears to be almost completely absent from biology. Instead phosphorus is utilized by life almost exclusively as phosphate, apart from a small contingent of other pentavalent phosphorus compounds containing structurally similar chemical groups. In this work, we address four previously stated arguments as to why life does not explore trivalent phosphorus: (1) precedent (lack of confirmed instances of trivalent phosphorus in biochemicals suggests that life does not have the means to exploit this chemistry), (2) thermodynamic limitations (synthesizing trivalent phosphorus compounds is too energetically costly), (3) stability (phosphines are too reactive and readily oxidize in an oxygen (O₂)-rich atmosphere), and (4) toxicity (the trivalent phosphorus compounds are broadly toxic). We argue that the first two of these arguments are invalid, and the third and fourth arguments only apply to the O₂-rich environment of modern Earth. Specifically, both the reactivity and toxicity of phosphines are specific to aerobic life and strictly dependent on O₂-rich environment. We postulate that anaerobic life persisting in anoxic (O₂-free) environments may exploit trivalent phosphorus chemistry much more extensively. We review the production of trivalent phosphorus compounds by anaerobic organisms, including phosphine gas and an alkyl phosphine, phospholane. We suggest that the failure to find more such compounds in modern terrestrial life may be a result of the strong bias of the search for natural products toward aerobic organisms. We postulate that a more thorough identification of metabolites of the anaerobic biosphere could reveal many more trivalent phosphorus compounds. We conclude with a discussion of the implications of our work for the origin and early evolution of life, and suggest that trivalent phosphorus compounds could be valuable markers for both extraterrestrial life and the Shadow Biosphere on Earth.

An Apparent Binary Choice in Biochemistry: Mutual Reactivity Implies Life Chooses Thiols or Nitrogen-Sulfur Bonds, but Not Both

A fundamental goal of biology is to understand the rules behind life's use of chemical space. Established work focuses on why life uses the chemistry that it does. Given the enormous scope of possible chemical space, we postulate that it is equally important to ask why life largely avoids certain areas of chemical space. The nitrogen-sulfur bond is a prime example, as it rarely appears in natural molecules, despite the very rich N-S bond chemistry applied in various branches of industry (e.g., industrial materials, agrochemicals, pharmaceuticals). We find that, out of more than 200,000 known, unique compounds made by life, only about 100 contain N-S bonds. Furthermore, the limited number of N-S bond-containing molecules that life produces appears to fall into a few very distinctive structural groups. One may think that industrial processes are unrelated to biochemistry because of a greater possibility of solvents, catalysts, and temperatures available to industry than to the cellular environment. However, the fact that life does rarely make N-S bonds, from the plentiful precursors available, and has evolved the ability to do so independently several times, suggests that the restriction on life's use of N-S chemistry is not in its synthesis. We present a hypothesis to explain life's extremely limited usage of the N-S bond: that the N-S bond chemistry is incompatible with essential segments of biochemistry, specifically with thiols. We support our hypothesis by (1) a quantitative analysis of the occurrence of N-S bond-containing natural products and (2) reactivity experiments between selected N-S compounds and key biological molecules. This work provides an example of a reason why life nearly excludes a distinct region of chemical space. Combined with future examples, this potentially new field of research may provide fresh insight into life's evolution through chemical space and its origin and early evolution.

Halogenated anilines as novel natural products from a marine biofilm forming microalga

A microalga produces the toxic halogenated anilines 2,4,6-tribromoaniline, 2,4,6-trichloroaniline and their dibromochloro and bromodichloro derivatives that were considered as compounds of exclusive synthetic origin.

The Alterations of Biofilm Formation and EPS Characteristics of a Diatom by a Sponge-Associated Bacterium Psychrobacter sp

A sponge-associated bacterium, which was identified as Psychrobacter sp. in this study, was found with high activity against biofilm formation of benthic diatoms, including Amphora sp., Nitzschia closterium, Nitzschia frustulum, and Stauroneis sp. The activity against diatom biofilm formation by the tested strain was confirmed mostly in the culture supernatant and could be extracted using organic solvents. Treatment with its supernatant crude extract significantly reduced the cells of Stauroneis sp. forming biofilm and slightly increased the cells floating in the culture medium, which results in the ratio of biofilm cell/floating cell altering from 0.736 in control to 0.414 in treatment. Use of the supernatant crude extract led to increased production of extracellular polymeric substances (EPSs) by diatom Stauroneis sp. from 16.66 to 41.59 (g/g cell dry weight). The increase in EPS production was mainly contributed by soluble EPS (SL-EPS) and followed by the EPS that was tightly bound to biofilm cells (BF-TB-EPS). In addition, the supernatant crude extract caused significant changes in the monosaccharides composition of the EPS of Stauroneis sp. Specifically, glucuronic acid (Glc-A) and N-acetyl-D-glucosamine (Glc-NAc) in BF-TB-EPS were 55% fold decreased and 1219% fold increased, respectively. Based on our findings, we proposed that these changes in monosaccharides composition might lead to a decreased biofilm formation efficiency of diatom.

Marine Vanadium-Dependent Haloperoxidases, Their Isolation, Characterization, and Application

Vanadium-dependent haloperoxidases in seaweeds, cyanobacteria, fungi, and possibly phytoplankton play an important role in the release of halogenated volatile compounds in the environment. These halocarbons have effects on atmospheric chemistry since they cause ozone depletion. In this chapter, a survey is given of the different sources of these enzymes, some of their properties, the various methods to isolate them, and the bottlenecks in purification. The assays to detect and quantify haloperoxidase activity are described as well as their kinetic properties. Several practical tips and pitfalls are given which have not yet been published explicitly. Recent developments in research on structure and function of these enzymes are reviewed. Finally, the application of vanadium-dependent haloperoxidases in the biosynthesis of brominated and other compounds is discussed.

Strategies and ecological roles of algicidal bacteria

In both freshwater and marine ecosystems, phytoplankton are the most dominant primary producers, contributing substantially to aquatic food webs. Algicidal bacteria that can associate to microalgae from the phytoplankton have the capability to control the proliferation and even to lyse them. These bacteria thus play an important role in shaping species composition in pelagic environments. In this review, we discuss and categorise strategies used by algicidal bacteria for the attack on microalgae. We highlight the complex regulation of algicidal activity and defence responses that govern alga-bacteria interactions. We also discuss how algicidal bacteria impact algal physiology and metabolism and survey the existing algicidal metabolites and enzymes. The review illustrates that the ecological role of algicidal bacteria is not yet fully understood and critically discusses the challenges in obtaining ecologically relevant data.

Reaction Product Variability and Biological Activity of the Lactoperoxidase System Depending on Medium Ionic Strength and pH, and on Substrate Relative Concentration

The potential of ions produced in water by the lactoperoxidase system against plant pests has shown promising results. We tested the bioactivity of ions produced by the lactoperoxidase oxidation of I^- and SCN^- in several buffers or in tap water and characterized the ions produced. In vitro biological activity was tested against Penicillium expansum, the causal agent of mold in fruits, and the major cause of patulin contamination of fruit juices and compotes. In buffers, the ionic concentration was increased 3-fold, and pathogen inhibition was obtained down to the 1:15 dilution. In tap water, the ionic concentration was weaker, and pathogen inhibition was obtained only down to the 1:3 dilution. Acidic buffer increased ion concentrations as compared to less acidic (pH 5.6 or 6.2) or neutral buffers, as do increased ionic strength. ¹³ C-labelled SCN^- and MS showed that different ions were produced in water and in buffers. In specific conditions the ion solution turned yellow and a product was formed, probably diiodothiocyanate (I₂ SCN^- ), giving an intense signal at 49.7 ppm in ¹³ C-NMR. The formation of the signal was unambiguously favored in acidic media and disadvantaged or inhibited in neutral or basic conditions. It was enhanced at a specific SCN^- : I^- ratio of 1:4.5, but decreased when the ratio was 1:2, and was inhibited at ratio SCN^- >I^- . We demonstrated that the formation of the signal required the interaction between I₂ and SCN^- , and MS showed the presence of I₂ SCN^- .

Minireview: algal natural compounds and extracts as antifoulants

Marine biofouling is a paramount phenomenon in the marine environment and causes serious problems to maritime industries worldwide. Marine algae are known to produce a wide variety of chemical compounds with antibacterial, antifungal, antialgal, and anti-macrofouling properties, inhibiting the settlement and growth of other marine fouling organisms. Significant investigations and progress have been made in this field in the last two decades and several antifouling extracts and compounds have been isolated from micro- and macroalgae. In this minireview, we have summarized and evaluated antifouling compounds isolated and identified from macroalgae and microalgae between January 2010 and June 2016. Future directions for their commercialization through metabolic engineering and industrial scale up have been discussed. Upon comparing biogeographical regions, investigations from Southeast Asian waters were found to be rather scarce. Thus, we have also discussed the need to conduct more chemical ecology based research in relatively less explored areas with high algal biodiversity like Southeast Asia.

Volatile Metabolites Emission by In Vivo Microalgae-An Overlooked Opportunity?

Fragrances and malodors are ubiquitous in the environment, arising from natural and artificial processes, by the generation of volatile organic compounds (VOCs). Although VOCs constitute only a fraction of the metabolites produced by an organism, the detection of VOCs has a broad range of civilian, industrial, military, medical, and national security applications. The VOC metabolic profile of an organism has been referred to as its 'volatilome' (or 'volatome') and the study of volatilome/volatome is characterized as 'volatilomics', a relatively new category in the 'omics' arena. There is considerable literature on VOCs extracted destructively from microalgae for applications such as food, natural products chemistry, and biofuels. VOC emissions from living (in vivo) microalgae too are being increasingly appreciated as potential real-time indicators of the organism's state of health (SoH) along with their contributions to the environment and ecology. This review summarizes VOC emissions from in vivo microalgae; tools and techniques for the collection, storage, transport, detection, and pattern analysis of VOC emissions; linking certain VOCs to biosynthetic/metabolic pathways; and the role of VOCs in microalgae growth, infochemical activities, predator-prey interactions, and general SoH.

Powdery polymer and carbon aerogels with high surface areas for high-performance solid phase microextraction coatings

A novel powdery polymer aerogel (PPA) with a hierarchical pore structure was prepared via hypercrosslinking of monodisperse poly(styrene-co-divinylbenzene) nanoparticles. Subsequently, the PPA was carbonized to obtain a powdery carbon aerogel (PCA) with a well-inherited pore structure and a much higher surface area (2354 m² g^-1). The PPA-coated and PCA-coated fibers were easily fabricated benefiting from the powdery morphologies of PPA and PCA, and demonstrated high extraction efficiencies towards hydrophobic analytes owing to their functional groups, unique three-dimensional (3D) porous nanonetworks and high surface areas.

Allelopathic interactions involving benthic phototrophic microorganisms

As a way to prevent resource depletion by other species, many phototrophic aquatic microorganisms produce inhibitory compounds. This process, known as allelopathy, has been widely studied in planktonic environments, where it is recognized as being a driving force of planktonic communities. However, in benthic environments, biofilms provide very particular micro-environments. The present review focuses on allelopathic interactions involving benthic phototrophic prokaryotes and micro-eukaryotes ('microalgae'), which generally form biofilms, and includes any interaction involving benthic microalgae either as the emitter or as the target in both marine and freshwater habitats. To support our hypothesis on the importance of allelopathy in biofilms due to the particularities of biofilms, we show that (i) reported allelopathic species and compounds are diverse and numerous in the three major groups of benthic phototrophic microorganisms, (ii) allelopathic benthic species could affect community composition, (iii) allelopathy in biofilms is currently underestimated because of the lack of suitable methods. As benthic primary producers represent an important source of organic carbon in some streams and littoral areas, these interactions could impact the whole ecosystem in these areas, probably more than in areas dominated by planktonic communities.

The possible role of bacterial signal molecules N-acyl homoserine lactones in the formation of diatom-biofilm (Cylindrotheca sp.)

Bacterial quorum sensing signal molecules N-acyl homoserine lactones (AHLs) (C10-HSL, 3-OXO-C10-HSL and 3-OH-C10-HSL) as possible chemical cues were employed to investigate the role in the formation of fouling diatom-biofilm (Cylindrotheca sp.). Results showed that AHLs promoted Chlorophyll a (Chl.a) and extracellular polymeric substance (EPS) contents in the diatom-biofilm. In the presence of AHLs-inhibitor 3, 4-Dibromo-2(5)H-furanone, which was used to avoid the possible interference of AHLs from bacteria, AHLs also increased the Chl.a and EPS contents. Scanning electron microscope and confocal laser scanning microscope analysis further demonstrated that AHLs promoted the formation of the diatom-biofilm. Non-invasive micro-test technique showed that AHLs promoted Ca(2+) efflux in Cylindrotheca sp., which implied that Ca(2+) might be correlated with AHLs-induced positive effect on the formation of diatom-biofilm. This study provides direct evidences that AHLs play an important role in developing the diatom-biofilm and AHLs-inhibitors might be promising active agents in marine antifouling.

Microbial transcriptome profiling of black band disease in a Faviid coral during a seasonal disease peak

The etiology of black band disease (BBD), a persistent, globally distributed coral disease characterized by a dark microbial mat, is still unclear. A metatranscriptomics approach was used to unravel the roles of the major mat constituents in the disease process. By comparing the transcriptomes of the mat constituents with those of the surface microbiota of diseased and healthy corals, we showed a shift in bacterial composition and function in BBD-affected corals. mRNA reads of Cyanobacteria, Bacteroidetes and Firmicutes phyla were prominent in the BBD mat. Cyanobacterial adenosylhomocysteinase, involved in cyanotoxin production, was the most transcribed gene in the band consortium. Pathogenic and non-pathogenic forms of Vibrio spp., mainly transcribing the thiamine ABC transporter, were abundant and highly active in both the band and surface tissues. Desulfovibrio desulfuricans was the primary producer of sulfide in the band. Members of the Bacilli class expressed high levels of rhodanese, an enzyme responsible for cyanide and sulfide detoxification. These results offer a first look at the varied functions of the microbiota in the disease mat and surrounding coral surface and enabled us to develop an improved functional model for this disease.

Zoosporic parasites infecting marine diatoms - A black box that needs to be opened

Living organisms in aquatic ecosystems are almost constantly confronted by pathogens. Nevertheless, very little is known about diseases of marine diatoms, the main primary producers of the oceans. Only a few examples of marine diatoms infected by zoosporic parasites are published, yet these studies suggest that diseases may have significant impacts on the ecology of individual diatom hosts and the composition of communities at both the producer and consumer trophic levels of food webs. Here we summarize available ecological and morphological data on chytrids, aphelids, stramenopiles (including oomycetes, labyrinthuloids, and hyphochytrids), parasitic dinoflagellates, cercozoans and phytomyxids, all of which are known zoosporic parasites of marine diatoms. Difficulties in identification of host and pathogen species and possible effects of environmental parameters on the prevalence of zoosporic parasites are discussed. Based on published data, we conclude that zoosporic parasites are much more abundant in marine ecosystems than the available literature reports, and that, at present, both the diversity and the prevalence of such pathogens are underestimated.

Formation of cyanogen iodide by lactoperoxidase

The haem protein lactoperoxidase (LPO) is an important component of the anti-microbial immune defence in external secretions and is also applied as preservative in food, oral care and cosmetic products. Upon oxidation of SCN(-) and I(-) by the LPO-hydrogen peroxide system, oxidised species are formed with bacteriostatic and/or bactericidal activity. Here we describe the formation of the inter(pseudo)halogen cyanogen iodide (ICN) by LPO. This product is formed when both, thiocyanate and iodide, are present together in the reaction mixture. Using (13)C nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry we could identify this inter(pseudo)halogen after applying iodide in slight excess over thiocyanate. The formation of ICN is based on the reaction of oxidised iodine species with thiocyanate. Further, we could demonstrate that ICN is also formed by the related haem enzyme myeloperoxidase and, in lower amounts, in the enzyme-free system. As I(-) is not competitive for SCN(-) under physiologically relevant conditions, the formation of ICN is not expected in secretions but may be relevant for LPO-containing products.

A Chemical Perspective on Microalgal-Microbial Interactions

The exchange of chemical compounds is central to the interactions of microalgae with other microorganisms. Although foundational for many food webs, these interactions have been poorly studied compared with higher plant-microbe interactions. Emerging insights have begun to reveal how these interactions and the participating chemical compounds shape microbial communities and broadly impact biogeochemical processes.

Metabolomics in chemical ecology

Chemical ecology elucidates the nature and role of natural products as mediators of organismal interactions. The emerging techniques that can be summarized under the concept of metabolomics provide new opportunities to study such environmentally relevant signaling molecules. Especially comparative tools in metabolomics enable the identification of compounds that are regulated during interaction situations and that might play a role as e.g. pheromones, allelochemicals or in induced and activated defenses. This approach helps overcoming limitations of traditional bioassay-guided structure elucidation approaches. But the power of metabolomics is not limited to the comparison of metabolic profiles of interacting partners. Especially the link to other -omics techniques helps to unravel not only the compounds in question but the entire biosynthetic and genetic re-wiring, required for an ecological response. This review comprehensively highlights successful applications of metabolomics in chemical ecology and discusses existing limitations of these novel techniques. It focuses on recent developments in comparative metabolomics and discusses the use of metabolomics in the systems biology of organismal interactions. It also outlines the potential of large metabolomics initiatives for model organisms in the field of chemical ecology.

Impairment of benthic diatom adhesion and photosynthetic activity by allelopathic compounds from a green alga: involvement of free fatty acids?

The role of chemical interactions in shaping microbial communities has raised increasing interest over the last decade. Many benthic microorganisms are known to develop chemical strategies to overcome competitors, but the real importance of chemical interactions within freshwater biofilm remains unknown. This study focused on the biological and chemical mechanisms of an interaction involving two benthic microorganisms, an allelopathic filamentous green alga, Uronema confervicolum, and a common diatom, Fistulifera saprophila. Our results showed that functions critical for benthic phototrophic microorganisms were inhibited by U. confervicolum extracts. Growth, cell motility, adhesion, and photosynthetic activity were impaired at extract concentrations ranging between 5 and 20 μg ml(-1). The adhesion inhibition was mediated by intracellular nitric oxide (NO) induction. A bioassay-guided fractionation of the extract with HPLC helped to identify two C18 fatty acids present in the growth-inhibiting fractions: linoleic (LA) and α-linolenic (LNA) acids. These compounds represented 77% of the total free fatty acids of U. confervicolum and were present in the culture medium (1.45 μg l(-1) in total). Both could inhibit the diatom growth at concentrations higher than 0.25 μg ml(-1), but had no effect on cell adhesion. The discrepancy between the effective concentrations of fatty acids and the concentration found in culture medium may be explained by the presence of high-concentration microenvironments. The compounds involved in adhesion inhibition remain to be identified. Though further experiments with complex biofilms are needed, our results suggest that U. confervicolum may participate to the control of biofilm composition by inhibiting diatom adhesion.

Low-Molecular-Weight Metabolites from Diatoms: Structures, Biological Roles and Biosynthesis

Diatoms are abundant and important biological components of the marine environment that biosynthesize diverse natural products. These microalgae are rich in various lipids, carotenoids, sterols and isoprenoids, some of them containing toxins and other metabolites. Several groups of diatom natural products have attracted great interest due to their potential practical application as energy sources (biofuel), valuable food constituents, and prospective materials for nanotechnology. In addition, hydrocarbons, which are used in climate reconstruction, polyamines which participate in biomineralization, new apoptotic agents against tumor cells, attractants and deterrents that regulate the biochemical communications between marine species in seawaters have also been isolated from diatoms. However, chemical studies on these microalgae are complicated by difficulties, connected with obtaining their biomass, and the influence of nutrients and contaminators in their environment as well as by seasonal and climatic factors on the biosynthesis of the corresponding natural products. Overall, the number of chemically studied diatoms is lower than that of other algae, but further studies, particularly those connected with improvements in the isolation and structure elucidation technique as well as the genomics of diatoms, promise both to increase the number of studied species with isolated biologically active natural products and to provide a clearer perception of their biosynthesis.

The role of vanadium in biology

Vanadium is special in at least two respects: on the one hand, the tetrahedral anion vanadate(v) is similar to the phosphate anion; vanadate can thus interact with various physiological substrates that are otherwise functionalized by phosphate. On the other hand, the transition metal vanadium can easily expand its sphere beyond tetrahedral coordination, and switch between the oxidation states +v, +iv and +iii in a physiological environment. The similarity between vanadate and phosphate may account for the antidiabetic potential of vanadium compounds with carrier ligands such as maltolate and picolinate, and also for vanadium's mediation in cardiovascular and neuronal defects. Other potential medicinal applications of more complex vanadium coordination compounds, for example in the treatment of parasitic tropical diseases, may also be rooted in the specific properties of the ligand sphere. The ease of the change in the oxidation state of vanadium is employed by prokarya (bacteria and cyanobacteria) as well as by eukarya (algae and fungi) in respiratory and enzymatic functions. Macroalgae (seaweeds), fungi, lichens and Streptomyces bacteria have available haloperoxidases, and hence enzymes that enable the 2-electron oxidation of halide X(-) with peroxide, catalyzed by a Lewis-acidic V(V) center. The X(+) species thus formed can be employed to oxidatively halogenate organic substrates, a fact with implications also for the chemical processes in the atmosphere. Vanadium-dependent nitrogenases in bacteria (Azotobacter) and cyanobacteria (Anabaena) convert N2 + H(+) to NH4(+) + H2, but are also receptive for alternative substrates such as CO and C2H2. Among the enigmas to be solved with respect to the utilization of vanadium in nature is the accumulation of V(III) by some sea squirts and fan worms, as well as the purport of the nonoxido V(IV) compound amavadin in the fly agaric.

Pheromone signaling during sexual reproduction in algae

Algae are found in all aquatic and many terrestrial habitats. They are dominant in phytoplankton and biofilms thereby contributing massively to global primary production. Since algae comprise photosynthetic representatives of the various protoctist groups their physiology and appearance is highly diverse. This diversity is also mirrored in their characteristic life cycles that exhibit various facets of ploidy and duration of the asexual phase as well as gamete morphology. Nevertheless, sexual reproduction in unicellular and colonial algae usually has as common motive that two specialized, sexually compatible haploid gametes establish physical contact and fuse. To guarantee mating success, processes during sexual reproduction are highly synchronized and regulated. This review focuses on sex pheromones of algae that play a key role in these processes. Especially, the diversity of sexual strategies as well as of the compounds involved are the focus of this contribution. Discoveries connected to algal pheromone chemistry shed light on the role of key evolutionary processes, including endosymbiotic events and lateral gene transfer, speciation and adaptation at all phylogenetic levels. But progress in this field might also in the future provide valid tools for the manipulation of aquaculture and environmental processes.

Early perturbation in mitochondria redox homeostasis in response to environmental stress predicts cell fate in diatoms

Diatoms are ubiquitous marine photosynthetic eukaryotes that are responsible for about 20% of global photosynthesis. Nevertheless, little is known about the redox-based mechanisms that mediate diatom sensing and acclimation to environmental stress. Here we used a redox-sensitive green fluorescent protein sensor targeted to various subcellular organelles in the marine diatom Phaeodactylum tricornutum, to map the spatial and temporal oxidation patterns in response to environmental stresses. Specific organelle oxidation patterns were found in response to various stress conditions such as oxidative stress, nutrient limitation and exposure to diatom-derived infochemicals. We found a strong correlation between the mitochondrial glutathione (GSH) redox potential (E_GSH) and subsequent induction of cell death in response to the diatom-derived unsaturated aldehyde 2E,4E/Z-decadienal (DD), and a volatile halocarbon (BrCN) that mediate trophic-level interactions in marine diatoms. Induction of cell death in response to DD was mediated by oxidation of mitochondrial E_GSH and was reversible by application of GSH only within a narrow time frame. We found that cell fate can be accurately predicted by a distinct life-death threshold of mitochondrial E_GSH (−335 mV). We propose that compartmentalized redox-based signaling can integrate the input of diverse environmental cues and will determine cell fate decisions as part of algal acclimation to stress conditions.

Haloperoxidase mediated quorum quenching by Nitzschia cf pellucida: study of the metabolization of N-acyl homoserine lactones by a benthic diatom

Diatoms are known to produce a variety of halogenated compounds, which were recently shown to have a role in allelopathic interactions between competing species. The production of these compounds is linked to haloperoxidase activity. This research, has shown that this system may also be involved in diatom-bacteria interactions via the H₂O₂ dependent inactivation of a type of quorum sensing (QS) molecule, i.e., N-β-ketoacylated homoserine lactones (AHLs), by a natural haloperoxidase system from the benthic diatom Nitzschia cf pellucida. The AHL degradation pathway towards corresponding halogenated derivatives was elucidated via HPLC-MS analysis and the synthesis of a broad series of novel halogenated AHL analogues as reference compounds. Furthermore, their biological activity as quorum sensing modulators was directly compared and evaluated against a series of naturally occurring β-keto-AHLs. It has been demonstrated that the loss of the QS activity results from the final cleavage of the halogenated N-acyl chain of the signal molecules.

Uptake of iodide in the marine haptophyte Isochrysis sp. (T.ISO) driven by iodide oxidation

Uptake of iodide was studied in the marine microalga Isochrysis sp. (isol. Haines, T.ISO) during short-term incubations with radioactive iodide ((125) I(-) ). Typical inhibitors of the sodium/iodide symporter (NIS) did not inhibit iodide uptake, suggesting that iodide is not taken up through this transport protein, as is the case in most vertebrate animals. Oxidation of iodide was found to be an essential step for its uptake by T.ISO and it seemed likely that hypoiodous acid (HOI) was the form of iodine taken up. Uptake of iodide was inhibited by the addition of thiourea and of other reducing agents, like L-ascorbic acid, L-glutathione and L-cysteine and increased after the addition of oxidized forms of the transition metals Fe and Mn. The simultaneous addition of both hydrogen peroxide (H2 O2 ) and a known iodide-oxidizing myeloperoxidase (MPO) significantly increased iodine uptake, but the addition of H2 O2 or MPO separately, had no effect on uptake. This confirms the observation that iodide is oxidized prior to uptake, but it puts into doubt the involvement of H2 O2 excretion and membrane-bound or extracellular haloperoxidase activity of T.ISO. The increase of iodide uptake by T.ISO upon Fe(III) addition suggests the nonenzymatic oxidation of iodide by Fe(III) in a redox reaction and subsequent influx of HOI. This is the first report on the mechanism of iodide uptake in a marine microalga.

Induction of protease release of the resistant diatom Chaetoceros didymus in response to lytic enzymes from an algicidal bacterium

Marine lytic bacteria can have a substantial effect on phytoplankton and are even capable to terminate blooms of microalgae. The bacterium Kordia algicida was reported to lyse cells of the diatom Skeletonema costatum and several other diatoms by a quorum sensing controlled excretion of proteases. However the diatom Chaetoceros didymus is fully resistant against the bacterial enzymes. We show that the growth curve of this diatom is essentially unaffected by addition of bacterial filtrates that are active against other diatoms. By monitoring proteases from the medium using zymography and fluorescence based activity assays we demonstrate that C. didymus responds to the presence of the lytic bacteria with the induced production of algal proteases. These proteases exhibit a substantially increased activity compared to the bacterial counterparts. The induction is also triggered by signals in the supernatant of a K. algicida culture. Size fractionation shows that only the >30 kD fraction of the bacterial exudates acts as an inducing cue. Implications for a potential induced defense of the diatom C. didymus are discussed.

Mass spectrometry-based metabolomics to elucidate functions in marine organisms and ecosystems

Marine systems are very diverse and recognized as being sources of a wide range of biomolecules. This review provides an overview of metabolite profiling based on mass spectrometry (MS) approaches in marine organisms and their environments, focusing on recent advances in the field. We also point out some of the technical challenges that need to be overcome in order to increase applications of metabolomics in marine systems, including extraction of chemical compounds from different matrices and data management. Metabolites being important links between genotype and phenotype, we describe added value provided by integration of data from metabolite profiling with other layers of omics, as well as their importance for the development of systems biology approaches in marine systems to study several biological processes, and to analyze interactions between organisms within communities. The growing importance of MS-based metabolomics in chemical ecology studies in marine ecosystems is also illustrated.