Biology subjects

Le Gac, M.

Publications and source records attributed to Le Gac, M..

4 recordsLinked to original sources

bioRxiv2026

Hydrodynamics structure the physiological trajectory and bloom dynamics of a harmful diatom species across neighboring coastal ecosystems

Hydrodynamics is a strong regulator of phytoplankton cell densities and population connectivity. However, the extent to which hydrodynamics, specifically dilution and advection, dictates the internal physiological state and overall bloom dynamics remains poorly understood. We conducted an integrated study of the harmful diatom Pseudo-nitzschia australis in two neighboring coastal bays with similar abiotic conditions but contrasting bloom trajectories. Using a combination of field observations, transcriptomics, domoic acid (DA) measurements, and hydrodynamic simulations, we show that a three-fold difference in dilution rates alone explains a 25-day shift in bloom timing and an order-of-magnitude difference in cell densities. Transcriptomic profiling revealed distinct metabolic and stress-associated gene modules that synchronized with hydrodynamically-mediated nutrient depletion rate, linking residence times to cellular mortality. Furthermore, directional connectivity between the bays may explain population-level toxin heterogeneity, as advection of older cells, releasing DA outside the cells, may have decreased the proportion of intracellular DA in the destination bay. Our results illustrate that hydrodynamics do not merely modulate population transport; they structure the full physiological life cycle of a bloom, from initiation to nutrient-driven demise. This highlights the necessity of considering hydrodynamics as a primary explanation for bloom dynamics with cascading effects on bloom onset, cell density, nutrient consumption, cell physiology and mortality.

ecology↗

bioRxiv2026

Paralytic Shellfish Toxin production in Alexandrium minutum (Dinophyceae): insights from omics integration using toxigenic and non-toxigenic recombinant progeny

Paralytic Shellfish Toxins (PSTs) are produced by certain species of cyanobacteria and dinoflagellates. Part of the PST biosynthetic pathway has been elucidated in cyanobacteria, and the implication of some sxt genes has been confirmed by experimental studies. Contrary to cyanobacteria, knowledge about PST biosynthesis in dinoflagellates is more limited and generally restricted to comparative studies with the cyanobacterial pathway. To investigate the specificity of the PST pathway in dinoflagellates, 16 toxic and non-toxic A. minutum strains from a recombinant cross were compared, without prior assumption on genes or metabolites involved in PST synthesis, using an integrative approach combining untargeted metabolomic and transcriptomic data. Among the 60 most distinguishing transcripts between toxic and non-toxic strains, only 3 sxt genes were present, sxtA4, sxtG, and sxtI. In contrast, non-sxt homologs were detected as highly discriminant between these two phenotypes. More specifically, a phyH homolog may act as the analog of sxtS found in cyanobacteria. Moreover, we identified four putative synthetic PST intermediates. Among these, Int-C2, correlated with the toxic phenotype, whereas 3 others were detected in both toxic and non-toxic strains, suggesting that these strains may share some parts of the biosynthetic pathway. Finally, our results showed that PST biosynthesis in dinoflagellate results from the activity of sxt genes, acquired by horizontal gene transfer from cyanobacteria, as well as from other genes not acquired from cyanobacteria, such as phyH.

genomics↗

bioRxiv2025

Two Spore Types in a Marine Parasite of Dinoflagellates

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. Significance StatementMarine 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.

microbiology↗

bioRxiv2021

Dinophyceae use exudates as weapons against the parasite Amoebophrya sp. (Syndiniales)

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.

microbiology↗