Trade-off between sex and growth in diatoms: Molecular mechanisms and demographic implications

Decoupling cell size homeostasis in diatoms from the geometrical constraints of the silica cell wall

Unicellular organisms are known to exert tight control over their cell size. In the case of diatoms, abundant eukaryotic microalgae, two opposing notions are widely accepted. On the one hand, the rigid silica cell wall that forms inside the parental cell is thought to enforce geometrical reduction of the cell size. On the other hand, numerous exceptions cast doubt on the generality of this model. Here, we monitored clonal cultures of the diatom Stephanopyxis turris for up to 2 yr, recording the sizes of thousands of cells, in order to follow the distribution of cell sizes in the population. Our results show that S. turris cultures above a certain size threshold undergo a gradual size reduction, in accordance with the postulated geometrical driving force. However, once the cell size reaches a lower threshold, it fluctuates around a constant size using the inherent elasticity of cell wall elements. These results reconcile the disparate observations on cell size regulation in diatoms by showing two distinct behaviors, reduction and homeostasis. The geometrical size reduction is the dominant driving force for large cells, but smaller cells have the flexibility to re-adjust the size of their new cell walls.

Transcriptional chronology reveals conserved genes involved in pennate diatom sexual reproduction

Sexual reproduction is a major driver of adaptation and speciation in eukaryotes. In diatoms, siliceous microalgae with a unique cell size reduction-restitution life cycle and among the world's most prolific primary producers, sex also acts as the main mechanism for cell size restoration through the formation of an expanding auxospore. However, the molecular regulators of the different stages of sexual reproduction and size restoration are poorly explored. Here, we combined RNA sequencing with the assembly of a 55 Mbp reference genome for Cylindrotheca closterium to identify patterns of gene expression during different stages of sexual reproduction. These were compared with a corresponding transcriptomic time series of Seminavis robusta to assess the degree of expression conservation. Integrative orthology analysis revealed 138 one-to-one orthologues that are upregulated during sex in both species, among which 56 genes consistently upregulated during cell pairing and gametogenesis, and 11 genes induced when auxospores are present. Several early, sex-specific transcription factors and B-type cyclins were also upregulated during sex in other pennate and centric diatoms, pointing towards a conserved core regulatory machinery for meiosis and gametogenesis across diatoms. Furthermore, we find molecular evidence that the pheromone-induced cell cycle arrest is short-lived in benthic diatoms, which may be linked to their active mode of mate finding through gliding. Finally, we exploit the temporal resolution of our comparative analysis to report the first marker genes for auxospore identity called AAE1-3 ("Auxospore-Associated Expression"). Altogether, we introduce a multi-species model of the transcriptional dynamics during size restoration in diatoms and highlight conserved gene expression dynamics during different stages of sexual reproduction.

The pennate diatom Pseudo-nitzschia multistriata as a model for diatom life cycles, from the laboratory to the sea

Phytoplankton dynamics are regulated by external cues, such as light and nutrients, as well as by biotic interactions and endogenous controls linked to life cycle characteristics. The planktonic pennate diatom Pseudo-nitzschia multistriata, with a heterothallic mating system with two opposite mating types (MTs), represents a model for the study of diatom life cycles. P. multistriata is a toxic species, able to produce the neurotoxin domoic acid. First described in Japan in 1993, it was detected at the long-term monitoring station MareChiara (Gulf of Naples, Italy) in 1995. Since then, P. multistriata has been reported from several worldwide coastal sites. A large body of knowledge has been produced on its ecology, genetic diversity, and life cycle characteristics. The availability of these data, the ecological relevance of the Pseudo-nitzschia genus, and its controllable life cycle with a short generation time made it an ideal species to develop a genetic model system for diatoms. To enable functional studies, a 59 Mb genome sequence and several transcriptomic data were produced, and genetic transformation was optimized. These tools allowed the discovery of the first mating-type determining gene for diatoms. Gene expression studies and metabolomics analyses defined genes and molecules underpinning different phases of the process of sexual reproduction. This model system, developed to explore the genetics of diatom life cycles, offers the opportunity to parallel experimental observations in the laboratory using in situ meta-omics analyses along space and time, empowering knowledge on the biology and ecology of the genus.

Sub-monthly prediction of harmful algal blooms based on automated cell imaging

Harmful algal blooms (HABs) are an increasing threat to global fisheries and human health. The mitigation of HABs requires management strategies to successfully forecast the abundance and distribution of harmful algal taxa. In this study, we attempt to characterize the dynamics of 2 phytoplankton genera (Pseudo-nitzschia spp. and Dinophysis spp.) in Narragansett Bay, Rhode Island, using empirical dynamic modeling. We utilize a high-resolution Imaging FlowCytobot dataset to generate a daily-resolution time series of phytoplankton images and then characterize the sub-monthly (1-30 days) timescales of univariate and multivariate prediction skill for each taxon. Our results suggest that univariate predictability is low overall, different for each taxon and does not significantly vary over sub-monthly timescales. For all univariate predictions, models can rely on the inherent autocorrelation within each time series. When we incorporated multivariate data based on quantifiable image features, we found that predictability increased for both taxa and that this increase was apparent on timescales >7 days. Pseudo-nitzschia spp. has distinctive predictive dynamics that occur on timescales of around 16 and 25 days. Similarly, Dinophysis spp. is most predictable on timescales of 25 days. The timescales of prediction for Pseudo-nitzschia spp. and Dinophysis spp. could be tied to environmental drivers such as tidal cycles, water temperature, wind speed, community biomass, salinity, and pH in Narragansett Bay. For most drivers, there were consistent effects between the environmental variables and the phytoplankton taxon. Our analysis displays the potential of utilizing data from automated cell imagers to forecast and monitor harmful algal blooms.

Strain-specific transcriptional responses overshadow salinity effects in a marine diatom sampled along the Baltic Sea salinity cline

The salinity gradient separating marine and freshwater environments represents a major ecological divide for microbiota, yet the mechanisms by which marine microbes have adapted to and ultimately diversified in freshwater environments are poorly understood. Here, we take advantage of a natural evolutionary experiment: the colonization of the brackish Baltic Sea by the ancestrally marine diatom Skeletonema marinoi. To understand how diatoms respond to low salinity, we characterized transcriptomic responses of acclimated S. marinoi grown in a common garden. Our experiment included eight strains from source populations spanning the Baltic Sea salinity cline. Gene expression analysis revealed that low salinities induced changes in the cellular metabolism of S. marinoi, including upregulation of photosynthesis and storage compound biosynthesis, increased nutrient demand, and a complex response to oxidative stress. However, the strain effect overshadowed the salinity effect, as strains differed significantly in their response, both regarding the strength and the strategy (direction of gene expression) of their response. The high degree of intraspecific variation in gene expression observed here highlights an important but often overlooked source of biological variation associated with how diatoms respond to environmental change.