Explore the vast range of titles available.

Adaptation of cell populations to environmental changes is mediated by phenotypic variability at the single-cell level. Enzyme activity is a key factor in cell phenotype and the expression of the alkaline phosphatase activity (APA) is a fundamental phytoplankton strategy for maintaining growth under phosphate-limited conditions. Our aim was to compare the APA among cells and species revived from sediments of the Bay of Brest (Brittany, France), corresponding to a pre-eutrophication period (1940’s) and a beginning of a post-eutrophication period (1990’s) during which phosphate concentrations have undergone substantial variations. Both toxic marine dinoflagellate Alexandrium minutum and the non-toxic dinoflagellate Scrippsiella acuminata were revived from ancient sediments. Using microfluidics, we measured the kinetics of APA at the single-cell level. Our results indicate that all S. acuminata strains had significantly higher APA than A. minutum strains. For both species, the APA in the 1990’s decade was significantly lower than in the 1940’s. For the first time, our results reveal both inter and intraspecific variabilities of dinoflagellate APA and suggest that, at a half-century timescale, two different species of dinoflagellate may have undergone similar adaptative evolution to face environmental changes and acquire ecological advantages.

Recurrent blooms of the toxic dinoflagellate Ostreopsis cf. ovata are frequently reported in the Northwestern Mediterranean Sea. The impact of these proliferations on other microalgal species inhabiting the same habitats is of interest from an ecological prospective. In vitro experiments were carried out to investigate the influence of O. cf. ovata on the growth of the co-occurring benthic diatoms Licmophora paradoxa, Navicula arenaria and the benthic dinoflagellates Prorocentrum lima and Coolia monotis. Overall, O. cf. ovata exhibited weak allelopathic effects towards these microalgal species, with a reduction in the cell abundance for L. paradoxa and P. lima only. Interestingly, dead cells of L. paradoxa and N. arenaria were observed embedded in the thick mucus surrounding O. cf. ovata cells, suggesting that the mucous layer could act as a toxic phycosphere, especially for non-motile cells. All competitors were further exposed for 24 h to ovatoxins, the major toxins produced by O. cf. ovata, and the maximum quantum yield efficiency of L. paradoxa, N. arenaria and P. lima was affected at a minimum concentration of 10 µg mL−1. We then hypothesized that the diffusion of solubilized ovatoxins in the culture medium affects only moderately the competitors’ growth, whereas their accumulation in the mucus would yield deleterious effects. More precisely, the competitors’ sensitivity to ovatoxins was enhanced in their stationary phase of growth and resulted from a rapid inhibition of an uncharacterized photosynthetic step downstream photosystem II. Altogether, these results emphasize the predominant role of the O. cf. ovata’s mucus in driving ecological interactions and suggest that it can affect the growth of several benthic microalgae by accumulating the potent ovatoxins.

Sabellaria alveolata is a sedentary gregarious tube-building species widely distributed from southwest Scotland to Morocco. This species builds what are currently considered the largest European biogenic reefs in the bay of Mont-Saint-Michel (France). As an ecosystem engineer, S. alveolata generates small to large scale topographic complexity, creating numerous spatial and trophic niches for other species to colonize. Sabellaria reefs are also under anthropogenic pressures, leading locally to massive degradation. However, stakeholders lack spatially explicit measures of reef ecological status, at adapted spatial resolution to provide key management information for this protected habitat. Traditional field surveys are extremely time-consuming and rely on expertise for visual ecological status assessment. The present study aims at using an automatic processing approach based on optical airborne data to (i) assess the potential of hyperspectral imagery to discriminate Sabellaria bioconstructions and its main ecosystem associated habitats, including different types of substrate as well as biological components and (ii) to use the combination of the hyperspectral and LiDAR signals to estimate the spatial structure of the different bioconstruction types (veneers vs hummocks and platforms) and ecological phases (retrograding and prograding). A reef from Mont-Saint-Michel was used as a test site. We built a processing chain based on supervised classification using Mahalanobis distance to generate an accurate distribution map (overall accuracy of 88% and a Kappa of 0.85) of ten Sabellaria-related benthic features, including large reef developing on sand and smaller veneers encrusting rocky shore areas. Specific spectral indices were used to define the spatial distribution of the main primary producers, in particular the microphytobenthos. Joining the hyperspectral and LiDAR data led characterizing the distribution of S. alvealata’s ecological status (prograding and retrograding phases) with an overall classification accuracy and Kappa coefficient that can respectively amount to up to 93% and 0.86. In our study site, the Sabellaria reef area (between 5.52 ha and 6.76 ha) was dominated by retrograding phases (between 53% and 58%). Our results showed that this automatic processing chain could be relevant for the spatial characterization of other Sabellaria reef sites. Study perspectives tend towards a quantitative estimation of their ecological status index.

Parasites in the genus Amoebophrya sp. infest dinoflagellate hosts in marine ecosystems and can be determining factors in the demise of blooms, including toxic red tides. These parasitic protists, however, rarely cause the total collapse of Dinophyceae blooms. Experimental addition of parasite-resistant Dinophyceae (Alexandrium minutum or Scrippsiella donghaienis) or exudates into a well-established host-parasite coculture (Scrippsiella acuminata-Amoebophrya sp.) mitigated parasite success and increased the survival of the sensitive host. This effect was mediated by waterborne molecules without the need for a physical contact. The strength of the parasite defenses varied between dinoflagellate species, and strains of A. minutum and was enhanced with increasing resistant host cell concentrations. The addition of resistant strains or exudates never prevented the parasite transmission entirely. Survival time of Amoebophrya sp. free-living stages (dinospores) decreased in presence of A. minutum but not of S. donghaienis. Parasite progeny drastically decreased with both species. Integrity of the dinospore membrane was altered by A. minutum, providing a first indication on the mode of action of anti-parasitic molecules. These results demonstrate that extracellular defenses can be an effective strategy against parasites that protects not only the resistant cells producing them, but also the surrounding community.

Summary Parasites of the genus Amoebophrya sp. are important contributors to marine ecosystems and can be determining factors in the demise of blooms of Dinophyceae, including microalgae commonly responsible for toxic red tides. Yet they rarely lead to the total collapse of Dinophyceae blooms. The addition of resistant Dinophyceae (Alexandrium minutum or Scrippsiella donghaienis) or their exudate into a well-established host-parasite culture (Scrippsiella acuminata-Amoebophrya sp.) mitigated the success of the parasite and increased the survival of the sensitive host. Effect were mediated via water-borne molecules without the need of a physical contact. Severity of the anti-parasitic defenses fluctuated depending on the species, the strain and its concentration, but never totally prevented the parasite transmission. The survival time of Amoebophrya sp. free-living stages (dinospores) decreased in presence of A. minutum but not of S. donghaienis. The progeny drastically decreased with both species. Integrity of the membrane of dinospores was altered by A. minutum which provided a first indication on the mode of action of these anti-parasitic molecules. These results demonstrate that extracellular defenses are an effective strategy against parasites that does not only protect the resistant cells but also have the potential to affect the whole surrounding community. Competing Interest Statement The authors have declared no competing interest.

Marine alveolates (MALVs) are diverse, primarily parasitic micro-eukaryotes that significantly impact marine ecosystems. The life cycles of most MALVs remain elusive and the role of sexual reproduction in these organisms is a key question that may determine their ecological success. In this study we focus on a widespread dinoflagellate parasite of bloom-forming dinoflagellates, Amoebophrya. After infection, we identified two distinct spores, differing in size, ultrastructure, swimming behavior, lifespan, gene expression, and metabolite composition. The smaller spores serve as infectious propagules, equipped with an apical complex for host invasion. They exhibit a distinct, shorter, and straighter swimming pattern, likely optimized for an extended lifespan while enhancing dispersion and chance for host encounters. Transcriptomic analysis reveals that these smaller spores are primed for efficient protein synthesis upon initiating a new infection. Conversely, the larger spores cannot infect new hosts and are characterized by the expression of meiotic genes, underscoring their sexual nature. They have a shorter lifespan, exhibit more tortuous movement, along display condensed chromosomes, signaling readiness for mating. Interestingly, infected hosts already express meiotic genes, and a single infected host only produces progeny of the same spore type, suggesting that cell fate is determined prior to spore release. Our study provides one of the first formal demonstrations of a sexually specialized cell in MALVs. Isolating compatible strains for cross-breeding and understanding how environmental conditions favor each reproductive route are the next key questions for elucidating the ecological success of MALVs in marine waters. Marine alveolates (MALVs) are ecologically significant parasites that impact carbon cycling, causing major disease outbreaks affecting fisheries and aquaculture, and influencing the dynamics of harmful algal blooms. Despite their diversity and wide host range, much of our knowledge comes from environmental DNA, leaving important aspects of their biology, such as their life cycles, largely unknown. This study provides the first evidence of sexual reproduction in MALVs, linking spore polymorphism to infective or sexual routes. This discovery is crucial as sexual reproduction increases genetic diversity and adaptability, aiding MALVs’ resilience in changing environments. Understanding MALVs’ reproductive strategies deepens our insight into their ecological roles and their broader impact on marine ecosystems.