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ABSTRACT As global temperatures are shifting, so too is the landscape of organismal fitness and, by extension, the role of the symbiotic microbes they house. As these host–microbe partnerships grapple with changing environments, current research struggles to keep pace with the complexity of microbial symbioses acclimating, adapting and evolving as environmental conditions change around them. Wild-caught organisms have been used to test adaptation to extreme environments, but extrapolating and interpreting data on how separate partners within a symbiosis respond to detrimental conditions is difficult. The beneficial association between bobtail squids and bioluminescent Vibrio bacteria is a model that has been used for over three decades to uncover evolutionary and ecological mechanisms of symbiogenesis. The system is highly amenable to a broad range of physiological and molecular techniques and has been used to study many dimensions of symbiotic interactions. This beneficial association has demonstrated that host selection of environmentally available Vibrio symbionts can be influenced by various abiotic conditions, such as temperature. Complex biochemical communication has been charted extensively between host and symbiont, revealing universally conserved traits that are temperature sensitive. Additionally, temperature can influence co-evolution of the partners, and this system can be used to predict symbiotic cooperation over evolutionary time scales. While one model system cannot provide exhaustive insight, the bobtail squid–Vibrio mutualism has laid extensive, pioneering groundwork that can be used to develop targeted questions about symbioses under changing climates. 

Abstract Bacteria often develop distinct phenotypes to adapt to environmental stress. In particular, they can produce biofilms, dense communities of bacteria that live in a complex extracellular matrix. While previous studies have investigated how bacterial biofilms are regulated under laboratory conditions, they have not considered (1) the data requirements necessary to estimate model parameters and (2) how bacteria respond to recurring stressors in their natural habitats. To address (1), we adapted a mechanistic population model to explore the dynamics of biofilm formation in the presence of predator stress, using synthetic data. We used a Maximum Likelihood Estimation framework to measure crucial parameters underpinning the biofilm formation dynamics. We used genetic algorithms to propose an optimal data collection schedule that minimised parameter identifiability confidence interval widths. Our sensitivity analysis revealed that, within the explored regimes, we could simplify the binding dynamics and eliminate biofilm detachment. To address (2), we proposed a structured version of our model to capture the long-term behaviour and evolutionary selection. In our extended model, the subpopulations feature different maximal rates of biofilm formation. We compared the selection under different predator types and amounts and identified key parameters that affected the speed of selection via sensitivity analysis. 

Biofilm formation is important for microbial survival, adaptation, and persistence within mutualistic and pathogenic systems in the Vibironaceae. Biofilms offer protection against environmental stressors, immune responses, and antimicrobial treatments by increasing host colonization and resilience. This review examines the mechanisms of biofilm formation in Vibrio species, focusing on quorum sensing, cyclic-di-GMP signaling, and host-specific adaptations that influence biofilm structure and function. We discuss how biofilms differ between mutualistic and pathogenic species based on environmental and host signals. Recent advances in omics technologies such as transcriptomics and metabolomics have enhanced research in biofilm regulation under different conditions. Horizontal gene transfer and phase variation promote the greater fitness of bacterial biofilms due to the diversity of environmental isolates that utilize biofilms to colonize host species. Despite progress, questions remain regarding the long-term effects of biofilm formation and persistence on host physiology and biofilm community dynamics. Research integrating multidisciplinary approaches will help advance our understanding of biofilms and their implications for influencing microbial adaptation, symbiosis, and disease. These findings have applications in biotechnology and medicine, where the genetic manipulation of biofilm regulation can enhance or disrupt microbiome stability and pathogen resistance, eventually leading to targeted therapeutic strategies. 

Interference competition, wherein bacteria actively antagonize and damage their microbial neighbors, is a key ecological strategy governing microbial community structure and composition. To gain a competitive edge, bacteria can deploy a diverse array of antimicrobial weapons—ranging from diffusible toxins to contact-mediated systems in order to eliminate their bacterial rivals. Among Gram-negative bacteria, the type VI secretion system (T6SS) has emerged as a potent and sophisticated contact-dependent mechanism that enables the delivery of toxic cargo into neighboring cells, thereby promoting the colonization and dominance of a bacterial taxon within an ecological niche. In this review, we examine the ecological significance of T6SS-mediated interference competition by members of the Vibrionaceae family across a range of marine habitats that include free-living microbial communities and host-associated niches such as coral and squid symbioses. Additionally, we explore the ecological impact of T6SS-mediated competition in modulating biofilm community structure and promoting horizontal gene transfer within those complex microbial populations. Together, these insights underscore the ecological versatility of the T6SS and emphasize its role in driving antagonistic bacterial interactions and shaping microbial community dynamics within marine ecosystems. 

ABSTRACT Many female squids and cuttlefishes have a symbiotic reproductive organ called the accessory nidamental gland (ANG) that hosts a bacterial consortium involved with egg defense against pathogens and fouling organisms. While the ANG is found in multiple cephalopod families, little is known about the global microbial diversity of these ANG bacterial symbionts. We used 16S rRNA gene community analysis to characterize the ANG microbiome from different cephalopod species and assess the relationship between host and symbiont phylogenies. The ANG microbiome of 11 species of cephalopods from four families (superorder: Decapodiformes) that span seven geographic locations was characterized. Bacteria of classAlphaproteobacteria, Gammaproteobacteria, andFlavobacteriiawere found in all species, yet analysis of amplicon sequence variants by multiple distance metrics revealed a significant difference between ANG microbiomes of cephalopod families (weighted/unweighted UniFrac, Bray–Curtis,P= 0.001). Despite being collected from widely disparate geographic locations, members of the family Sepiolidae (bobtail squid) shared many bacterial taxa including (~50%)Opitutae(Verrucomicrobia) andRuegeria(Alphaproteobacteria) species. Furthermore, we tested for phylosymbiosis and found a positive correlation between host phylogenetic distance and bacterial community dissimilarity (Mantel testr= 0.7). These data suggest that closely related sepiolids select for distinct symbionts from similar bacterial taxa. Overall, the ANGs of different cephalopod species harbor distinct microbiomes and thus offer a diverse symbiont community to explore antimicrobial activity and other functional roles in host fitness. IMPORTANCEMany aquatic organisms recruit microbial symbionts from the environment that provide a variety of functions, including defense from pathogens. Some female cephalopods (squids, bobtail squids, and cuttlefish) have a reproductive organ called the accessory nidamental gland (ANG) that contains a bacterial consortium that protects eggs from pathogens. Despite the wide distribution of these cephalopods, whether they share similar microbiomes is unknown. Here, we studied the microbial diversity of the ANG in 11 species of cephalopods distributed over a broad geographic range and representing 15–120 million years of host divergence. The ANG microbiomes shared some bacterial taxa, but each cephalopod species had unique symbiotic members. Additionally, analysis of host–symbiont phylogenies suggests that the evolutionary histories of the partners have been important in shaping the ANG microbiome. This study advances our knowledge of cephalopod–bacteria relationships and provides a foundation to explore defensive symbionts in other systems.