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ABSTRACT Functional studies of host-microbe interactions benefit from natural model systems that enable the exploration of molecular mechanisms at the host-microbe interface. BioluminescentVibrio fischericolonize the light organ of the Hawaiian bobtail squid,Euprymna scolopes, and this binary model has enabled advances in understanding host-microbe communication, colonization specificity,in vivobiofilms, intraspecific competition, and quorum sensing. The hummingbird bobtail squid,Euprymna berryi,can be generationally bred and maintained in lab settings and has had multiple genes deleted by CRISPR approaches. The prospect of expanding the utility of the light organ model system by producing multigenerational host lines led us to determine the extent to which theE. berryilight organ symbiosis parallels known processes inE. scolopes. However, the nature of theE. berryilight organ, including its microbial constituency and specificity for microbial partners, has not been examined. In this report, we isolated bacteria fromE. berryianimals and tank water. Assays of bacterial behaviors required in the host, as well as host responses to bacterial colonization, illustrate largely parallel phenotypes inE. berryiandE. scolopeshatchlings. This study revealsE. berryito be a valuable comparative model to complement studies inE. scolopes.IMPORTANCEMicrobiome studies have been substantially advanced by model systems that enable functional interrogation of the roles of the partners and the molecular communication between those partners. TheEuprymna scolopes-Vibrio fischerisystem has contributed foundational knowledge, revealing key roles for bacterial quorum sensing broadly and in animal hosts, for bacteria in stimulating animal development, for bacterial motility in accessing host sites, and forin vivobiofilm formation in development and specificity of an animal’s microbiome.Euprymna berryiis a second bobtail squid host, and one that has recently been shown to be robust to laboratory husbandry and amenable to gene knockout. This study identifiesE. berryias a strong symbiosis model host due to features that are conserved with those ofE. scolopes, which will enable the extension of functional studies in bobtail squid symbioses. 

Photosynthetic carbon (C) fixation by phytoplankton in the Southern Ocean (SO) plays a critical role in regulating air–sea exchange of carbon dioxide and thus global climate. In the SO, photosynthesis (PS) is often constrained by low iron, low temperatures, and low but highly variable light intensities. Recently, proton-pumping rhodopsins (PPRs) were identified in marine phytoplankton, providing an alternate iron-free, light-driven source of cellular energy. These proteins pump protons across cellular membranes through light absorption by the chromophore retinal, and the resulting pH energy gradient can then be used for active membrane transport or for synthesis of adenosine triphosphate. Here, we show that PPR is pervasive in Antarctic phytoplankton, especially in iron-limited regions. In a model SO diatom, we found that it was localized to the vacuolar membrane, making the vacuole a putative alternative phototrophic organelle for light-driven production of cellular energy. Unlike photosynthetic C fixation, which decreases substantially at colder temperatures, the proton transport activity of PPR was unaffected by decreasing temperature. Cellular PPR levels in cultured SO diatoms increased with decreasing iron concentrations and energy production from PPR photochemistry could substantially augment that of PS, especially under high light intensities, where PS is often photoinhibited. PPR gene expression and high retinal concentrations in phytoplankton in SO waters support its widespread use in polar environments. PPRs are an important adaptation of SO phytoplankton to growth and survival in their cold, iron-limited, and variable light environment. 

Abstract Southern Ocean (SO) diatoms play an important role in global carbon flux, and their influence on carbon export is directly linked to interactions with epiphytic bacteria. Bacterial symbionts that increase diatom growth promote atmospheric carbon uptake, while bacterial degraders divert diatom biomass into the microbial loop where it can then be released as carbon dioxide through respiration. To further explore SO diatom-bacterial associations, a natural model system is needed that is representative of these diverse and important interactions. Here, we use concurrent cultivation to isolate a species of the ecologically-important SO diatom, Pseudo-nitzschia subcurvata, and its co-occurring bacteria. Although vitamin-depleted, axenic Pseudo-nitzschia grew poorly in culture, addition of a co-isolated Roseobacter promoted diatom growth, while addition of a co-isolated Flavobacterium negatively impacted diatom growth. Microscopy revealed both bacterial isolates are physically associated with diatom cells and genome sequencing identified important predicted functions including vitamin synthesis, motility, cell attachment mechanisms, and diverse antimicrobial weapons that could be used for interbacterial competition. These findings revealed the natural coexistence of competing symbiotic strategies of diatom-associated bacteria in the SO, and the utility of this tripartite system, composed of a diatom and two bacterial strains, as a co-culture model to probe ecological-relevant interactions between diatoms and the bacteria that compete for access to the phycosphere.