Mating type specific transcriptomic response to sex inducing pheromone in the pennate diatom Seminavis robusta

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.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

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.

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

Diatoms are fast-growing and winning competitors in aquatic environments, possibly due to optimized growth performance. However, their life cycles are complex, heteromorphic, and not fully understood. Here, we report on the fine control of cell growth and physiology during the sexual phase of the marine diatom Pseudo-nitzschia multistriata. We found that mating, under nutrient replete conditions, induces a prolonged growth arrest in parental cells. Transcriptomic analyses revealed down-regulation of genes related to major metabolic functions from the early phases of mating. Single-cell photophysiology also pinpointed an inhibition of photosynthesis and storage lipids accumulated in the arrested population, especially in gametes and zygotes. Numerical simulations revealed that growth arrest affects the balance between parental cells and their siblings, possibly favoring the new generation. Thus, in addition to resources availability, life cycle traits contribute to shaping the species ecological niches and must be considered to describe and understand the structure of plankton communities.

Light intensity and spectral composition drive reproductive success in the marine benthic diatom Seminavis robusta

The properties of incident light play a crucial role in the mating process of diatoms, a group of ecologically important microalgae. While species-specific requirements for light intensity and photoperiod have been observed in several diatom species, little is known about the light spectrum that allows sexual reproduction. Here, we study the effects of spectral properties and light intensity on the initiation and progression of sexual reproduction in the model benthic diatom Seminavis robusta. We found that distinct stages of the mating process have different requirements for light. Vigorous mating pair formation occurred under a broad range of light intensities, ranging from 10 to 81 µE m⁻² s⁻¹, while gametogenesis and subsequent stages were strongly affected by moderate light intensities of 27 µE m⁻² s⁻¹ and up. In addition, light of blue or blue–green wavelengths was required for the formation of mating pairs. Combining flow cytometric analysis with expression profiling of the diatom-specific cyclin dsCyc2 suggests that progression through a blue light-dependent checkpoint in the G1 cell cycle phase is essential for induction of sexual reproduction. Taken together, we expand the current model of mating in benthic pennate diatoms, which relies on the interplay between light, cell cycle and sex pheromone signaling.

Diurnal transcript profiling of the diatom Seminavis robusta reveals adaptations to a benthic lifestyle

Coastal regions contribute an estimated 20% of annual gross primary production in the oceans, despite occupying only 0.03% of their surface area. Diatoms frequently dominate coastal sediments, where they experience large variations in light regime resulting from the interplay of diurnal and tidal cycles. Here, we report on an extensive diurnal transcript profiling experiment of the motile benthic diatom Seminavis robusta. Nearly 90% (23 328) of expressed protein-coding genes and 66.9% (1124) of expressed long intergenic non-coding RNAs showed significant expression oscillations and are predominantly phasing at night with a periodicity of 24 h. Phylostratigraphic analysis found that rhythmic genes are enriched in highly conserved genes, while diatom-specific genes are predominantly associated with midnight expression. Integration of genetic and physiological cell cycle markers with silica depletion data revealed potential new silica cell wall-associated gene families specific to diatoms. Additionally, we observed 1752 genes with a remarkable semidiurnal (12-h) periodicity, while the expansion of putative circadian transcription factors may reflect adaptations to cope with highly unpredictable external conditions. Taken together, our results provide new insights into the adaptations of diatoms to the benthic environment and serve as a valuable resource for the study of diurnal regulation in photosynthetic eukaryotes.

Pheromone Mediated Sexual Reproduction of Pennate Diatom Cylindrotheca closterium

Benthic diatoms dominate primary production in marine subtidal and intertidal environments. Their extraordinary species diversity and ecological success is thought to be linked with their predominantly heterothallic sexual reproduction. Little is known about pheromone involvement during mating of pennate diatoms. Here we describe pheromone guided mating in the coastal raphid diatom Cylindrotheca closterium. We show that the two mating types (mt⁺ and mt⁻) have distinct functions. Similar to other benthic diatoms, mt⁺ cells are searching for the mt⁻ cells to pair. To enhance mating efficiency mt⁻ exudes an attraction pheromone which we proved by establishing a novel capillary assay. Further, two more pheromones produced by mt⁻ promote the sexual events. One arrests the cell cycle progression of mt⁺ while the other induces gametogenesis of mt⁺. We suggest that C. closterium shares a functionally similar pheromone system with other pennate diatoms like Seminavis robusta and Pseudostaurosira trainorii which synchronize sexual events and mate attraction. Remarkably, we found no evidence of mt⁺ producing pheromones, which differentiates C. closterium from other pennates and suggests a less complex pheromone system in C. closterium.

Altering the Sex Pheromone Cyclo(l-Pro-l-Pro) of the Diatom Seminavis robusta towards a Chemical Probe

As a major group of algae, diatoms are responsible for a substantial part of the primary production on the planet. Pennate diatoms have a predominantly benthic lifestyle and are the most species-rich diatom group, with members of the raphid clades being motile and generally having heterothallic sexual reproduction. It was recently shown that the model species Seminavis robusta uses multiple sexual cues during mating, including cyclo(l-Pro-l-Pro) as an attraction pheromone. Elaboration of the pheromone-detection system is a key aspect in elucidating pennate diatom life-cycle regulation that could yield novel fundamental insights into diatom speciation. This study reports the synthesis and bio-evaluation of seven novel pheromone analogs containing small structural alterations to the cyclo(l-Pro-l-Pro) pheromone. Toxicity, attraction, and interference assays were applied to assess their potential activity as a pheromone. Most of our analogs show a moderate-to-good bioactivity and low-to-no phytotoxicity. The pheromone activity of azide- and diazirine-containing analogs was unaffected and induced a similar mating behavior as the natural pheromone. These results demonstrate that the introduction of confined structural modifications can be used to develop a chemical probe based on the diazirine- and/or azide-containing analogs to study the pheromone-detection system of S. robusta.