Tomohiro Nishimura

Sex determination mechanisms exhibit remarkable diversity across eukaryotic lineages. In the pennate diatom Pseudo-nitzschia multistriata , sex is determined by MRP3 , a gene with unknown function, which is monoallelically expressed in sexually competent mating type plus (MT+) cells and silenced in MT− cells. Here, we describe an allelic-specific methylation pattern at the sex locus, with a hypomethylated allele specific to the MT+ which is transcriptionally active, while the other hypermethylated MRP3 alleles are transcriptionally silent. In addition, the genome-wide methylation landscape remains stable throughout the life cycle. Treatment with a methylation inhibitor leads to MRP3 activation and partial sex reversal, suggesting an active role of DNA methylation in the sex determination mechanism. Long-read sequencing improved the MT-determining region resolution, highlighting a gene-poor, repeat-rich area that likely represents a primordial MT-determining locus. Our results provide insights into the mechanisms of sex determination and suggest that DNA methylation in diatoms has a more complex role than silencing repetitive regions.

Biological nitrogen fixation provides fixed nitrogen for microbes living in the oligotrophic open ocean. UCYN ‐ A2 , the previously known symbiont of Braarudosphaera bigelowii , now believed to be an early‐stage B. bigelowii organelle that exchanges fixed nitrogen for fixed carbon, is globally distributed. Indirect evidence suggested that B. bigelowii might be a mixotrophic (phagotrophic) phototrophic flagellate. The goal of this study was to determine if B. bigelowii can graze on bacteria using several independent approaches. The results showed that B. bigelowii grazed on co‐occurring bacteria at a rate of 5–7 cells/h/ B. bigelowii and that the overall grazing rate was significantly higher at nighttime than at daytime. Bacterial abundance changes, assessed with 16S rRNA gene amplicon sequencing analysis, may have indicated preferential grazing by B. bigelowii on specific bacterial genotypes. In addition, Lysotracker™ staining of B. bigelowii suggested digestive activity inside B. bigelowii . Carbon and nitrogen fixation measurements revealed that the carbon demand of B. bigelowii could not be fulfilled by photosynthesis alone, implying supplementation by heterotrophy. These independent lines of evidence together revealed that B. bigelowii engages in phagotrophy, which, beyond serving as a supplementary source of carbon and energy, may also facilitate the indirect assimilation of inorganic nutrients.

In this study, novel Calothrix-like strains NUACC09 and NUACC10 were isolated from the surfaces of rocks in Phayao Lake, Thailand. Morphological, molecular and ecological comparisons were investigated to characterize the taxonomic status of these novel strains. Under the light microscope, morphological studies indicated that these two strains were morphologically similar to Calothrix but could be differentiated by production of a non-hyaline hair at the terminal end, a low degree of tapering, twisted or loop-forming filaments, a knotted growth form, intertwined trichomes and presenting more than one trichome within the single sheath. In the 16S rRNA and rbcLX gene phylogenetic tree analyses, our strains formed a monophyletic clade with former freshwater/terrestrial Calothrix-like taxa separating distantly from other Calothrix-like genera. Furthermore, low 16S rRNA gene sequence similarity (<94.9%) to the closely related genera and species delimitation analyses (PTP/bPTP, GMYC, ABGD and ASAP methods) indicated that this clade should be considered as a different cyanobacterial lineage. The phylogenetic tree and secondary structures (D1–D1′, V2, Box-B and V3 helix) based on 16S–23S rRNA ITS regions suggested that multiple species might be contained in this clade. Therefore, here, Phayaothrix was proposed as a novel cyanobacterial genus with Phayaothrix lacustris sp. nov. as the type species following the International Code of Nomenclature for algae, fungi and plants.

Symbiotic interactions were key to the evolution of chloroplast and mitochondria organelles, which mediate carbon and energy metabolism in eukaryotes. Biological nitrogen fixation, the reduction of abundant atmospheric nitrogen gas (N₂) to biologically available ammonia, is a key metabolic process performed exclusively by prokaryotes.CandidatusAtelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N₂-fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga. Here we show that UCYN-A has been tightly integrated into algal cell architecture and organellar division and that it imports proteins encoded by the algal genome. These are characteristics of organelles and show that UCYN-A has evolved beyond endosymbiosis and functions as an early evolutionary stage N₂-fixing organelle, or “nitroplast.”

The genus Gambierdiscus produces an array of bioactive hydrophilic and lipophilic secondary metabolites that range in mode of action and toxicity. In this study, the metabolite fingerprint was mapped for thirteen Gambierdiscus, five Coolia and two Fukuyoa species (34 isolates) by assessing the production of 56 characterised secondary metabolites. Gambierdiscus polynesiensis was the only species to produce Pacific-ciguatoxin-3B (P-CTX3B), P-CTX3C, iso-P-CTX3B/C, P-CTX4A, P-CTX4B and iso-P-CTX4A/B. G. australes produced maitotoxin-1 (MTX-1) and MTX-5, G. cheloniae produced MTX-6 and G. honu produced MTX-7. Ubiquitous production of 44-methylgambierone was observed amongst all the Gambierdiscus isolates, with nine species also producing gambierone. Additional gambierone analogues, including anhydrogambierone (tentatively described herein), were also detected in all Gambierdiscus species, two Coolia and two Fukuyoa species. Gambieroxide was detected in G. lewisii and G. pacificus and gambieric acid A was detected in ten Gambierdiscus species, with G. australes (CAWD381) being the only isolate to produce gambieric acids A–D. This study has demonstrated that the isolates tested to date produce the known CTXs or MTXs, but not both, and highlighted several species that produced ‘unknown’ compounds displaying characteristics of cyclic polyethers, which will be the focus of future compound discovery efforts.

Diatoms are unicellular microalgae widely distributed in aquatic ecosystems. In diatoms, sexual reproduction is needed to counteract cell miniaturization imposed by the rigid silica shell, and only small cells, below a species-specific size threshold, are competent for sex. We performed a genome-wide Enzymatic Methyl-seq analysis in the heterothallic diatom Pseudo-nitzschia multistriata comparing cells of different size and opposite mating type (MT) to investigate potential epigenetic controls in life cycle transitions. We found an imprinting-like pattern of methylation at the sex locus: alleles of the gene responsible for the specification of the MT+, MRP3 , are hypermethylated in MT- and differentially methylated in MT+, with transcription occurring only on the MT+ hypomethylated variant. The methylation pattern is overall stable over the P. multistriata life cycle. Absence of methylation in MRP3 is necessary for its expression but not sufficient, since large non-sexual cells have the same methylation profile of small sexual cells but do not express MRP3 , suggesting that additional controls are involved in the mechanism of sex determination. The methylation profile of the mating type determining gene MRP3 of the diatom Pseudo-nitzschia multistriata is different between opposite mating types. Lack of methylation is necessary but not sufficient for MRP3 expression in the sexually competent MT+ strains. Highlights: - The mating type locus is differentially methylated in MT+ and MT- - DNA methylation is linked to MRP3 expression - DNA methylation does not play a role in the acquisition of sexual competence