Background: Many Trichoderma species are applied as biofungicides and biofertilizers to agricultural soils to enhance crop growth. These filamentous fungi have the ability to reduce plant diseases and promote plant growth and productivity through overlapping modes of action including induced systemic resistance, antibiosis, enhanced nutrient efficiency, and myco-parasitism. Trichoderma species are prolific producers of many small metabolites with antifungal, antibacterial, and anticancer properties. Volatile metabolites of Trichoderma also have the ability to induce resistance to plant pathogens leading to improved plant health. In this study, Arabidopsis plants were exposed to mixtures of volatile organic compounds (VOCs) emitted by growing cultures of Trichoderma from 20 strains, representing 11 different Trichoderma species.Results: We identified nine Trichoderma strains that produced plant growth promoting VOCs. Exposure to mixtures of VOCs emitted by these strains increased plant biomass (37.1-41.6 %) and chlorophyll content (82.5-89.3 %). Trichoderma volatile-mediated changes in plant growth were strain-and species-specific. VOCs emitted by T. pseudokoningii (CBS 130756) were associated with the greatest Arabidopsis growth promotion. One strain, T. atroviride (CBS 01-209), in our screen decreased growth (50.5 %) and chlorophyll production (13.1 %). Similarly, tomatoes exposed to VOCs from T. viride (BBA 70239) showed a significant increase in plant biomass (>99 %), larger plant size, and significant development of lateral roots. We also observed that the tomato plant growths were dependent on the duration of the volatile exposure. A GC-MS analysis of VOCs from Trichoderma strains identified more than 141 unique compounds including several unknown sesquiterpenes, diterpenes, and tetraterpenes. Conclusions:Plants grown in the presence of fungal VOCs emitted by different species and strains of Trichoderma exhibited a range of effects. This study demonstrates that the blend of volatiles produced by actively growing fungi and volatile exposure time in plant development both influence the outcome of volatile-mediated interactions. Only some of our growth promoting strains produced microbial VOCs known to enhance plant growth. Compounds such as 6-pentyl-2H-pyran-2-one were not common to all promoting strains. We found that biostimulatory strains tended to have a larger number of complex terpenes which may explain the variation in growth induced by different Trichoderma strains.

Interactions between phytoplankton and bacteria play important roles in shaping the microenvironment surrounding these organisms and in turn influence global biogeochemical cycles. This microenvironment, known as the phycosphere, is presumed to shape the bacterial diversity around phytoplankton and thus stimulate a diverse array of interactions between both groups. Although many studies have attempted to characterize bacterial communities that associate and interact with phytoplankton, bias in bacterial cultivation and consistency and persistence of bacterial communities across phytoplankton isolates likely impede the understanding of these microbial associations. Here, we isolate four strains of the diatom Asterionellopsis glacialis and three strains of the diatom Nitzschia longissima and show through metabarcoding of the bacterial 16S rDNA gene that though each species possesses a unique bacterial community, the bacterial composition across strains from the same species are highly conserved at the genus level. Cultivation of all seven strains in the laboratory for longer than 1 year resulted in only small changes to the bacterial composition, suggesting that despite strong pressures from laboratory culturing conditions associations between these diatoms and their bacterial communities are robust. Specific operational taxonomic units (OTUs) belonging to the Roseobacter-clade appear to be conserved across all strains and time, suggesting their importance to diatoms. In addition, we isolate a range of cultivable bacteria from one of these cultures, A. glacialis strain A3, including several strains of Shimia marina and Nautella sp. that appear closely related to OTUs conserved across all strains and times. Coculturing of A3 with some of its cultivable bacteria as well as other diatom-associated bacteria shows a wide range of responses that include enhancing diatom growth. Cumulatively, these findings suggest that phytoplankton possess unique microbiomes that are consistent across strains and temporal scales.

Unicellular eukaryotic phytoplankton, such as diatoms, rely on microbial communities for survival despite lacking specialized compartments to house microbiomes (e.g., animal gut). Microbial communities have been widely shown to benefit from diatom excretions that accumulate within the microenvironment surrounding phytoplankton cells, known as the phycosphere. However, mechanisms that enable diatoms and other unicellular eukaryotes to nurture specific microbiomes by fostering beneficial bacteria and repelling harmful ones are mostly unknown. We hypothesized that diatom exudates may tune microbial communities and employed an integrated multiomics approach using the ubiquitous diatom Asterionellopsis glacialis to reveal how it modulates its naturally associated bacteria. We show that A. glacialis reprograms its transcriptional and metabolic profiles in response to bacteria to secrete a suite of central metabolites and two unusual secondary metabolites, rosmarinic acid and azelaic acid. While central metabolites are utilized by potential bacterial symbionts and opportunists alike, rosmarinic acid promotes attachment of beneficial bacteria to the diatom and simultaneously suppresses the attachment of opportunists. Similarly, azelaic acid enhances growth of beneficial bacteria while simultaneously inhibiting growth of opportunistic ones. We further show that the bacterial response to azelaic acid is numerically rare but globally distributed in the world’s oceans and taxonomically restricted to a handful of bacterial genera. Our results demonstrate the innate ability of an important unicellular eukaryotic group to modulate select bacteria in their microbial consortia, similar to higher eukaryotes, using unique secondary metabolites that regulate bacterial growth and behavior inversely across different bacterial populations.

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22 23 † Current affiliation: Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, 24 USA 25 26 27 Keywords 28 phycospheres | microbiomes | signaling | diatom | phytoplankton-bacteria interactions | 29 secondary metabolites 30 Abstract 31 32Unicellular eukaryotic phytoplankton, such as diatoms, rely on microbial communities for survival 33 despite lacking specialized compartments to house microbiomes (e.g., animal gut). Microbial 34 communities have been widely shown to benefit from diatom excretions that accumulate within the 35 microenvironment surrounding phytoplankton cells, known as the phycosphere. However, 36 mechanisms that enable diatoms and other unicellular eukaryotes to nurture specific microbiomes 37 by fostering beneficial bacteria and repelling harmful ones are mostly unknown. We hypothesized 38 that diatom exudates may attune microbial communities and employed an integrated multi-omics 39 approach using the ubiquitous diatom Asterionellopsis glacialis to reveal how it modulates its 40 naturally associated bacteria. We show that A. glacialis reprograms its transcriptional and metabolic 41 profiles in response to bacteria to secrete a suite of central metabolites and two unusual secondary 42 metabolites, rosmarinic acid and azelaic acid. While central metabolites are utilized by potential 43 bacterial symbionts and opportunists alike, rosmarinic acid promotes attachment of beneficial 44 bacteria to the diatom and simultaneously suppresses the attachment of opportunists. Similarly, 45 azelaic acid enhances growth of beneficial bacteria, while simultaneously inhibiting growth of 46 opportunistic ones. We further show that the bacterial response to azelaic acid is widespread in the 47 world's oceans and taxonomically restricted to a handful of bacterial genera. Our results 48 demonstrate the innate ability of an important unicellular eukaryotic group to modulate their 49 microbial consortia, similar to higher eukaryotes, using unique secondary metabolites that regulate 50 bacterial growth and behavior inversely in different bacterial populations. 52 53Large swaths of eukaryotic lineages possess associated microbiomes that play central roles in 54 maintaining host survival and ecological success (1). Several biotic and abiotic factors have been 55 shown to drive microbiome assembly and modulation in special compartments and organelles of 56 multicellular eukaryotes such as squid light organs (2), coral skeletons (3), mammalian guts (4), 57and roots and leaves of terrestrial plants (5). Contrarily, unicellular eukaryotes such as diatoms lack 58 developmental features that can harbor microbes, yet rely heavily on essential bacterial growth 59 factors (6-8) to proliferate and thrive in their environment. Diatoms are ubiquitous primary producers 60 in aquatic environments that excrete up to 50% of their fixed carbon (9-11) into a diffusive boundary 61 layer that surrounds individual cells. This physically sheltered microscale region, known as the 62 phycosphere, is highly enriched in dis...