Lack of Phylogeographic Structure in the Freshwater Cyanobacterium Microcystis aeruginosa Suggests Global Dispersal

Background : Free-living microorganisms have long been assumed to have ubiquitous distributions with little biogeographic signature because they typically exhibit high dispersal potential and large population sizes. However, molecular data provide contrasting results and it is far from clear to what extent dispersal limitation determines geographic structuring of microbial populations. We aimed to determine biogeographical patterns of the bloom-forming freshwater cyanobacterium Microcystis aeruginosa. Being widely distributed on a global scale but patchily on a regional scale, this prokaryote is an ideal model organism to study microbial dispersal and biogeography. Methodology/Principal Findings : The phylogeography of M. aeruginosa was studied based on a dataset of 311 rDNA internal transcribed spacer (ITS) sequences sampled from six continents. Richness of ITS sequences was high (239 ITS types were detected). Genetic divergence among ITS types averaged 4% (maximum pairwise divergence was 13%). Preliminary analyses revealed nearly completely unresolved phylogenetic relationships and a lack of genetic structure among all sequences due to extensive homoplasy at multiple hypervariable sites. After correcting for this, still no clear phylogeographic structure was detected, and no pattern of isolation by distance was found on a global scale. Concomitantly, genetic differentiation among continents was marginal, whereas variation within continents was high and was mostly shared with all other continents. Similarly, no genetic structure across climate zones was detected. Conclusions/Significance : The high overall diversity and wide global distribution of common ITS types in combination with the lack of phylogeographic structure suggest that intercontinental dispersal of M. aeruginosa ITS types is not rare, and that this species might have a truly cosmopolitan distribution

Luminescence characteristics of natural building stones from the Falcon Monastery (Antwerp, Belgium) : potential for establishing (de)construction chronologies

Over the past few years, luminescence dating of consolidated rocks (as opposed to loose sediments) has been revitalized, leading to new methods for unraveling their light exposure and/or burial history. In this study, we examine if the approach may aid in unravelling the (de)construction history of monuments built out of Lede stones. Lede stone is a sandy limestone, that has been widely used for stonemasonry in NW Belgium from the Roman times onwards. The samples of Lede stone investigated here were collected during an archaeological excavation of the Falcon Monastery (Antwerp, NW Belgium). The Monastery has a rich building history, with various construction phases starting as early as the 14th century CE, until it was demolished (and buried) by Napoleon Bonaparte in the beginning of the 19th century CE. While some of the successive phases can be well constrained in time, it is unknown when some of the other buildings were added to the complex. Given the mineralogical composition of the stones, the IR50-signal from feldspar was used and we document the characteristics of this signal (as observed from rock slices and using the SAR protocol) in terms of signal intensity, dose response, recuperation, recycling, and dose recovery. We assess completeness of resetting by examining the variability in Ln/Tn for single surficial slices taken from multiple cores in the same rock samples; the variation in signal with depth into the rock is investigated as well. We then use the Ln/Tn values, which are finite, to establish burial or deconstruction ages, while exposure ages are derived from the luminescence depth profiles. The accuracy of the former is assessed by comparison with the known demolition date, while that of the latter is evaluated by treating one of the known-age phases as an unknown

Natural hybrids in the marine diatom Pseudo-Nitzschia pungens (Bacillariophyceae): genetic and morphological evidence

Hybridization between genetically distinguishable taxa provides opportunities for investigating speciation. While hybridization is a common phenomenon in various macro-organisms, natural hybridization among micro-eukaryotes is barely studied. Here we used a nuclear and a chloroplast molecular marker and morphology to demonstrate the presence of natural hybrids between two genetically and morphologically distinct varieties of the marine planktonic diatom Pseudo-nitzschia pungens (vars. pungens and cingulata) in a contact zone in the northeast Pacific. Cloning and sequencing of the rDNA internal transcribed spacer region revealed strains containing ribotypes from both varieties, indicating hybridization. Both varieties were found to also have different chloroplast-encoded rbcL sequences. Hybrid strains were either hetero- or homoplastidial, as demonstrated by denaturing gradient gel electrophoresis, which is in accordance with expectations based on the mode of chloroplast inheritance in Pseudo-nitzschia. While most hybrids are probably first generation, there are also indications for further hybridization. Morphologically, the hybrids resembled var. pungens for most characters rather than having an intermediate morphology. Further research should focus on the hybridization frequency, by assessing the spatial and temporal extent of the contact zone, and hybrid fitness, to determine the amount of gene flow between the two varieties and its evolutionary consequences

Optical dating of prehistoric and historic anthropogenic features at Ninove Doorn Noord (East Flanders, Belgium)

Archaeological excavations prior to the construction of the new Business Park “Doorn Noord” (Ninove, East Flanders, Belgium) revealed a complex of traces of human activity and occupation, spanning several millennia. The oldest traces are burial mounds and pits dating from the Final Neolithic / Metal Ages, followed by various remains of a Roman settlement; the youngest traces are the remains of three camps, dating from the seventeenth and eighteenth centuries. While a broad chronological framework for the site could be established on the basis of archaeological evidence, 14C and archaeomagnetic dating, and historical sources, this remains either circumstantial and/or lacks precision. In this case study, we report on the potential of quartz-based optically stimulated luminescence (OSL) dating of unheated and heated sediments that are associated with the archaeological traces. Given the availability of independent age information, we consider it a test of both accuracy and precision. At the same time, our study seeks to contribute to the development of methodologies that may aid in an improved understanding and care of the regional heritage. Samples were collected from the infills of both a circular ditch surrounding a relict burial mound and a loam extraction pit, and from the heated sedimentary remains of fireplaces. We first document the OSL characteristics of sand-sized (63-250 m) quartz and evaluate the performance of a single-aliquot regenerative-dose (SAR) protocol for equivalent dose (ED) determination through laboratory procedural tests in the usual manner (e.g. dose recovery). We then examine the distribution of ED in all samples using small 2 mm diameter aliquots. In most samples, the majority of the results appears to belong to a single dose population of which the average is then used to obtain the optical age. The resulting OSL dates are confronted with the available independent age, demonstrating that no inaccurate conclusions were drawn from the ED distributions. In terms of both accuracy and precision, the results are most promising for the heated sediments associated with post-Medieval encampments. For 6 samples from 3 features optical ages of around 1750 ± 30 CE were obtained, where independent information dates them to 1745 CE. As all features are comparable, and sources of systematic uncertainty are (largely) shared, this implies the possibility of distinguishing them with a time-resolution that is governed by measurement uncertainties only; for the investigated samples that is on timescales of 5 - 10 years

Luminescence investigations of consolidated rocks : experiences at MINPET (UGent, Belgium)

Over the past few years, luminescence dating of consolidated rocks (as opposed to loose sediments) has been revitalized, leading to new methods for unravelling their light exposure and/or burial history. This poster presents a brief status report on some of the work that has been carried out at the UGent Laboratory for Mineralogy and Petrology (MINPET): (i) unravelling the (de)construction history of monuments built out of Lede stones. Lede stone is a sandy limestone, that has been widely used for stonemasonry in NW Belgium from the Roman times onwards; (ii) luminescence properties of Sarsen stones, which are silicified sandstones that have been used for constructing many megalithic monuments in southern England; (iii) erosive processes that shaped the Navajo-sandstone landscape along the Sand Island rock art site in Utah (USA). The focus is on materials, methods and observations, and a whole lot of complexities. The latter include sample collection and preparation, calibrants and calibration, rock inhomogeneity at our scale of analysis, understanding observed luminescence depth-profiles in terms of the chronological information they preserve, and the validity of local and/or sample specific assumptions underlying quantitative chronometric analysis

Limits to gene flow in a cosmopolitan marine planktonic diatom

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

Systematic high-sampling resolution OSL dating of a well-preserved river dune in the Lys valley (Sint-Martens-Latem, NW Belgium)

During the Late Glacial recolonization of NW Europe, hunter-gatherers preferably settled in sheltered environments and along river valleys and lake edges. However, in the Scheldt basin of NW Belgium (which includes tributaries such as the Lys river) remains of these Late Glacial settlers are scarce. Due to the lack of an in-depth assessment of Final Palaeolithic sites in the Scheldt basin, it is not clear at present whether this scarcity is related to specific taphonomic factors and/or corresponds to a prehistoric reality. Moreover, the few known sites in the floodplains of the Scheldt and its tributaries all seem to date to the Younger Dryas (final stage of the Late Glacial), which recently led to the hypothesis of a possible population shift from the inland lakes towards the rivers at the abrupt transition from the warm AllerØd to the extreme cold Younger Dryas. We seek to investigate this hypothesis through intensive geoarchaeological research of aeolian river dunes in this area, as they represent dry elevations bordering a wet landscape, making them attractive spots for occupation. This study reports on quartz-based optically stimulated luminescence (OSL) dating of an exceptionally well-preserved dune, as one of those potential contexts of sealed Final Palaeolithic sites. The dune, locally called “Molenberg” (Lys valley, municipality of Sint-Martens-Latem, NW Belgium), is more than 10 m high (top is 24.82 m a.s.l.). Samples for OSL dating were collected systematically at 50 cm vertical intervals by hand coring at two localities, allowing for a complete sequence from top to base and resulting in a total of 31 samples. These were processed in the usual manner for OSL dating using sand-sized (125-180 m) quartz and the single-aliquot regenerative-dose (SAR) protocol. All samples exhibit satisfactory luminescence characteristics (in terms of purity, signal intensity and composition, and SAR procedural tests). The optical ages range from 11.6 ± 1.1 ka at the top, to 14.9 ± 1.4 ka at the base of the dune, placing its formation in the Late Glacial. In general, the dataset is internally consistent although, further down the sequence, the observed variability appears somewhat larger than expected from measurement uncertainties (2 sigma) only. We consider additional sources of (partial) random uncertainty (such as water content) and use Bayesian chronological age-depth modeling to improve the chronological precision and connect the sedimentation ages with the different climatic and environmental changes during the Late Glacial. Our case study not only illustrates the importance of OSL dating for prospecting potential contexts for hunter-gatherer finds dating to the Final Palaeolithic; it also contributes to a chronological framework for the changing aeolian landscape in the Lys valley during the Late Glacial, which is somewhat contentious untill now. Finally, we show that results from a high-sampling resolution strategy contribute to debates on expected and observed variability, and hence the time-resolution that can reasonably be achieved for the sandy archives under consideration here

Genotypic diversity and differentiation among populations of two benthic freshwater diatoms as revealed by microsatellites

Given their large population sizes and presumed high dispersal capacity, protists are expected to exhibit homogeneous population structure over large spatial scales. On the other hand, the fragmented and short-lived nature of the lentic freshwater habitats that many protists inhabit promotes strong population differentiation. We used microsatellites in two benthic freshwater diatoms, Eunotia bilunaris 'robust' and Sellaphora capitata, sampled from within a pond and connected ponds, through isolated ponds from the same region to western Europe to determine the spatial scale at which differentiation appears. Because periods of low genotypic diversity contribute to population differentiation, we also assessed genotypic diversity. While genotypic diversity was very high to maximal in most samples of both species, some had a markedly lower diversity, with up to half (Eunotia) and over 90% (Sellaphora) of the strains having the same multilocus genotype. Population differentiation showed an isolation-by-distance pattern with very low standardized FST values between samples from the same or connected ponds but high values between isolated ponds, even when situated in the same region. Partial rbcL sequences in Eunotia were consistent with this pattern as isolated ponds in the same region could differ widely in haplotype composition. Populations identified by Structure corresponded to the source ponds, confirming that 'pond' is the main factor structuring these populations. We conclude that freshwater benthic diatom populations are highly fragmented on a regional scale, reflecting either less dispersal than is often assumed or reduced establishment success of immigrants, so that dispersal does not translate into gene flow.status: publishe

Data from: Genotypic diversity and differentiation among populations of two benthic freshwater diatoms as revealed by microsatellites

Given their large population sizes and presumed high dispersal capacity, protists are expected to exhibit homogeneous population structure over large spatial scales. On the other hand, the fragmented and short-lived nature of the lentic freshwater habitats that many protists inhabit promotes strong population differentiation. We used microsatellites in two benthic freshwater diatoms, Eunotia bilunaris ‘robust’ and Sellaphora capitata, sampled from within a pond and connected ponds, through isolated ponds from the same region to western Europe to determine the spatial scale at which differentiation appears. Because periods of low genotypic diversity contribute to population differentiation, we also assessed genotypic diversity. While genotypic diversity was very high to maximal in most samples of both species, some had a markedly lower diversity, with up to half (Eunotia) and over 90% (Sellaphora) of the strains having the same multilocus genotype. Population differentiation showed an isolation-by-distance pattern with very low standardized FST values between samples from the same or connected ponds but high values between isolated ponds, even when situated in the same region. Partial rbcL sequences in Eunotia were consistent with this pattern as isolated ponds in the same region could differ widely in haplotype composition. Populations identified by Structure corresponded to the source ponds, confirming that ‘pond’ is the main factor structuring these populations. We conclude that freshwater benthic diatom populations are highly fragmented on a regional scale, reflecting either less dispersal than is often assumed or reduced establishment success of immigrants, so that dispersal does not translate into gene flow

BI_consensus_phylogeny_rbcL_Eunotia

Tree file of full rbcL Bayesian Inference phylogeny of the Eunotia bilunaris/flexuosa species complex