Algal volatiles - the overlooked chemical language of aquatic primary producers

The bioconversion of dietary α-linolenic acid to eicosapentaenoic acid in Bombyx mori

n-3 Long-chain polyunsaturated fatty acids (n-3 LC-PUFAs), including eicosapentaenoic acid (EPA), are essential multifunctional nutrients in animals. Microorganisms such as microalgae are known to be n-3 LC-PUFA producers in aquatic environments. Various aquatic invertebrates, including Harpacticoida copepods, and a few terrestrial invertebrates, such as the nematode Caenorhabditis elegans, possess n-3 LC-PUFA biosynthetic enzymes. However, the capacity for n-3 LC-PUFA biosynthesis and the underlying molecular mechanisms in terrestrial insects are largely unclear. In this study, we investigated the fatty acid biosynthetic pathway in the silkworm Bombyx mori and found that EPA was present in silkworms throughout their development. Stable isotope tracing revealed that dietary α-linolenic acid (ALA) was metabolized to EPA in silkworm larvae. These results indicated that silkworms synthesize EPA from ALA. Given that EPA is enriched in the central nervous system, we propose that EPA confers optimal neuronal functions, similar to docosahexaenoic acid, in the mammalian nervous system.

Volatile Organic Compounds Produced by a Deep-Sea Bacterium Efficiently Inhibit the Growth of Pseudomonas aeruginosa PAO1

The deep-sea bacterium Spongiibacter nanhainus CSC3.9 has significant inhibitory effects on agricultural pathogenic fungi and human pathogenic bacteria, especially Pseudomonas aeruginosa, the notorious multidrug-resistant pathogen affecting human public health. We demonstrate that the corresponding antibacterial agents against P. aeruginosa PAO1 are volatile organic compounds (VOCs, namely VOC-3.9). Our findings show that VOC-3.9 leads to the abnormal cell division of P. aeruginosa PAO1 by disordering the expression of several essential division proteins associated with septal peptidoglycan synthesis. VOC-3.9 hinders the biofilm formation process and promotes the biofilm dispersion process of P. aeruginosa PAO1 by affecting its quorum sensing systems. VOC-3.9 also weakens the iron uptake capability of P. aeruginosa PAO1, leading to reduced enzymatic activity associated with key metabolic processes, such as reactive oxygen species (ROS) scavenging. Overall, our study paves the way to developing antimicrobial compounds against drug-resistant bacteria by using volatile organic compounds.

The biosynthesis of the odorant 2-methylisoborneol is compartmentalized inside a protein shell

Terpenoids are the largest class of natural products, found across all domains of life. One of the most abundant bacterial terpenoids is the volatile odorant 2-methylisoborneol (2-MIB), partially responsible for the earthy smell of soil and musty taste of contaminated water. Many bacterial 2-MIB biosynthetic gene clusters were thought to encode a conserved transcription factor, named EshA in the model soil bacterium Streptomyces griseus . Here, we revise the function of EshA, now referred to as Sg Enc, and show that it is a Family 2B encapsulin shell protein. Using cryo-electron microscopy, we find that Sg Enc forms an icosahedral protein shell and encapsulates 2-methylisoborneol synthase (2-MIBS) as a cargo protein. Sg Enc contains a cyclic adenosine monophosphate (cAMP) binding domain (CBD)-fold insertion and a unique metal-binding domain, both displayed on the shell exterior. We show that Sg Enc CBDs do not bind cAMP. We find that 2-MIBS cargo loading is mediated by an N-terminal disordered cargo-loading domain and that 2-MIBS activity and Sg Enc shell structure are not modulated by cAMP. Our work redefines the function of EshA and establishes Family 2B encapsulins as cargo-loaded protein nanocompartments involved in secondary metabolite biosynthesis.

Evaluation of lipid extraction methods for fatty acid quantification and compound-specific δ13C and δ2Hn analyses

Lipids, with fatty acids (FA) as a crucial subset, have become a focal point for diverse medical, physiological, and ecological studies. However, a comprehensive assessment of the various pre-analytical FA extraction methods published in the scientific literature remains lacking. In this study, we examined the efficacy of seven well-established sample preparation methods, specifically focusing on their effectiveness in total lipid and fatty acid extraction and their impact on compound-specific stable hydrogen (δ2H) and carbon (δ13C) isotope values. We also considered the repercussions of FA removal efficacy on residual bulk tissue δ2Hn analysis, because lipids typically have low δ2H values. Our findings showed that in most cases chloroform-based extraction methods outperformed those without chloroform. While discrepancies were not as evident for smaller organisms, such as plankton, marked variations were discernible in the extraction efficiencies for muscle and liver samples, which was also manifested in the residual bulk tissue δ2Hn results. Notably, most extraction methods had little effect on specific δ13C or δ2H isotope values of FA; instead, an emphasis should be on using an extraction method that achieves optimal baseline peak separation of the chromatograms for C and H isotope measurements.

Volatilome: Smells like microbial spirit

In recent years, the study of volatile compounds has sparked interest due to their implications in signaling and the enormous variety of bioactive properties attributed to them. Despite the absence of analysis methods standardization, there are a multitude of tools and databases that allow the identification and quantification of volatile compounds. These compounds are chemically heterogeneous and their diverse properties are exploited by various fields such as cosmetics, the food industry, agriculture and medicine, some of which will be discussed here. In virtue of volatile compounds being ubiquitous and fast chemical messengers, these molecules mediate a large number of interspecific and intraspecific interactions, which are key at an ecological level to maintaining the balance and correct functioning of ecosystems. This review briefly summarized the role of volatile compounds in inter- and intra-specific relationships as well as industrial applications associated with the use of these compounds that is emerging as a promising field of study.

β-cyclocitral, a novel AChE inhibitor, contributes to the defense of Microcystis aeruginosa against Daphnia grazing

β-cyclocitral is one of the major compounds in cyanobacterial volatile organic compound (VOCs) and can poison other aquatic organisms. To investigate the effect of β-cyclocitral on cyanobacterial-grazer interactions, Daphnia sinensis was fed Microcystis aeruginosa and exposed to β-cyclocitral. Our present study demonstrated that M. aeruginosa could significantly inhibit D. sinensis grazing. And the grazing inhibition by Microcystis aeruginosa results from the suppression of feeding rate, heart rate, thoracic limb activity and swimming speed of D. sinensis. In addition, M. aeruginosa could also induce intestinal peristalsis and emptying in D. sinensis. Interestingly, our present study found that the exposure to β-cyclocitral could mimic a range of phenotypes induced by M. aeruginosa in D. sinensis. These results suggested that M. aeruginosa could release β-cyclocitral to inhibit Daphnia grazing. To further examine the toxic mechanism of β-cyclocitral in Daphnia, several in vivo and in vitro experiments displayed that β-cyclocitral was a novel inhibitor of acetylcholinesterase (AChE). It could induce the accumulation of acetylcholine (ACh) by inhibiting AchE activity in D. sinensis. High level of endogenous Ach could inhibit feeding rate and induce intestinal peristalsis and emptying in D. sinensis.

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.

Algae communication, conspecific and interspecific: the concepts of phycosphere and algal-bacteria consortia in a photobioreactor (PBR)

Microalgae in the wild often form consortia with other species promoting their own health and resource foraging opportunities. The recent application of microalgae cultivation and deployment in commercial photobioreactors (PBR) so far has focussed on single species of algae, resulting in multi-species consortia being largely unexplored. Reviewing the current status of PBR ecological habitat, this article argues in favor of further investigation into algal communication with conspecifics and interspecifics, including other strains of microalgae and bacteria. These mutualistic species form the 'phycosphere': the microenvironment surrounding microalgal cells, potentiating the production of certain metabolites through biochemical interaction with cohabitating microorganisms. A better understanding of the phycosphere could lead to novel PBR configurations, capable of incorporating algal-microbial consortia, potentially proving more effective than single-species algal systems.

Distinct emissions of biogenic volatile organic compounds from temperate benthic taxa

INTRODUCTION

Biogenic volatile organic compounds (BVOCs) are emitted by all organisms as intermediate or end-products of metabolic processes. Individual BVOCs perform important physiological, ecological and climatic functions, and collectively constitute the volatilome-which can be reflective of organism taxonomy and health. Although BVOC emissions of tropical benthic reef taxa have recently been the focus of multiple studies, emissions derived from their temperate counterparts have never been characterised.

OBJECTIVES

Characterise the volatilomes of key competitors for benthic space among Australian temperate reefs.

METHODS

Six fragments/fronds of a temperate coral (Plesiastrea versipora) and a macroalga (Ecklonia radiata) from a Sydney reef site were placed within modified incubation chambers filled with seawater. Organism-produced BVOCs were captured on thermal desorption tubes using a purge-and-trap methodology, and were then analysed using GC × GC - TOFMS and multivariate tests.

RESULTS

Analysis detected 55 and 63 BVOCs from P. versipora and E. radiata respectively, with 30 of these common between species. Each taxon was characterised by a similar relative composition of chemical classes within their volatilomes. However, 14 and 10 volatiles were distinctly emitted by either E. radiata or P. versipora respectively, including the halogenated compounds iodomethane, tribromomethane, carbon tetrachloride and trichloromonofluoromethane. While macroalgal cover was 3.7 times greater than coral cover at the sampling site, P. versipora produced on average 17 times more BVOCs per cm2 of live tissue, resulting in an estimated contribution to local BVOC emission that was 4.7 times higher than E. radiata.

CONCLUSION

Shifts in benthic community composition could disproportionately impact local marine chemistry and affect how ecosystems contribute to broader BVOC emissions.

Volatile sensation: The chemical ecology of the earthy odorant geosmin

Geosmin may be the most familiar volatile compound, as it lends the earthy smell to soil. The compound is a member of the largest family of natural products, the terpenoids. The broad distribution of geosmin among bacteria in both terrestrial and aquatic environments suggests that this compound has an important ecological function, for example, as a signal (attractant or repellent) or as a protective specialized metabolite against biotic and abiotic stresses. While geosmin is part of our everyday life, scientists still do not understand the exact biological function of this omnipresent natural product. This minireview summarizes the current general observations regarding geosmin in prokaryotes and introduces new insights into its biosynthesis and regulation, as well as its biological roles in terrestrial and aquatic environments.

Airborne dimethyl sulfide (DMS) cues dimethylsulfoniopropionate (DMSP) increases in the intertidal green alga Ulva fenestrata

Although the use of airborne molecules as infochemicals is common in terrestrial plants, it has not been shown to occur in an ecologically relevant context in marine seaweeds. Like terrestrial plants, intertidal plants spend part of their lives emersed at low tide and release volatile organic compounds (VOCs) into the air when they are grazed or physiologically stressed. We hypothesized seaweeds could use airborne VOCs as infochemicals and respond to them by upregulating a keystone defensive metabolite, dimethylsulfoniopropionate (DMSP). We conducted laboratory and field experiments in which Ulva fenestrata was exposed to airborne dimethyl sulfide (DMS), a volatile antiherbivore and antioxidant metabolite released when the seaweed is grazed or physiologically stressed. In the laboratory, U. fenestrata exposed to DMS had 43-48% higher DMSP concentrations, relative to controls, 6-9 days after exposure. In the field, U. fenestrata 1 m downwind of DMS emitters had 19% higher DMSP concentrations than upwind seaweeds after 11 days. To our knowledge, this is the first demonstration of a marine plant using an airborne molecule released when damaged to elicit defensive responses. Our study suggests that the ability to detect airborne compounds has evolved multiple times or before the divergence of terrestrial plants and green algae.

Impact of Marine Chemical Ecology Research on the Discovery and Development of New Pharmaceuticals

Diverse ecologically important metabolites, such as allelochemicals, infochemicals and volatile organic chemicals, are involved in marine organismal interactions. Chemically mediated interactions between intra- and interspecific organisms can have a significant impact on community organization, population structure and ecosystem functioning. Advances in analytical techniques, microscopy and genomics are providing insights on the chemistry and functional roles of the metabolites involved in such interactions. This review highlights the targeted translational value of several marine chemical ecology-driven research studies and their impact on the sustainable discovery of novel therapeutic agents. These chemical ecology-based approaches include activated defense, allelochemicals arising from organismal interactions, spatio-temporal variations of allelochemicals and phylogeny-based approaches. In addition, innovative analytical techniques used in the mapping of surface metabolites as well as in metabolite translocation within marine holobionts are summarized. Chemical information related to the maintenance of the marine symbioses and biosyntheses of specialized compounds can be harnessed for biomedical applications, particularly in microbial fermentation and compound production. Furthermore, the impact of climate change on the chemical ecology of marine organisms-especially on the production, functionality and perception of allelochemicals-and its implications on drug discovery efforts will be presented.

Emission of cyanobacterial volatile organic compounds and their roles in blooms

Cyanobacteria are photosynthetic prokaryotes and one of dominant species in eutrophicated waters, which easily burst blooms in summer with high irradiance and temperature conditions. In response to high irradiance, high temperature, and nutrient conditions, cyanobacteria release abundant of volatile organic compounds (VOCs) by up-regulating related gene expression and oxidatively degrading β-carotene. These VOCs not only increase offensive odor in waters, but also transfer allelopathic signals to algae and aquatic plants, resulting in cyanobacteria dominating eutrophicated waters. Among these VOCs, β-cyclocitral, α-ionone, β-ionone, limonene, longifolene, and eucalyptol have been identified as the main allelopathic agents, which even directly kill algae by inducing programmed cell death (PCD). The VOCs released from cyanobacteria, especially the ruptured cells, exhibit repelling effects on the herbivores, which is beneficial to survival of the population. Cyanobacterial VOCs might transfer aggregating information among homogeneous species, so the acceptors initiate aggregation to resist the coming stresses. It can be speculated that the adverse conditions can promote VOC emission from cyanobacteria, which play important roles in cyanobacteria dominating eutrophicated waters and even bursting blooms.