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ABSTRACT Diverse marine animals undergo a metamorphic larval-to-juvenile transition in response to surface-bound bacteria. Although this host-microbe interaction is critical to establishing and maintaining marine animal populations, the functional activity of bacterial products and how they activate the host’s metamorphosis program has not yet been defined for any animal. The marine bacteriumPseudoalteromonas luteoviolaceastimulates the metamorphosis of a tubeworm calledHydroides elegansby producing a molecular syringe called metamorphosis-associated contractile structures (MACs). MACs stimulate metamorphosis by injecting a protein effector termed metamorphosis-inducing factor 1 (Mif1) into tubeworm larvae. Here, we show that MACs bind to tubeworm cilia and form visible pores on the cilia membrane surface, which are smaller and less numerous in the absence of Mif1.In vitro, Mif1 associates with eukaryotic lipid membranes and possesses phospholipase activity. MACs can also deliver Mif1 to human cell lines and cause parallel phenotypes, including cell surface binding, membrane disruption, calcium flux, and mitogen-activated protein kinase activation. Finally, MACs can also stimulate metamorphosis by delivering two unrelated membrane-disrupting proteins, MLKL and RegIIIɑ. Our findings demonstrate that membrane disruption by MACs and Mif1 is necessary forHydroidesmetamorphosis, connecting the activity of a bacterial protein effector to the developmental transition of a marine animal. IMPORTANCEThis research describes a mechanism wherein a bacterium prompts the metamorphic development of an animal from larva to juvenile form by injecting a protein that disrupts membranes in the larval cilia. Specifically, results show that a bacterial contractile injection system and the protein effector it injects form pores in larval cilia, influencing critical signaling pathways like mitogen-activated protein kinase and calcium flux, ultimately driving animal metamorphosis. This discovery sheds light on how a bacterial protein effector exerts its activity through membrane disruption, a phenomenon observed in various bacterial toxins affecting cellular functions, and elicits a developmental response. This work reveals a potential strategy used by marine organisms to respond to microbial cues, which could inform efforts in coral reef restoration and biofouling prevention. The study’s insights into metamorphosis-associated contractile structures’ delivery of protein effectors to specific anatomical locations highlight prospects for future biomedical and environmental applications. 

ABSTRACT Prochlorococcusis an abundant photosynthetic bacterium in the open ocean, where nitrogen (N) often limits phytoplankton growth. In the low-light-adapted LLI clade ofProchlorococcus, nearly all cells can assimilate nitrite (NO₂⁻), with a subset capable of assimilating nitrate (NO₃⁻). LLI cells are maximally abundant near the primary NO₂⁻maximum layer, an oceanographic feature that may, in part, be due to incomplete assimilatory NO₃⁻reduction and subsequent NO₂⁻release by phytoplankton. We hypothesized that someProchlorococcusexhibit incomplete assimilatory NO₃⁻reduction and examined NO₂⁻accumulation in cultures of threeProchlorococcusstrains (MIT0915, MIT0917, and SB) and twoSynechococcusstrains (WH8102 and WH7803). Only MIT0917 and SB accumulated external NO₂⁻during growth on NO₃⁻. Approximately 20–30% of the NO₃⁻transported into the cell by MIT0917 was released as NO₂⁻, with the rest assimilated into biomass. We further observed that co-cultures using NO₃⁻as the sole N source could be established for MIT0917 andProchlorococcusstrain MIT1214 that can assimilate NO₂⁻but not NO₃⁻. In these co-cultures, the NO₂⁻released by MIT0917 is efficiently consumed by its partner strain, MIT1214. Our findings highlight the potential for emergent metabolic partnerships that are mediated by the production and consumption of N cycle intermediates withinProchlorococcuspopulations. IMPORTANCEEarth’s biogeochemical cycles are substantially driven by microorganisms and their interactions. Given that N often limits marine photosynthesis, we investigated the potential for N cross-feeding within populations ofProchlorococcus, the numerically dominant photosynthetic cell in the subtropical open ocean. In laboratory cultures, someProchlorococcuscells release extracellular NO₂⁻during growth on NO₃⁻. In the wild,Prochlorococcuspopulations are composed of multiple functional types, including those that cannot use NO₃⁻but can still assimilate NO₂⁻. We show that metabolic dependencies arise whenProchlorococcusstrains with complementary NO₂⁻production and consumption phenotypes are grown together on NO₃⁻. These findings demonstrate the potential for emergent metabolic partnerships, possibly modulating ocean nutrient gradients, that are mediated by cross-feeding of N cycle intermediates. 

ABSTRACT Endosymbionts can influence host reproduction and fitness to favor their maternal transmission. For example, endosymbiotic Wolbachia bacteria often cause cytoplasmic incompatibility (CI) that kills uninfected embryos fertilized by Wolbachia -modified sperm. Infected females can rescue CI, providing them a relative fitness advantage. Wolbachia -induced CI strength varies widely and tends to decrease as host males age. Since strong CI drives Wolbachia to high equilibrium frequencies, understanding how fast and why CI strength declines with male age is crucial to explaining age-dependent CI’s influence on Wolbachia prevalence. Here, we investigate if Wolbachia densities and/or CI gene ( cif ) expression covary with CI-strength variation and explore covariates of age-dependent Wolbachia -density variation in two classic CI systems. w Ri CI strength decreases slowly with Drosophila simulans male age (6%/day), but w Mel CI strength decreases very rapidly (19%/day), yielding statistically insignificant CI after only 3 days of Drosophila melanogaster adult emergence. Wolbachia densities and cif expression in testes decrease as w Ri-infected males age, but both surprisingly increase as w Mel-infected males age, and CI strength declines. We then tested if phage lysis, Octomom copy number (which impacts w Mel density), or host immune expression covary with age-dependent w Mel densities. Only host immune expression correlated with density. Together, our results identify how fast CI strength declines with male age in two model systems and reveal unique relationships between male age, Wolbachia densities, cif expression, and host immunity. We discuss new hypotheses about the basis of age-dependent CI strength and its contributions to Wolbachia prevalence. IMPORTANCE Wolbachia bacteria are the most common animal-associated endosymbionts due in large part to their manipulation of host reproduction. Many Wolbachia cause cytoplasmic incompatibility (CI) that kills uninfected host eggs. Infected eggs are protected from CI, favoring Wolbachia spread in natural systems and in transinfected mosquito populations where vector-control groups use strong CI to maintain pathogen-blocking Wolbachia at high frequencies for biocontrol of arboviruses. CI strength varies considerably in nature and declines as males age for unknown reasons. Here, we determine that CI strength weakens at different rates with age in two model symbioses. Wolbachia density and CI gene expression covary with w Ri-induced CI strength in Drosophila simulans , but neither explain rapidly declining w Mel-induced CI in aging D. melanogaster males. Patterns of host immune gene expression suggest a candidate mechanism behind age-dependent w Mel densities. These findings inform how age-dependent CI may contribute to Wolbachia prevalence in natural systems and potentially in transinfected systems. 

ABSTRACT In the ocean surface layer and cell culture, the polyamine transport protein PotD of SAR11 bacteria is often one of the most abundant proteins detected. Polyamines are organic cations at seawater pH produced by all living organisms and are thought to be an important component of dissolved organic matter (DOM) produced in planktonic ecosystems. We hypothesized that SAR11 cells uptake and metabolize multiple polyamines and use them as sources of carbon and nitrogen. Metabolic footprinting and fingerprinting were used to measure the uptake of five polyamine compounds (putrescine, cadaverine, agmatine, norspermidine, and spermidine) in two SAR11 strains that represent the majority of SAR11 cells in the surface ocean environment, “ Candidatus Pelagibacter” strain HTCC7211 and “ Candidatus Pelagibacter ubique” strain HTCC1062. Both strains took up all five polyamines and concentrated them to micromolar or millimolar intracellular concentrations. Both strains could use most of the polyamines to meet their nitrogen requirements, but polyamines did not fully substitute for their requirements of glycine (or related compounds) or pyruvate (or related compounds). Our data suggest that potABCD transports all five polyamines and that spermidine synthase, speE, is reversible, catalyzing the breakdown of spermidine and norspermidine, in addition to its usual biosynthetic role. These findings provide support for the hypothesis that enzyme multifunctionality enables streamlined cells in planktonic ecosystems to increase the range of DOM compounds they metabolize. IMPORTANCE Genome streamlining in SAR11 bacterioplankton has resulted in a small repertoire of genes, yet paradoxically, they consume a substantial fraction of primary production in the oceans. Enzyme multifunctionality, referring to enzymes that are adapted to have broader substrate and catalytic range than canonically defined, is hypothesized to be an adaptation that increases the range of organic compounds metabolized by cells in environments where selection favors genome minimization. We provide experimental support for this hypothesis by demonstrating that SAR11 cells take up and metabolize multiple polyamine compounds and propose that a small set of multifunctional enzymes catalyze this metabolism. We report that polyamine uptake rates can exceed metabolic rates, resulting in both high intracellular concentrations of these nitrogen-rich compounds (in comparison to native polyamine levels) and an increase in cell size. 

ABSTRACT The increase in prevalence and severity of coral disease outbreaks produced by Vibrio pathogens, and related to global warming, has seriously impacted reef-building corals throughout the oceans. The coral Oculina patagonica has been used as a model system to study coral bleaching produced by Vibrio infection. Previous data demonstrated that when two coral pathogens ( Vibrio coralliilyticus and Vibrio mediterranei ) simultaneously infected the coral O. patagonica , their pathogenicity was greater than when each bacterium was infected separately. Here, to understand the mechanisms underlying this synergistic effect, transcriptomic analyses of monocultures and cocultures as well as experimental infection experiments were performed. Our results revealed that the interaction between the two vibrios under culture conditions overexpressed virulence factor genes (e.g., those encoding siderophores, the type VI secretion system, and toxins, among others). Moreover, under these conditions, vibrios were also more likely to form biofilms or become motile through induction of lateral flagella. All these changes that occur as a physiological response to the presence of a competing species could favor the colonization of the host when they are present in a mixed population. Additionally, during coral experimental infections, we showed that exposure of corals to molecules released during V. coralliilyticus and V. mediterranei coculture induced changes in the coral microbiome that favored damage to coral tissue and increased the production of lyso-platelet activating factor. Therefore, we propose that competition sensing, defined as the physiological response to detection of harm or to the presence of a competing Vibrio species, enhances the ability of Vibrio coral pathogens to invade their host and cause tissue necrosis. IMPORTANCE Vibrio coralliilyticus and Vibrio mediterranei are important coral pathogens capable of inducing serious coral damage, which increases severely when they infect the host simultaneously. This has consequences related to the dispersion of these pathogens among different locations that could enhance deleterious effects on coral reefs. However, the mechanisms underlying this synergistic interaction are unknown. The work described here provides a new perspective on the complex interactions among these two Vibrio coral pathogens, suggesting that coral infection could be a collateral effect of interspecific competition. Major implications of this work are that (i) Vibrio virulence mechanisms are activated in the absence of the host as a response to interspecific competition and (ii) release of molecules by Vibrio coral pathogens produces changes in the coral microbiome that favor the pathogenic potential of the entire Vibrio community. Thus, our results highlight that social cues and competition sensing are crucial determinants of development of coral diseases.