Thorbjørn Joest Andersen

Hundred years of genetic structure in a sediment revived diatom population

This paper presents research on the genetic structure and diversity of populations of a common marine protist and their changes over time. The bloom-forming diatom Skeletonema marinoi was used as a model organism. Strains were revived from anoxic discrete layers of a 210Pb-dated sediment core accumulated over more than 100 y, corresponding to >40,000 diatom mitotic generations. The sediment core was sampled from the highly eutrophic Mariager Fjord in Denmark. The genetic structure of S. marinoi was examined using microsatellite markers, enabling exploration of changes through time and of the effect of environmental fluctuations. The results showed a stable population structure among and within the examined sediment layers, and a similar genetic structure has been maintained over thousands of generations. However, established populations from inside the fjord were highly differentiated from open-sea populations. Despite constant water exchange and influx of potential colonizers into the fjord, the populations do not mix. One fjord population, accumulated in 1980, was significantly differentiated from the other groups of strains isolated from the fjord. This differentiation could have resulted from the status of Mariager Fjord, which was considered hypereutrophic, around 1980. There was no significant genetic difference between pre- and posteutrophication groups of strains. Our data show that dispersal potential and generation time do not have a large impact on the genetic structuring of the populations investigated here. Instead, the environmental conditions, such as the extreme eutrophication of the Mariager Fjord, are deemed more important.

Phytoplankton growth after a century of dormancy illuminates past resilience to catastrophic darkness

Photosynthesis evolved in the oceans more than 3 billion years ago and has persisted throughout all major extinction events in Earths history. The most recent of such events is linked to an abrupt collapse of primary production due to darkness following the Chicxulub asteroid impact 65.5 million years ago. Coastal phytoplankton groups (particularly dinoflagellates and diatoms) appear to have been resilient to this biotic crisis, but the reason for their high survival rates is still unknown. Here we show that the growth performance of dinoflagellate cells germinated from resting stages is unaffected by up to a century of dormancy. Our results clearly indicate that phytoplankton resting stages can endure periods of darkness far exceeding those estimated for the Cretaceous-Paleogene extinction and may effectively aid the rapid resurgence of primary production in coastal areas after events of prolonged photosynthesis shut-down.

Deposition and mixing depths on some European intertidal mudflats based on and activities

Abstract Vertical profiles of 210 Pb and 137 Cs have been collected at micro-, meso- and macrotidal mudflats in order to calculate present accumulation rates. Sediment cores were taken at the mudflat at Kongsmark in the microtidal Lister Dyb tidal area (Denmark), at the BOA bridge in the mesotidal Dollard estuary (Netherlands), and at the Skeffling mudflat in the macrotidal Humber estuary (UK). Except for the Kongsmark site and site A in the Humber estuary, no accurate calculation of accumulation rates was possible for the investigated sites. The accumulation rate obtained for the Kongsmark site is confirmed by other independent data and shows that the accumulation is not supply limited at this site at present. For site A in the Humber estuary a modified CIC method was applied to the 210 Pb -profile and the result agrees with results from the nearby Welwick Marsh. The salt marsh at Skeffling is advancing out onto the fringing mudflat with accumulation at the innermost part of the mudflat but with erosion taking place further offshore. The reasons for the generally poor dating results seems to be either erosion (two Humber sites) or postdepositional reworking of the deposited material (bioturbation and continued resuspension) and the data set suggests that the mixing depth may increase with tidal range and thereby hydrodynamic forcing.

Planktonic aggregates of Staphylococcus aureus protect against common antibiotics.

Bacterial cells are mostly studied during planktonic growth although in their natural habitats they are often found in communities such as biofilms with dramatically different physiological properties. We have examined another type of community namely cellular aggregates observed in strains of the human pathogen Staphylococcus aureus. By laser-diffraction particle–size analysis (LDA) we show, for strains forming visible aggregates, that the aggregation starts already in the early exponential growth phase and proceeds until post-exponential phase where more than 90% of the population is part of the aggregate community. Similar to some types of biofilm, the structural component of S. aureus aggregates is the polysaccharide intercellular adhesin (PIA). Importantly, PIA production correlates with the level of aggregation whether altered through mutations or exposure to sub-inhibitory concentrations of selected antibiotics. While some properties of aggregates resemble those of biofilms including increased mutation frequency and survival during antibiotic treatment, aggregated cells displayed higher metabolic activity than planktonic cells or cells in biofilm. Thus, our data indicate that the properties of cells in aggregates differ in some aspects from those in biofilms. It is generally accepted that the biofilm life style protects pathogens against antibiotics and the hostile environment of the host. We speculate that in aggregate communities S. aureus increases its tolerance to hazardous environments and that the combination of a biofilm-like environment with mobility has substantial practical and clinical importance.

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a shortterm laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10000 and 50000 ind m 2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10000 to 50000 ind m 2 . The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions. D 2002 Elsevier Science B.V. All rights reserved.

Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier

The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate, and several numerical models suggest that unstable and irreversible retreat of the grounding line—which marks the boundary between grounded ice and floating ice shelf—is underway. Understanding this recent retreat requires a detailed knowledge of grounding-line history, but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated. In particular, a history of grounding-line retreat is required to understand the relative roles of contemporaneous ocean-forced change and of ongoing glacier response to an earlier perturbation in driving ice-sheet loss. Here we show that the present thinning and retreat of Pine Island Glacier in West Antarctica is part of a climatically forced trend that was triggered in the 1940s. Our conclusions arise from analysis of sediment cores recovered beneath the floating Pine Island Glacier ice shelf, and constrain the date at which the grounding line retreated from a prominent seafloor ridge. We find that incursion of marine water beyond the crest of this ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years); final ungrounding of the ice shelf from the ridge occurred in 1970 (±4 years). The initial opening of this ocean cavity followed a period of strong warming of West Antarctica, associated with El Niño activity. Thus our results suggest that, even when climate forcing weakened, ice-sheet retreat continued.

Buried alive – germination of up to a century-old marine protist resting stages

Lundholm N., Ribeiro S., Andersen T.J., Koch T., Godhe A., Ekelund F. and Ellegaard M. 2011. Buried alive – germination of up to a century-old marine protist resting stages. Phycologia 50: 629–640. DOI: 10.2216/11-16.1 We report on the survival and germination of up to a century-old marine protist resting stages naturally preserved in sediments from Koljö Fjord on the west coast of Sweden. This work has focused on germination of dinoflagellate cysts, but diatom resting stages were also observed. We record the longest known survival of dormant dinoflagellate cells. We individually isolated more than 1200 cysts of the three most abundant dinoflagellate taxa: Pentapharsodinium dalei, Lingulodinium polyedrum and Scrippsiella spp. Germination success decreased with core depth, and all successful germinations took place within the first 2 wk of incubation. Pentapharsodinium dalei had the highest germination success rate, with a maximum of up to 80% in 28-yr-old sediment, and could successfully germinate from core sediments dated to 1920 ± 12. Scrippsiella spp. cysts with cell contents occurred down to c. 90-yr-old sediment and could germinate from down to ca. 40-yr-old sediments, with a maximum germination rate of 50–60% in recent sediments. Cysts of L. polyedrum germinated frequently down to 20 yr and rarely to c. 80 yr, with a maximum of 20–50% germination success in recent sediments. Cyst isolation under cooled conditions rather than at room temperature resulted in a significantly higher germination success in P. dalei, while no effect was observed for L. polyedrum. The time elapsed since slicing of the core affected survival of L. polyedrum cysts negatively, most likely due to the effect of oxygen. The long-term survival potential of benthic resting stages that we report here has important implications, as viable resting stages accumulated in bottom sediments can be transported back to the water column by, for example, bioturbation and human-mediated sediment dredging. Hence, the sediment may to a higher degree than previously considered play a role as seed bank. This is important in a changing climate and might have particularly severe impacts in the case of harmful species.

Optical Dating of Dune Ridges on Rømø, a Barrier Island in the Wadden Sea, Denmark

Abstract The application of optically stimulated luminescence (OSL) to the dating of recent aeolian sand ridges on Rømø, an island off the southwest coast of Denmark, is tested. These sand ridges began to form approximately 300 years ago, and estimates of the ages are available from historical records. Samples for OSL dating were taken ∼0.5 m below the crests of four different dune ridges; at least five samples were recovered from each ridge to test the internal consistency of the ages. Additional samples were recovered from the low lying areas in the swales and from the scattered dune formations in a broad hummocky area landward of the well-defined ridges. Despite low luminescence signals, we were able to obtain a mean age for the youngest ridge of 17 ± 2 years, consistent with the known age of >28 years. Optical ages of individual samples in our study ranged between 10 ± 3 years and 690 ± 50 years, and all ages were broadly consistent with those expected from historical information. The oldest aeolian surface on Rømø appears to be 370 ± 30 years. This is built on what we interpret as a marine sandbank, whose surface is ∼700 years old. The sand ridges seaward of the hummocky dune field have well-defined building phases separated by inactive periods and the first major ridge formed ∼235 years ago. This study demonstrates that optical dating can be successfully applied to these young aeolian sand deposits, and we conclude that OSL dating is a powerful chronological tool in studies of coastal change.

Sedimentation and chronology of heavy metal pollution in Oslo harbor, Norway

Stratigraphic profiles of Cu, Cd and Hg in ten sediment cores from the Oslo harbor, Norway, combined with results of radiometric dating demonstrate that pollution by these metals peaked between 1940 and 1970. Dating results indicate that Hg discharges peaked between 1940 and 1950, Cd reached maximum ca. 1955-1960, and Cu has the highest concentration in sediment interval corresponding to ca. 1970. Geochemical profiles and maxima of Cu, Cd and Hg concentrations can be used as chronostratigraphic markers for sediment cores from the Oslo harbor. Acoustic backscatter and sediment core data indicate that propeller wash affects the seabed in the Oslo harbor. The propeller-induced turbulence causes erosion, and in places exposes and remobilizes contaminated sediments that accumulated in the harbor during previous decades. Such re-exposure of contaminated sediments could be detrimental to local ecosystems and offset remediation efforts, warranting further impact studies and potential mitigation strategies to prevent redistribution.

Sedimentation of helminth eggs in water

Helminth parasite eggs in low quality water represent health risks when used for irrigation of crops. The settling velocities of helminth eggs (Ascaris suum, Trichuris suis, and Oesophagostomum spp.) and wastewater particles were experimentally determined in tap water and in wastewater using Owen tubes. The settling velocities of eggs in tap water was compared with theoretical settling velocities calculated by Stokes law using measurements of size and density of eggs as well as density and viscosity of tap water. The mean settling velocity in tap water of 0.0612 mm s(-1) found for A. suum eggs was significantly lower than the corresponding values of 0.1487 mm s(-1) for T. suis and 0.1262 mm s(-1) for Oesophagostomum spp. eggs. For T. suis and Oesophagostomum spp. eggs the theoretical settling velocities were comparable with the observed velocities in the Owen tubes, while it was three times higher for A. suum eggs. In wastewater, the mean settling velocity for A. suum eggs (0.1582 mm s(-1)) was found to be different from T. suis (0.0870 mm s(-1)), Oesophagostomum spp. (0.1051 mm s(-1)), and wastewater particles (0.0474 mm s(-1)). This strongly indicates that in low quality water the eggs are incorporated into particle flocs with different settling velocities and that the settling velocity of eggs and particles is closely associated. Our results document that there is a need to differentiate the sedimentation of different types of helminth eggs when assessing the quality of low quality water, e.g. for irrigation usage. The results can also be used to improve existing models for helminth egg removal.