Elie Verleyen

Historical processes constrain patterns in global diatom diversity

There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom data set, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere.

The diversity and distribution of diatoms: from cosmopolitanism to narrow endemism

It has been claimed that microbial taxa will not exhibit endemism because their enormous populations remove dispersal as an effective constraint on geographical range. Here we review evidence that challenges this ubiquity hypothesis for the most speciose group of microbial eukaryotes, the diatoms. Detailed taxonomic inventories using fine-grained morphological characteristics, molecular markers, and crossing experiments have revealed that the geographic distribution of diatoms ranges from global to narrow endemic. Records of human-mediated introductions of exotic species further provide a strong indication that geographic dispersal was limiting in the past. Finally, recent studies have revealed that diatom community structure and diversity are influenced by geographical factors independent of environmental conditions. Diatom communities are thus regulated by the same processes that operate in macro-organisms, although possibly to a different degree, implying that dispersal limitation is significant and the endemism observed in isolated areas is real. These results underscore the pressing need to (1) continue research into diatom biology, ecology and the factors driving diatom species diversity and geographic distributions, and (2) protect relatively isolated areas against further introductions of exotic species.

Were the Larsemann Hills ice-free through the Last Glacial Maximum?

Lake sediments in the Larsemann Hills contain a great diversity of biological and physical markers from which past environments can be inferred. In order to determine the timing of environmental changes it is essential to have accurate dating of sediments. We used radiometric (210Pb and 137Cs), radiocarbon (AMS 14C) and uranium series (238U) methods to date cores from eleven lakes. These were sampled on coastal to inland transects across the two main peninsulas, Broknes and Stornes, together with a single sample from the Bolingen Islands. Radiometric dating of recent sediments yielded 210Pb levels below acceptable detection limits. However, a relatively well-defined peak in 137Cs gave a date marker which corresponds to the fallout maximum from the atmospheric testing of atomic weapons in 1964/65. Radiocarbon (AMS 14C) measurements showed stratigraphical consistency in the age-depth sequences and undisturbed laminae in some cores provides evidence that the sediments have remained undisturbed by glacial action. In addition, freshwater surface sediments were found to be in near-equilibrium with modern 14CO2 and not influenced by radiocarbon contamination processes. This dating program, together with geomorphological records of ice flow directions and glacial sediments, indicates that parts of Broknes were ice-free throughout the Last Glacial Maximum and that some lakes have existed continuously since at least 44 ka bp. Attempts to date sediments older than 44 ka bp using 128U dating were inconclusive. However, supporting evidence for Broknes being ice-free is provided by an Optically Stimulated Luminescence date from a glaciofluvial deposit. In contrast, Stornes only became ice-free in the mid to late Holocene. This contrasting glacial history results from the Dålk Glacier which diverts ice around Broknes. Lakes on Broknes and some offshore islands therefore contain the oldest known lacustrine sediment records from eastern Antarctica, with the area providing an ice-free oasis and refuge for plants and animals throughout the Last Glacial Maximum. These sediments are therefore well placed to unravel a unique limnological sequence of environmental and climate changes in East Antarctica from the late Pleistocene to the present. This information may help better constrain models of current climate changes and ensure the adequate protection of these lakes and their catchments from the impacts of recent human occupation.

Benthic diatom flora of freshwater and saline lakes in the Larsemann Hills and Rauer Islands, East Antarctica

A floristic and taxonomic survey was made of the diatom communities of sediments and microbial mats in 66 freshwater and saline lakes and pools in the Larsemann Hills, Rauer Islands and Bølingen Islands (continental eastern Antarctica). A total of 31 taxa were distinguished, 10 of which could not be identified to species (nine) or even generic (one) level, either because they have most probably not yet been described or because they belong to species complexes that are in need of revision. Four new combinations are proposed; three species are reported for the first time from continental Antarctica, while another three are confirmed for eastern Antarctica for the first time. Analysis of literature data on Antarctic lacustrine diatoms shows that taxonomic practice has a profound influence on the assessment of distribution patterns. Force-fitting of European and North American names to Antarctic taxa and erroneous identifications have contributed to an underestimation of endemism in the diatom flora of Antarctic inland waters. In addition, changing concepts on species boundaries during the last decade influence the interpretation of biogeographic patterns. The application of a more fine grained taxonomy will almost certainly reveal a higher degree of endemism in Antarctica, and especially continental Antarctica. The present case study shows that in the Larsemann Hills Antarctic endemics account for about 40% of all freshwater and brackish taxa, while the biogeographic distribution of about 26% is unknown, mainly due to their uncertain taxonomic identity. This contradicts the view that cosmopolitanism prevails in Antarctic diatoms.

Hidden levels of phylodiversity in Antarctic green algae: further evidence for the existence of glacial refugia

Recent data revealed that metazoans such as mites and springtails have persisted in Antarctica throughout several glacial–interglacial cycles, which contradicts the existing paradigm that terrestrial life was wiped out by successive glacial events and that the current inhabitants are recent colonizers. We used molecular phylogenetic techniques to study Antarctic microchlorophyte strains isolated from lacustrine habitats from maritime and continental Antarctica. The 14 distinct chlorophycean and trebouxiophycean lineages observed point to a wide phylogenetic diversity of apparently endemic Antarctic lineages at different taxonomic levels. This supports the hypothesis that long-term survival took place in glacial refugia, resulting in a specific Antarctic flora. The majority of the lineages have estimated ages between 17 and 84 Ma and probably diverged from their closest relatives around the time of the opening of Drake Passage (30–45 Ma), while some lineages with longer branch lengths have estimated ages that precede the break-up of Gondwana. The variation in branch length and estimated age points to several independent but rare colonization events.

Sexual reproduction, mating system, chloroplast dynamics and abrupt cell size reduction in Pseudo-nitzschia pungens from the North Sea (Bacillariophyta)

Clonal cultures of Pseudo-nitzschia pungens were isolated at various times from seven sites in the North Sea. During the mitotic cell cycle, the two plate-shaped chloroplasts were girdle-appressed during interphase and mitosis. After cytokinesis, the chloroplasts moved onto the parental valve and remained there during the formation of the new hypovalve and until separation and re-arrangement of the sibling cells within the cell chain had been completed. Clones were almost always heterothallic and cultures of opposite mating type isolated from different localities were compatible. Meiosis I was cytokinetic and accompanied by chloroplast division. Meiosis II involved karyokinesis but not cytokinesis and preceded the rearrangement and contraction of the two gametes. Sexual reproduction involved physiological anisogamy. With one exception, gamete behaviour was clone-specific, gametes being active in clones of one mating type but passive in clones of the other mating type. Auxospore development was accompanied by deposition of a transverse and then a longitudinal perizonium. Infrequently, triploid auxospores and presumably haploid auxospores were produced. The four chloroplasts of diploid auxospores did not divide, and behaved synchronously during the two acytokinetic mitotic cycles accompanying the deposition of the initial thecae. Just before the first division of the initial cell, the chloroplasts shifted onto the valves (two per valve). The division of the initial cell was not accompanied by chloroplast division and so the two daughter cells received two chloroplasts each. Two modes of abrupt cell size reduction were detected. One occurred during initial cell formation when part of the expanded auxospore aborted. The other pattern was more gradual and was observed in growing cultures; during successive cell divisions a frustule constriction appeared and intensified, one chloroplast split into two, and part of the protoplast aborted. A simple naming system is proposed for mating types in pennate diatoms.

Modelling diatom responses to climate induced fluctuations in the moisture balance in continental Antarctic lakes

The water chemistry of lake systems on the edge of the Antarctic continent responds quickly to changes in the moisture balance. This is expressed as increasing salinity and decreasing lake water level during dry periods, and the opposite during wet periods. The diatom composition of the lakes also changes with these fluctuations in salinity and lake water depth. This is important, as their siliceous remains become incorporated into lake sediments and can provide long-term records of past salinity using transfer functions. In order to develop transfer functions, diatoms and water chemistry data were inter-calibrated from five different East Antarctic oases, namely the Larsemann Hills, the Bølingen Islands, the Vestfold Hills, the Rauer Islands and the Windmill Islands. Results indicate that salinity is the most important environmental variable explaining the variance in the diatom flora in East Antarctic lakes. In oligo- saline lakes the variance is mainly explained by lake water depth. This dataset was used to construct a weighted averaging transfer function for salinity in order to infer historical changes in the moisture balance. This model has a jack-knifed r2 of 0.83 and a RMSEP of 0.31. The disadvantage of this transfer function is that salinity changes in oligo-saline lakes are reconstructed inaccurately due to the ‘edge effect’ and due to the low species turnover along the salinity gradient at its lower end. In order to infer changes in the moisture balance in these lakes, a second transfer function using weighted averaging partial least squares (with two components) for depth was constructed. This model has a jack-knifed r2 of 0.76 and a RMSEP of 0.22. Both transfer functions can be used to infer climate driven changes in the moisture balance in lake sediment cores from oligo-, hypo-, meso- and hyper-saline lakes in East Antarctic oases between 102–75°E. The transfer function for lake water depth is promising to track trends in the moisture balance of small freshwater lakes, where changes in shallow and deep-water sediments are readily reflected in changing diatom composition.

Molecular evidence for distinct Antarctic lineages in the cosmopolitan terrestrial diatoms Pinnularia borealis and Hantzschia amphioxys

Recent morphology-based studies indicate that freshwater diatom floras in the Antarctic comprise a significant share of endemics among a majority of apparently cosmopolitan species. Given the widespread (pseudo)cryptic species diversity in diatoms, we assessed the molecular divergence and temperature-dependent growth characteristics between Antarctic and non-Antarctic strains for two presumed species with a cosmopolitan distribution, namely Pinnularia borealis and Hantzschia amphioxys. Molecular phylogenies based on the plastid gene rbcL and the nuclear 28S rDNA (D1-D3 region) revealed that both taxa consist of multiple lineages, each including a distinct Antarctic lineage. A molecular clock estimates the origin of P. borealis at 35.8 (30-47) million years (Ma) ago, making this the oldest known diatom species complex. The Antarctic P. borealis lineage is estimated to have diverged 7.8 (2-15) Ma ago, after the geographical and thermal isolation of the Antarctic continent. Despite not being psychrophilic, the Antarctic lineages of P. borealis and H. amphioxys have a lower optimal growth temperature and upper lethal temperature than most lineages from more temperate regions, indicating niche differentiation. Together, this suggests that many presumed cosmopolitan Antarctic diatom species are in fact species complexes, possibly containing Antarctic endemics with low temperature preferences.

Tolerance of benthic diatoms from temperate aquatic and terrestrial habitats to experimental desiccation and temperature stress

Souffreau C., Vanormelingen P., Verleyen E., Sabbe K. and Vyverman W. 2010. Tolerance of benthic diatoms from temperate aquatic and terrestrial habitats to experimental desiccation and temperature stress. Phycologia 49: 309–324. DOI: 10.2216/09-30.1 Soils differ from aquatic sediments in environmental characteristics such as moisture availability and temperature fluctuations, and it is therefore believed that soil-inhabiting diatoms have a broader tolerance range to these stresses than aquatic diatoms. To test this hypothesis, we assessed the survival capacities of vegetative cells of 34 benthic diatom species from terrestrial and aquatic habitats in Belgium when exposed to desiccation and temperature stress. Six different stress conditions were studied: gradual heating up to +30°C and +40°C, abrupt heating to +40°C, freezing to −20°C and desiccation with and without preconditioning at +30°C. All six conditions resulted in a significantly decreased survival of cells compared to control conditions. Desiccation killed all tested strains, freezing was survived by only three species and abrupt heating was significantly more lethal than gradual heating, suggesting a generally high sensitivity of vegetative diatom cells to these three stress factors. While tolerance to temperature extremes (+40°C and −20°C) was to a large extent species-specific, habitat-specific differences in cell survival were also detected. Only terrestrial species survived freezing, and aquatic diatoms were less tolerant to gradual heating to +40°C, both pointing at a higher tolerance of terrestrial diatoms to temperature extremes. Moreover, in two species with both aquatic and terrestrial isolates, only the terrestrial strains survived +40°C. We conclude that vegetative cells of benthic diatoms (1) are very sensitive to desiccation, freezing and abrupt heating and (2) have a habitat-dependent tolerance to temperature extremes. The consequences of these observations for the dispersal capacities and the subsequent biogeographical patterns of diatoms are discussed.

Bacterial Diversity Assessment in Antarctic Terrestrial and Aquatic Microbial Mats: A Comparison between Bidirectional Pyrosequencing and Cultivation

The application of high-throughput sequencing of the 16S rRNA gene has increased the size of microbial diversity datasets by several orders of magnitude, providing improved access to the rare biosphere compared with cultivation-based approaches and more established cultivation-independent techniques. By contrast, cultivation-based approaches allow the retrieval of both common and uncommon bacteria that can grow in the conditions used and provide access to strains for biotechnological applications. We performed bidirectional pyrosequencing of the bacterial 16S rRNA gene diversity in two terrestrial and seven aquatic Antarctic microbial mat samples previously studied by heterotrophic cultivation. While, not unexpectedly, 77.5% of genera recovered by pyrosequencing were not among the isolates, 25.6% of the genera picked up by cultivation were not detected by pyrosequencing. To allow comparison between both techniques, we focused on the five phyla (Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes and Deinococcus-Thermus) recovered by heterotrophic cultivation. Four of these phyla were among the most abundantly recovered by pyrosequencing. Strikingly, there was relatively little overlap between cultivation and the forward and reverse pyrosequencing-based datasets at the genus (17.1–22.2%) and OTU (3.5–3.6%) level (defined on a 97% similarity cut-off level). Comparison of the V1–V2 and V3–V2 datasets of the 16S rRNA gene revealed remarkable differences in number of OTUs and genera recovered. The forward dataset missed 33% of the genera from the reverse dataset despite comprising 50% more OTUs, while the reverse dataset did not contain 40% of the genera of the forward dataset. Similar observations were evident when comparing the forward and reverse cultivation datasets. Our results indicate that the region under consideration can have a large impact on perceived diversity, and should be considered when comparing different datasets. Finally, a high number of OTUs could not be classified using the RDP reference database, suggesting the presence of a large amount of novel diversity.