An inordinate fondness?: the number, distributions, and origins of diatom species

The number of extant species of diatoms is estimated here to be at least 30,000 and probably ca. 100,000, by extrapolation from an eclectic sample of genera and species complexes. Available data, although few, indicate that the pseudocryptic species being discovered in many genera are not functionally equivalent. Molecular sequence data show that some diatom species are ubiquitously dispersed. A good case can be made that at least some diatom species and even a few genera are endemics, but many such claims are still weak. The combination of very large species numbers and relatively rapid dispersal in diatoms is inconsistent with some versions of the ubiquity hypothesis of protist biogeography, and appears paradoxical. However, population genetic data indicate geographical structure in all the (few) marine and freshwater species that have been examined in detail, sometimes over distances of a few tens of kilometres. The mode of speciation may often be parapatric, in the context of a constantly shifting mosaic of temporarily isolated (meta) populations, but if our intermediate dispersal hypothesis is true (that long-distance dispersal is rare, but not extremely rare), allopatric speciation could also be maximized

Local genetic adaptation to grazing pressure of the green alga Desmodesmus armatus in a strongly connected pond system

Dispersal potentially homogenizes genetic variation among populations and thus may prevent local genetic adaptation. If selection gradients are strong and the selection response efficient, however, local genetic adaptation may persist in the face of high dispersal rates. We compared grazing-resistance traits among populations of the green microalga Desmodesmus armatus, which inhabit ponds that are ecologically different but part of a strongly interconnected pond system. Desmodesmus clones were isolated from a clear-water and a turbid pond. For 16 clones from an internal transcribed spacer 2 clade with low sequence variation (1.3%) corresponding morphologically to D. armatus, coenobial dimensions and the average number of cells per coenobium, in both the absence and the presence of water conditioned by their main grazer, the waterflea Daphnia, were determined. Clones from the clear-water pond had four-celled coenobia with a higher greatest axial linear dimension and an increased average number of cells per coenobium in response to Daphnia kairomone, contrary to clones from the turbid pond. Unexpectedly, they were also characterized by a lower average number of cells per coenobium. No differences among populations were detected for cell length. Genetic variation was present in both populations for all traits, except for the response to kairomone. Continuous dispersal through overflows and rivulets in this pond system is thus incapable of preventing strong among-population genetic differentiation for ecological relevant traits, testifying both to the capacity of phytoplankton populations to adapt to local conditions and to the importance of grazing as a structuring factor in natural phytoplankton populations

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

A Hotspot of Amoebae Diversity: 8 New Naked Amoebae Associated with the Planktonic Bloom-forming Cyanobacterium Microcystis

The colonies of Microcystis, one of the most common bloom-forming cyanobacteria worldwide, harbor a diverse community of microorganisms. Among these, naked amoebae feeding on Microcystis cells can strongly influence natural Microcystis population dynamics. In this study, we investigated the species diversity of these amoebae based on 26 Microcystis-associated amoebae (MAA) strains from eutrophied water bodies in Belgium and elsewhere in western Europe. A detailed morphological characterization in combination with 18S rDNA (SSU) phylogenies revealed the presence of no less than 10 species. Nine of these belonged to the known genera Vannella (2 species), Korotnevella (2), Copromyxa (2), Vexillifera (1), Cochliopodium (1) and the recently described Angulamoeba (1). Only two were previously described, the others were new to science. One taxon could not be assigned to a known genus and is here described as Schoutedamoeba gen. n., representing a new variosean lineage. The discovery of so many new species from only one very specific habitat (Microcystis colonies) from a rather restricted geographical area indicates that the diversity of planktonic naked amoebae is much higher than previously appreciated and that only a tiny fraction of the total diversity of naked amoebae is currently known

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