Rapid diversification underlying the global dominance of a cosmopolitan phytoplankton

Phagocytosis in Marine Coccolithophore Gephyrocapsa huxleyi: Comparison between Calcified and Non-Calcified Strains

Coccolithophores play a significant role in marine calcium carbonate production and carbon cycles, attributing to their unique feature of producing calcareous plates, coccoliths. Coccolithophores also possess a haplo-diplontic life cycle, presenting distinct morphology types and calcification states. However, differences in nutrient acquisition strategies and mixotrophic behaviors of the two life phases remain unclear. In this study, we conducted a series of phagocytosis experiments of calcified diploid and non-calcified haploid strains of coccolithophore Gephyrocapsa huxleyi under light and dark conditions. The phagocytosis capability of each strain was examined based on characteristic fluorescent signals from ingested beads using flow cytometry and fluorescence microscopy. The results show a significantly higher phagocytosis percentage on fluorescent beads in the bacterial prey surrogates of the non-calcified haploid Gephyrocapsa huxleyi strain, than the calcified diploid strain with or without light. In addition, the non-calcified diploid cells seemingly to presented a much higher phagocytosis percentage in darkness than under light. The differential phagocytosis capacities between the calcified diploid and non-calcified haploid Gephyrocapsa huxleyi strains indicate potential distinct nutritional strategies at different coccolithophore life and calcifying stages, which may further shed light on the potential strategies that coccolithophore possesses in unfavorable environments such as twilight zones and the expanding coccolithophore niches in the natural marine environment under the climate change scenario.

Bacteria contribute exopolysaccharides to an algal-bacterial joint extracellular matrix

Marine ecosystems are influenced by phytoplankton aggregation, which affects processes like marine snow formation and harmful events such as marine mucilage outbreaks. Phytoplankton secrete exopolymers, creating an extracellular matrix (ECM) that promotes particle aggregation. This ECM attracts heterotrophic bacteria, providing a nutrient-rich and protective environment. In terrestrial environments, bacterial colonization near primary producers relies on attachment and the formation of multidimensional structures like biofilms. Bacteria were observed attaching and aggregating within algal-derived exopolymers, but it is unclear if bacteria produce an ECM that contributes to this colonization. This study, using Emiliania huxleyi algae and Phaeobacter inhibens bacteria in an environmentally relevant model system, reveals a shared algal-bacterial ECM scaffold that promotes algal-bacterial aggregation. Algal exudates play a pivotal role in promoting bacterial colonization, stimulating bacterial exopolysaccharide (EPS) production, and facilitating a joint ECM formation. A bacterial biosynthetic pathway responsible for producing a specific EPS contributing to bacterial ECM formation is identified. Genes from this pathway show increased expression in algal-rich environments. These findings highlight the underestimated role of bacteria in aggregate-mediated processes in marine environments, offering insights into algal-bacterial interactions and ECM formation, with implications for understanding and managing natural and perturbed aggregation events.

Impact of airborne algicidal bacteria on marine phytoplankton blooms

Ocean microbes are involved in global processes such as nutrient and carbon cycling. Recent studies indicated diverse modes of algal-bacterial interactions, including mutualism and pathogenicity, which have a substantial impact on ecology and oceanic carbon sequestration, and hence, on climate. However, the airborne dispersal and pathogenicity of bacteria in the marine ecosystem remained elusive. Here, we isolated an airborne algicidal bacterium, Roseovarius nubinhibens, emitted to the atmosphere as primary marine aerosol (referred also as sea spray aerosols) and collected above a coccolithophore bloom in the North Atlantic Ocean. The aerosolized bacteria retained infective properties and induced lysis of Gephyrocapsa huxleyi cultures.This suggests that the transport of marine bacteria through the atmosphere can effectively spread infection agents over vast oceanic regions, highlighting its significance in regulating the cell fate in algal blooms.

Transport-Limited Growth of Coccolith Crystals

Biogenic crystals present a variety of complex morphologies that form with exquisite fidelity. In the case of the intricate morphologies of coccoliths, calcite crystals produced by marine algae, only a single set of crystallographic facets is utilized. It is unclear which growth process can merge this simple crystallographic habit with the species-specific architectures. Here, a suite of state-of-the-art electron microscopies is used to follow both the growth trajectories of the crystals ex situ, and the cellular environment in situ, in the species Emiliania huxleyi. It is shown that crystal growth alternates between a space filling and a skeletonized growth mode, where the crystals elongate via their stable crystallographic facets, but the final morphology is a manifestation of growth arrest. This process is reminiscent of the balance between reaction-limited and transport-limited growth regimes underlying snowflake formation. It is suggested that localized ion transport regulates the kinetic instabilities that are required for transport-limited growth, leading to reproducible morphologies.

Gephyrocapsa huxleyi (Emiliania huxleyi) as a model system for coccolithophore biology

Coccolithophores are the most abundant calcifying organisms in modern oceans and are important primary producers in many marine ecosystems. Their ability to generate a cellular covering of calcium carbonate plates (coccoliths) plays a major role in marine biogeochemistry and the global carbon cycle. Coccolithophores also play an important role in sulfur cycling through the production of the climate-active gas dimethyl sulfide. The primary model organism for coccolithophore research is Emiliania huxleyi, now named Gephyrocapsa huxleyi. G. huxleyi has a cosmopolitan distribution, occupying coastal and oceanic environments across the globe, and is the most abundant coccolithophore in modern oceans. Research in G. huxleyi has identified many aspects of coccolithophore biology, from cell biology to ecological interactions. In this perspective, we summarize the key advances made using G. huxleyi and examine the emerging tools for research in this model organism. We discuss the key steps that need to be taken by the research community to advance G. huxleyi as a model organism and the suitability of other species as models for specific aspects of coccolithophore biology.

Taxonomy and morphology of Calciopappus curvus sp. nov. (Syracosphaeraceae, Prymnesiophyceae), a novel appendage-bearing coccolithophore

Based on scanning electron microscopy observations, a new species of the coccolithophore genus Calciopappus (Syracosphaeraceae, Prymnesiophyceae) is described from the surface waters off Bergen and from the lower photic zone of sub-tropical and tropical waters. Morphological, coccolith rim structure and biometric analyses strongly support separation of this morphotype from the two described Calciopappus species, but inclusion of it within the genus. The new form differs from the other species in being noticeably smaller and in morpho-structural details of each of the three coccolith types that form the coccosphere: (1) the body coccoliths have an open central area; (2) the whorl coccoliths have a wide central opening and two thumb-like protrusions; and (3) the appendage coccoliths are curved. On this basis, the species is formally described as Calciopappus curvus sp. nov., its systematic affinity is discussed and compared with other extant coccolithophores.

Aerobic bacteria produce nitric oxide via denitrification and promote algal population collapse

Microbial interactions govern marine biogeochemistry. These interactions are generally considered to rely on exchange of organic molecules. Here we report on a novel inorganic route of microbial communication, showing that algal-bacterial interactions between Phaeobacter inhibens bacteria and Gephyrocapsa huxleyi algae are mediated through inorganic nitrogen exchange. Under oxygen-rich conditions, aerobic bacteria reduce algal-secreted nitrite to nitric oxide (NO) through denitrification, a well-studied anaerobic respiratory mechanism. The bacterial NO is involved in triggering a cascade in algae akin to programmed cell death. During death, algae further generate NO, thereby propagating the signal in the algal population. Eventually, the algal population collapses, similar to the sudden demise of oceanic algal blooms. Our study suggests that the exchange of inorganic nitrogen species in oxygenated environments is a potentially significant route of microbial communication within and across kingdoms.