The role of geographic isolation in marine microbial speciation is hotly debated because of the high dispersal potential and large population sizes of planktonic microorganisms and the apparent lack of strong dispersal barriers in the open sea. Here, we show that gene flow between distant populations of the globally distributed, bloom-forming diatom species Pseudo-nitzschia pungens (clade I) is limited and follows a strong isolation by distance pattern. Furthermore, phylogenetic analysis implies that under appropriate geographic and environmental circumstances, like the pronounced climatic changes in the Pleistocene, population structuring may lead to speciation and hence may play an important role in diversification of marine planktonic microorganisms. A better understanding of the factors that control population structuring is thus essential to reveal the role of allopatric speciation in marine microorganisms.allopatric speciation | dispersal | marine cosmopolitan planktonic microorganisms | population structure | microsatellites

ABSTRACT1. Even though beach nourishment is generally considered as an environment-friendly option for coastal protection and beach restoration, sizeable impacts on several beach ecosystem components (microphytobenthos, vascular plants, terrestrial arthropods, marine zoobenthos and avifauna) are described in the literature, as reviewed in this paper.2. Negative, ecosystem-component specific effects of beach nourishment dominate in the short to medium term, with the size of the impact being determined by (1) activities during the construction phase, (2) the quality and (3) the quantity of the nourishment sand, (4) the timing, place and size of project, and (5) the nourishment technique and strategy applied. Over the long term the speed and degree of ecological recovery largely depend on the physical characteristics of the beach habitat, mainly determined by (1) sediment quality and quantity, (2) the nourishment technique and strategy applied, (3) the place and the size of nourishment and (4) the physical environment prior to nourishment.3. The limited information available on indirect and cumulative ecological effects indicates that these effects cannot be neglected in an overall impact assessment. Hence, for ecologically good practice of beach nourishment it is advised (1) to choose nourishment sands with a sediment composition comparable to that of the natural sediment, (2) to avoid short-term compaction by ploughing immediately after construction, (3) to execute the nourishment in a period of low beach use by birds and other mobile organisms, (4) to choose a number of smaller projects rather than a single large nourishment project and (5) to select the nourishment technique with respect to the local natural values. *Correspondence to: J. Speybroeck, Ghent University, Biology Department, Marine Biology Section, Krijgslaan 281, Building S8, B-9000 Ghent, Belgium. E-mail: jeroen.speybroeck@UGent.be 4. In order to allow an objective, scientifically sound, ecological adjustment of future nourishments, research should aim at (1) taking into account the full sandy beach ecosystem, (2) avoiding strategic imperfections in experimental design and (3) elucidating the biological processes behind impact and recovery of all ecosystem components.

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 cDNAamplified 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 fucoxanthinchlorophyll 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.

Salinity is generally considered to be the dominant environmental factor regulating aquatic community structure in hydrologically closed lakes and wetlands, but it is not well known whether community response to long‐term trends in hydrological balance is driven primarily by the direct physiological effect of salinity stress or by the habitat restructuring that accompanies changes in lake level and salinity. Attempts to separate the effects of various environmental factors on invertebrate populations in shallow fluctuating lakes through field study are hampered by the typically large temporal and spatial variation in species abundances and the long time scale of climate‐driven habitat restructuring relative to the period of study. We used paleolimnological techniques to document long‐term dynamics of the benthic invertebrate community inhabiting a shallow fluctuating lake in Kenya where during the period ∼1870–1991 lake depth fluctuated between 4 and 19 m, and lakewater conductivity between ∼250 and 14000 μS/cm. Analyses of sediment texture, plant macrofossils, and fossil diatom assemblages in a 210Pb‐dated sediment core were combined with historical lake‐level data to reconstruct continuous records of past changes in salinity and the distribution of various types of benthic habitat. Fossil invertebrate faunas recovered from the same core comprised 58 species of Ostracoda, chydorid Cladocera, and Chironomidae, with 26 common species together accounting for >95% of the recovered fossils. Uni‐ and multivariate statistical analyses of the faunal and environmental data revealed distinct species‐specific responses to lake level, salinity, and papyrus‐swamp development. Redundancy analysis and variation partitioning showed that salinity and swamp development together explained 51% of the observed historical variation in benthic community composition, and that their effects were independent. In the univariate analyses, six species (23%) showed a strong response to salinity. Nine species (35%) responded strongly to swamp development, which determines availability of submerged vegetated substrates and turbulence‐free, littoral mud bottoms. Five species (19%) responded primarily to lake level, which affects the general distribution of sand and mud bottoms and the intensity of subaqueous spring discharge. The remaining six species (23%) showed no distinct pattern or correlation with the selected environmental factors. These results suggest that a significant portion of the documented correlation between salinity and invertebrate community structure along the full gradient of inland aquatic ecosystems may be an indirect effect of broad but diffuse relationships between salinity and the distribution of various types of benthic microhabitat. Decade‐scale environmental change experienced by individual closed‐basin lakes tends to remain limited to a relatively narrow portion of this gradient, where covariance between salinity and other ecological determinants may be weak. Therefore, local populations of benthic invertebrates will...