Physiological and transcriptomic evidence for a close coupling between chloroplast ontogeny and cell cycle progression in the pennate diatom Seminavis robusta

Despite the growing interest in diatom genomics, detailed time series of gene expression in relation to key cellular processes are still lacking. Here, we investigated the relationships between the cell cycle and chloroplast development in the pennate diatom Seminavis robusta. This diatom possesses two chloroplasts with a well-orchestrated developmental cycle, common to many pennate diatoms. By assessing the effects of induced cell cycle arrest with microscopy and flow cytometry, we found that division and reorganization of the chloroplasts are initiated only after S-phase progression. Next, we quantified the expression of the S. robusta FtsZ homolog to address the division status of chloroplasts during synchronized growth and monitored microscopically their dynamics in relation to nuclear division and silicon deposition. We show that chloroplasts divide and relocate during the S/G2 phase, after which a girdle band is deposited to accommodate cell growth. Synchronized cultures of two genotypes were subsequently used for a cDNA-amplified fragment length polymorphism-based genome-wide transcript profiling, in which 917 reproducibly modulated transcripts were identified. We observed that genes involved in pigment biosynthesis and coding for light-harvesting proteins were up-regulated during G2/M phase and cell separation. Light and cell cycle progression were both found to affect fucoxanthin-chlorophyll a/c-binding protein expression and accumulation of fucoxanthin cell content. Because chloroplasts elongate at the stage of cytokinesis, cell cycle-modulated photosynthetic gene expression and synthesis of pigments in concert with cell division might balance chloroplast growth, which confirms that chloroplast biogenesis in S. robusta is tightly regulated

Global radiation in a rare biosphere soil diatom

Soil micro-organisms drive the global carbon and nutrient cycles that underlie essential ecosystem functions. Yet, we are only beginning to grasp the drivers of terrestrial microbial diversity and biogeography, which presents a substantial barrier to understanding community dynamics and ecosystem functioning. This is especially true for soil protists, which despite their functional significance have received comparatively less interest than their bacterial counterparts. Here, we investigate the diversification of Pinnularia borealis, a rare biosphere soil diatom species complex, using a global sampling of >800 strains. We document unprecedented high levels of species-diversity, reflecting a global radiation since the Eocene/Oligocene global cooling. Our analyses suggest diversification was largely driven by colonization of novel geographic areas and subsequent evolution in isolation. These results illuminate our understanding of how protist diversity, biogeographical patterns, and members of the rare biosphere are generated, and suggest allopatric speciation to be a powerful mechanism for diversification of micro-organisms

Diatom distributions in space and time – a case study from the polar regions

第3回極域科学シンポジウム/第34回極域生物シンポジウム 11月27日（火） 統計数理研究所 ３階セミナー

Tolerance of resting cells of freshwater and terrestrial benthic diatoms to experimental desiccation and freezing is habitat-dependent

For a wide range of organisms, dormancy is a strategy to overcome adverse conditions in time and space, and one may expect that the stress tolerance of dormant stages is tuned to the habitat in which they occur. We assessed the tolerance of vegetative and resting cells of 17 benthic diatom morphospecies from habitats with contrasting permanency, ranging from moist soils to permanent lakes. Vegetative cells of all morphospecies were highly sensitive to desiccation and, except for strains of some terrestrial taxa, freezing. In contrast, resting cells of several morphospecies tolerated desiccation, especially when preceded by a heat treatment, and resting cells of more strains and morphospecies survived freezing, albeit often with low survival percentages and a large interstrain variation. Strikingly, all strains surviving desiccation and/or freezing belonged to terrestrial morphospecies, i.e. diatoms occurring mainly in wet and moist or temporary dry habitats outside water bodies. These results emphasize the importance of resting cells of terrestrial diatoms for the survival of stress tolerance, especially desiccation, and indicate specific adaptations of terrestrial diatoms to their highly variable habitats

Pinnularia borealis: disentangling the evolutionary history of a terrestrial diatom using genetics, fossils and ecophysiological data

Pinnularia borealis Ehrenberg is a terrestrial diatom, commonly found in moist soils and temporary freshwater habitats all over the world, including the (sub)Antarctic. Given the widespread (pseudo)cryptic species diversity in diatoms and the restricted distribution patterns of many (sub)Antarctic diatoms revealed by dedicated morphological studies, we asked whether Pinnularia borealis is indeed a single species and started reconstructing the evolutionary history of this cosmopolitan diatom. We observed that, unlike freshwater diatoms, resting cells of P. borealis are tolerant for at least short periods of desiccation, suggesting that airborne dispersal is possible which may limit opportunities for allopatric speciation. Nevertheless, molecular phylogenies based on the plastid gene rbcL and the nuclear 28S rDNA (D1-D3 region) revealed that P. borealis consists of multiple lineages, including a distinct continental Antarctic lineage. Ongoing work includes the addition of isolates from the maritime Antarctic and the (sub)Antarctic Marion Island. A molecular clock for Pinnularia calibrated by fossil data estimates the age of P. borealis at 35.8 (30-47) million years (Ma), and the continental Antarctic lineage at 7.8 (2-15) Ma. Compared to the lineages from non-polar regions, the continental Antarctic lineage of P. borealis has a lower optimal growth temperature and upper lethal temperature, indicating niche differentiation. The distinct molecular lineages, the old age of these lineages and the (partial) thermal niche differentiation suggest that long-distance dispersal is not common enough in P. borealis to prevent speciation, despite the desiccation tolerance of the resting cells, and indicate that besides freshwater diatoms, also terrestrial diatoms are not as ubiquitous as previously thought