Arctic Diatom Research Articles

Taxon‐specific dark survival of diatoms and flagellates affects Arctic phytoplankton composition during the polar night and early spring

AbstractEffects of prolonged darkness on Arctic phytoplankton composition were investigated with lab experiments and a pigment time series in Kongsfjorden, Spitsbergen (78°55′N). Chlorophyll a (Chl a), pigment composition, particulate organic carbon, cell numbers, and photosynthetic characteristics were studied in Arctic diatoms (Thalassiosira antarctica, Thalassiosira nordenskioeldii) and flagellates (Rhodomonas sp., Micromonas sp.) during 8 weeks of darkness and subsequent recovery in irradiance. Loss of photosynthetic functionality after 2 weeks of darkness was reversible in all species when returned to irradiance. Diatoms were more resistant to prolonged darkness (> 2 weeks) compared to the flagellates, with lower decline rates of Chl a and maximum quantum yield of PSII. T. nordenskioeldii showed rapid growth during recovery throughout 8 weeks of dark incubation, whereas recovery of flagellates diminished within 4 weeks. Ratios of taxonomic marker pigments relative to Chl a of all species showed limited variation during 8 weeks of dark incubation. The experimentally observed enhanced dark survival of diatoms was in agreement with pigment observations during four polar nights (2013–2017) in Kongsfjorden, which showed increased relative diatom abundance during declining biomass (down to 0.02 mg Chl a m−3). Therefore, a period of prolonged darkness gives Arctic diatoms a head start during the early stages of the spring bloom. The taxon‐specific survival traits can influence the geographical distribution of diatoms and flagellates within the polar oceans and their phenology. Furthermore, the persistence of Chl a of nonviable phytoplankton during darkness might influence biomass estimates during the polar night.

Company matters: The presence of other genotypes alters traits and intraspecific selection in an Arctic diatom under climate change.

Arctic phytoplankton and their response to future conditions shape one of the most rapidly changing ecosystems on the planet. We tested how much the phenotypic responses of strains from the same Arctic diatom population diverge and whether the physiology and intraspecific composition of multistrain populations differs from expectations based on single strain traits. To this end, we conducted incubation experiments with the diatom Thalassiosira hyalina under present‐day and future temperature and pCO2 treatments. Six fresh isolates from the same Svalbard population were incubated as mono‐ and multistrain cultures. For the first time, we were able to closely follow intraspecific selection within an artificial population using microsatellites and allele‐specific quantitative PCR. Our results showed not only that there is substantial variation in how strains of the same species cope with the tested environments but also that changes in genotype composition, production rates, and cellular quotas in the multistrain cultures are not predictable from monoculture performance. Nevertheless, the physiological responses as well as strain composition of the artificial populations were highly reproducible within each environment. Interestingly, we only detected significant strain sorting in those populations exposed to the future treatment. This study illustrates that the genetic composition of populations can change on very short timescales through selection from the intraspecific standing stock, indicating the potential for rapid population level adaptation to climate change. We further show that individuals adjust their phenotype not only in response to their physicochemical but also to their biological surroundings. Such intraspecific interactions need to be understood in order to realistically predict ecosystem responses to global change.

Decoupling light harvesting, electron transport and carbon fixation during prolonged darkness supports rapid recovery upon re-illumination in the Arctic diatom Chaetoceros neogracilis

During winter in the Arctic marine ecosystem, diatoms have to survive long periods of darkness caused by low sun elevations and the presence of sea ice covered by snow. To better understand how diatoms survive in the dark, we subjected cultures of the Arctic diatom Chaetoceros neogracilis to a prolonged period of darkness (1 month) and to light resupply. Chaetoceros neogracilis was not able to grow in the dark but cell biovolume remained constant after 1 month in darkness. Rapid resumption of photosynthesis and growth recovery was also found when the cells were transferred back to light at four different light levels ranging from 5 to 154 µmol photon m−2 s−1. This demonstrates the remarkable ability of this species to re-initiate growth over a wide range of irradiances even after a prolonged period in the dark with no apparent lag period or impact on survival. Such recovery was possible because C. neogracilis cells preserved their Chl a content and their light absorption capabilities. Carbon fixation capacity was down-regulated (ninefold dark decrease in $$P_{\text{m}}^{\text{C}}$$ ) much more than was the photochemistry in PSII (2.3-fold dark decrease in ETRm). Rubisco content, which remained unchanged after one month in the dark, was not responsible for the decrease in $$P_{\text{m}}^{\text{C}}$$ . The decrease in PSII activity was partially related to the induction of sustained non-photochemical quenching (NPQ) as we observed an increase in diatoxanthin content after one month in the dark.

Late spring bloom development of pelagic diatoms in Baffin Bay

The Arctic Ocean is particularly affected by climate change, with changes in sea ice cover expected to impact phytoplankton primary production. During the Green Edge expedition, the development of the late spring–early summer diatom bloom was studied in relation with the sea ice retreat by multiple transects across the marginal ice zone. Biogenic silica concentrations and uptake rates were measured. In addition, diatom assemblage structures and their associated carbon biomass were determined, along with taxon-specific contributions to total biogenic silica production using the fluorescent dye PDMPO. Results indicate that a diatom bloom developed in open waters close to the ice edge, following the alleviation of light limitation, and extended 20–30 km underneath the ice pack. This actively growing diatom bloom (up to 0.19 μmol Si L–1 d–1) was associated with high biogenic silica concentrations (up to 2.15 μmol L–1), and was dominated by colonial fast-growing centric (Chaetoceros spp. and Thalassiosira spp.) and ribbon-forming pennate species (Fragilariopsis spp./Fossula arctica). The bloom remained concentrated over the shallow Greenland shelf and slope, in Atlantic-influenced waters, and weakened as it moved westwards toward ice-free Pacific-influenced waters. The development resulted in a near depletion of all nutrients eastwards of the bay, which probably induced the formation of resting spores of Melosira arctica. In contrast, under the ice pack, nutrients had not yet been consumed. Biogenic silica and uptake rates were still low (respectively <0.5 μmol L–1 and <0.05 μmol L–1 d–1), although elevated specific Si uptake rates (up to 0.23 d–1) probably reflected early stages of the bloom. These diatoms were dominated by pennate species (Pseudo-nitzschia spp., Ceratoneis closterium, and Fragilariopsis spp./Fossula arctica). This study can contribute to predictions of the future response of Arctic diatoms in the context of climate change.

The Role of Sustained Photoprotective Non-photochemical Quenching in Low Temperature and High Light Acclimation in the Bloom-Forming Arctic Diatom Thalassiosira gravida

Thalassiosira gravida is a major Arctic diatom responsible for the under-ice spring bloom. We investigated T. gravida physiological plasticity growing it at two temperatures (0°C and 5°C) and under different light intensities typically found in its natural environment. T. gravida showed remarkable thermal- and photo-acclimatory plasticity including: low light saturation parameter for growth (KE) and photosynthesis (EK), low μmax but relatively high Chl a/C, low C/N and decreasing light-saturated carbon fixation rate (PCm) with increasing growth irradiance. T. gravida also showed remarkable photoprotective features, namely a strong sustained non photochemical quenching (NPQs, hour kinetics relaxation) supported by a high amount of xanthophyll cycle pigments. T. gravida growth remained possible under a wide range of irradiances but photosynthetic plasticity was higher at moderately low light (up to ~50 µmol photon m-2 s-1), nevertheless corresponding to the mean in situ conditions under which it predominates, i.e. underneath the spring thin-ice punctuated with melting ponds. The potential role of NPQs in the photophysiological plasticity of T. gravida is discussed.

Petroleum hydrocarbon and microbial community structure successions in marine oil-related aggregates associated with diatoms relevant for Arctic conditions

Oil-related aggregates (ORAs) may contribute to the fate of oil spilled offshore. However, our understanding about the impact of diatoms and associated bacteria involved in the formation of ORAs and the fate of oil compounds in these aggregates is still limited. We investigated these processes in microcosm experiments with defined oil dispersions in seawater at 5 °C, employing the Arctic diatom Fragilariopsis cylindrus and its associated bacterial assemblage to promote ORA formation. Accumulation of oil compounds, as well as biodegradation of naphthalenes in ORAs and corresponding water phases, was enhanced in the presence of diatoms. Interestingly, the genus Nonlabens was predominating the bacterial communities in diatom-supplemented microcosms, while this genus was not abundant in other samples. This work elucidates the relevance of diatom biomass for the formation of ORAs, microbial community structures and biodegradation processes in chemically dispersed oil at low temperatures relevant for Arctic conditions.

Structural and functional analyses of Rubisco from arctic diatom species reveal unusual posttranslational modifications

The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Natural diversity in the catalytic properties of Rubisco indicates possibilities for improvement. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO2 sequestration in polar cold waters. Until now, the biochemical properties and three-dimensional structures of Rubisco from diatoms were unknown. Here, diatoms from arctic waters were collected, cultivated, and analyzed for their CO2-fixing capability. We characterized the kinetic properties of five and determined the crystal structures of four Rubiscos selected for their high CO2-fixing efficiency. The DNA sequences of the rbcL and rbcS genes of the selected diatoms were similar, reflecting their close phylogenetic relationship. The Vmax and Km for the oxygenase and carboxylase activities at 25 °C and the specificity factors (Sc/o) at 15, 25, and 35 °C were determined. The Sc/o values were high, approaching those of mono- and dicot plants, thus exhibiting good selectivity for CO2 relative to O2 Structurally, diatom Rubiscos belong to form I C/D, containing small subunits characterized by a short βA-βB loop and a C-terminal extension that forms a β-hairpin structure (βE-βF loop). Of note, the diatom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxyproline, β-hydroxyleucine, hydroxylated and nitrosylated cysteine, mono- and dihydroxylated lysine, and trimethylated lysine. Our studies suggest adaptation toward achieving efficient CO2 fixation in arctic diatom Rubiscos.

Distribution of viable resting stage cells of diatoms in sediments and water columns of the Chukchi Sea, Arctic Ocean

:The distribution of resting stage cells of diatoms in the surface sediments and water samples collected in the Chukchi Sea was investigated using the most probable number (MPN) analysis to understand the mechanism underlying diatom blooms in the Arctic region. High densities (3.5 × 105 to 6.8 × 106 MPN cells cm−3 wet sediment) of viable resting stage cells of typical Arctic diatom species were found in the surface sediments of the Chukchi Sea. Resting stage cells of the sea-ice–related diatom genus Fragilariopsis were more abundant (2.6 × 106 MPN cells cm−3 wet sediment) than Chaetoceros (2.18 × 106 MPN cells cm−3 wet sediment) or Thalassiosira (1.76 × 106 MPN cells cm−3 wet sediment) in the southern Chukchi Sea. This reflected a previous bloom of Fragilariopsis ahead of typical planktonic blooms of Chaetoceros and Thalassiosira and suggests that sea-ice–related blooms are a substantial factor for primary production in this area. During the sampling period, diatom assemblages in the in situ water columns were dominated by Pseudo-nitzschia delicatissima, Cylindrotheca closterium, Proboscia alata, and Thalassionema spp. After storage of water samples for 6 months in the dark, viable diatom resting stage cells of typical Arctic species such as Attheya longicornis and Chaetoceros socialis were detected. These autochthonous species formed resting stage cells that remained viable in the dark for more than 6 months, while diatom species of Pacific origins could not survive under conditions of extremely limited light. The resting stage cells provide an advantage for autochthonous diatoms to endure the unfavourable light conditions of the Arctic winter. In the Chukchi Sea, resting stage cells of diatoms were supplied by current inflows from shallow sea areas. Densely distributed viable resting stages at the bottom are also a possible source for seeding the diatom cells into phytoplankton communities and ice assemblages in shallow marginal ice zones.

Resilience by diversity: Large intraspecific differences in climate change responses of an Arctic diatom

AbstractThe potential for adaptation of phytoplankton to future climate is often extrapolated based on single strain responses of a representative species, ignoring variability within and between species. The aim of this study was to approximate the range of strain‐specific reaction patterns within an Arctic diatom population, which selection can act upon. In a laboratory experiment, we first incubated natural communities from an Arctic fjord under present and future conditions. In a second step, single strains of the diatom Thalassiosira hyalina were isolated from these selection environments and exposed to a matrix of temperature (3°C and 6°C) and pCO2 levels (180 μatm, 370 μatm, 1000 μatm, 1400 μatm) to establish reaction norms for growth, production rates, and elemental quotas. The results revealed interactive effects of temperature and pCO2 as well as wide tolerance ranges. Between strains, however, sensitivities and optima differed greatly. These strain‐specific responses corresponded well with their respective selection environments of the previous community incubation. We therefore hypothesize that intraspecific variability and the selection between coexisting strains may pose an underestimated source of species' plasticity. Thus, adaptation of phytoplankton assemblages may also occur by selection within rather than only between species, and species‐wide inferences from single strain experiments should be treated with caution.

FTIR imaging analysis of cell content in sea-ice diatom taxa during a spring bloom in the lower Northwest Passage of the Canadian Arctic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 569:77-88 (2017) - DOI: https://doi.org/10.3354/meps12088 FTIR imaging analysis of cell content in sea-ice diatom taxa during a spring bloom in the lower Northwest Passage of the Canadian Arctic Nicole M. Pogorzelec1, C. J. Mundy1,*, Catherine R. Findlay2, Karley Campbell1, Aura Diaz1, Jens K. Ehn1, Søren Rysgaard1, Kathleen M. Gough2,* 1Centre for Earth Observation Science (CEOS), and 2Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada *Corresponding authors: cj.mundy@umanitoba.ca, kathleen.gough@umanitoba.ca ABSTRACT: During their vernal bloom, ice algae respond to variable light and nutrient availability by changing their production of cellular lipids and proteins. Fourier transform infrared (FTIR) spectrochemical imaging was used to quantify relative amounts of saturated lipids and proteins within algal cells from high and low light conditions, i.e. thin and thick snow cover, respectively. Samples were collected within the lower Northwest Passage, near Cambridge Bay, Nunavut, Canada during the 2014 ICE-CAMPS field campaign. Diatom taxa analyzed included Nitzschia frigida, pennate ribbon colonies, and Attheya spp. Saturated lipid content and lipid to protein ratios were significantly greater under thin snow covers relative to those under thick; cell protein content remained relatively stable throughout the study under both snow covers. Results have been interpreted relative to a concurrent, related study wherein under-ice nutrients were concluded to be limited throughout the study. Taxa responses reported herein differed according to size, with N. frigida exhibiting the earliest and most dramatic changes. The increase in lipid to protein ratio highlighted the transitioning allocation of carbon from protein to lipid algal biomass through the spring bloom progression, with increasing light under nutrient limiting conditions in this location. KEY WORDS: Arctic diatom · FTIR · Sea-ice algae · Light and nutrient limitation · Biomass composition · Proteins · Saturated lipids Full text in pdf format Supplementary material PreviousNextCite this article as: Pogorzelec NM, Mundy CJ, Findlay CR, Campbell K and others (2017) FTIR imaging analysis of cell content in sea-ice diatom taxa during a spring bloom in the lower Northwest Passage of the Canadian Arctic. Mar Ecol Prog Ser 569:77-88. https://doi.org/10.3354/meps12088 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 569. Online publication date: April 07, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Molecular and morphological characterization of the Achnanthidium minutissimum complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of A. digitatum sp. nov.

ABSTRACTRecent morphology-based investigations of freshwater Arctic diatoms suggest that many species remain to be discovered, reflecting a unique polar flora. During a survey of the freshwater diatom flora of northern Billefjorden, including the Petuniabukta fjord region (Spitsbergen), several morphodemes belonging to the Achnanthidium minutissimum species complex were recorded. Molecular phylogenies based on rbcL, 28S and 18S sequences, including single cells from Canada and strains from Marion Island (sub-Antarctica) and GenBank revealed the presence of 12 distinct A. minutissimum complex lineages, of which three contained strains from Spitsbergen. One Arctic lineage is described as a new species. Achnanthidium digitatum sp. nov. is morphologically characterized by narrow, linear to only slightly lanceolate valves and usually two areolae per stria. The two remaining Arctic lineages are in need of a more complete morphological and molecular comparison with other representatives of the A. minutissimum complex to clarify their taxonomic identity. It is argued that implementation of molecular data in the taxonomy of Achnanthidium will be essential to solve the taxonomic problems associated with this group, eventually resulting in a better understanding of the biogeography and niche differentiation of different species belonging to the A. minutissimum complex. Ideally, this should be based on more variable genes than the currently widely used 18S, which does not have a species level resolution in the A. minutissimum complex.

Thermal source Fourier transform infrared microtomography applied to Arctic sea ice diatoms

We have used thermal source Fourier Transform Infrared (FTIR) microtomographic imaging to compare sea ice diatoms growing under different light conditions. A prototype tomography accessory was designed to have sufficient degrees of freedom to align any tilted cylindrical sample relative to the axis of rotation, minimizing the off-axis path traced during rotation. The lightweight device rests on the motorized stage to position the sample in the field-of-view and enable mosaic imaging. Reconstruction routines were tested with simulated and real phantoms, to assess limitations in the Radon back-projection method employed. The distribution and abundance of biochemicals is analysed for targets larger than a single FPA tile. Two and three dimensional (2D and 3D) FTIR spectrochemical images were obtained with a Focal Plane Array (FPA, nominal 1.1 μm pixel edges) for phantoms (polystyrene beads in polyvinyl alcohol matrix) and diatom cells harvested from land fast, first-year ice sites in Resolute Passage (74 43.628'N; 95 33.330'W) and Dease Strait (69° 1.11'N; 105° 21.29'W), Nunavut, Canada. The analysis of relative concentrations of organic matter within the encapsulating silica frustules of diatoms is important for a better understanding of both the physiological state and the individual cellular response to environmental pressures. Analysis of 3D FTIR images of Nitzschia frigida collected from beneath high (17-19 cm) and low (3-7 cm) snow depth revealed higher concentrations of lipids in diatoms collected under low snow cover, uniquely based on spectroscopically determined total 3D cell volume and biochemical content.

Morphological and genetic diversity of Beaufort Sea diatoms with high contributions from the Chaetoceros neogracilis species complex.

Seventy-five diatom strains isolated from the Beaufort Sea (Canadian Arctic) in the summer of 2009 were characterized by light and electron microscopy (SEM and TEM), as well as 18S and 28S rRNA gene sequencing. These strains group into 20 genotypes and 17 morphotypes and are affiliated with the genera Arcocellulus, Attheya, Chaetoceros, Cylindrotheca, Eucampia, Nitzschia, Porosira, Pseudo-nitzschia, Shionodiscus, Thalassiosira, and Synedropsis. Most of the species have a distribution confined to the northern/polar area. Chaetoceros neogracilis and Chaetoceros gelidus were the most represented taxa. Strains of C.neogracilis were morphologically similar and shared identical 18S rRNA gene sequences, but belonged to four distinct genetic clades based on 28S rRNA, ITS-1 and ITS-2 phylogenies. Secondary structure prediction revealed that these four clades differ in hemi-compensatory base changes (HCBCs) in paired positions of the ITS-2, suggesting their inability to interbreed. Reproductively isolated C.neogracilis genotypes can thus co-occur in summer phytoplankton communities in the Beaufort Sea. C.neogracilis generally occurred as single cells but also formed short colonies. It is phylogenetically distinct from an Antarctic species, erroneously identified in some previous studies as C.neogracilis, but named here as Chaetoceros sp. This work provides taxonomically validated sequences for 20 Arctic diatom taxa, which will facilitate future metabarcoding studies on phytoplankton in this region.

A (too) bright future? Arctic diatoms under radiation stress

Decreasing Arctic sea ice cover and increasing stratification of ocean surface waters make the exposure of pelagic microalgae to high irradiances more likely. Apart from light being a necessary prerequisite for photosynthesis, rapidly changing and/or high irradiances are potentially detrimental. An in situ study was performed in the high Arctic (79°N) to determine the effect of high irradiances in general, and ultraviolet radiation (UVR, 280–400 nm) in particular, on cell concentrations, fatty acid composition, and photoprotective pigments of three diatom species isolated from seawater around Svalbard. Unialgal cultures were exposed in situ at 0.5- and 8 m-depth. After 40 h, cell concentrations of Synedropsis hyperborea and Thalassiosira sp., were lower at 0.5 than at 8 m, and the content of the photoprotective xanthophyll-cycle pigment diatoxanthin in all species (S. hyperborea, Thalassiosira sp., Porosira glacialis) was higher in the 0.5 m exposure compared to 8 m. In S. hyperborea, growth was additionally inhibited by UVR at 0.5-m depth. In situ radiation conditions led, furthermore, to a significant decrease in polyunsaturated fatty acids (PUFAs) in all three species, but UVR had no additional effect. Hence, we conclude that natural radiation conditions close to the surface could reduce growth and PUFA concentrations, but the effects are species specific. The diatoms’ potential to acclimate to these conditions over time has to be evaluated.

Diversity, ecology and community structure of the freshwater littoral diatom flora from Petuniabukta (Spitsbergen)

Despite the frequent application of diatoms in palaeoecological research in the High Arctic, our knowledge concerning the diversity, ecology, community associations, community dynamics and survival strategies of High Arctic freshwater littoral diatom floras is still rather limited. In the present study, the diversity, ecological characteristics and community structures of the freshwater littoral diatom flora from ponds and lakes of Petuniabukta (Spitsbergen, Svalbard Archipelago) were studied. In total, 87 samples from the littoral zone of 53 freshwater ponds and lakes were examined using light and scanning electron microscopy. A highly diverse diatom flora dominated by Achnanthidium and Nitzschia was observed. In total, 310 diatom taxa belonging to 59 genera were found, of which about one-third could not be identified beyond the genus level. Based on species composition, four main diatom communities were distinguished. Differences in environmental characteristics, such as habitat type, habitat diversity, habitat stability, streams, water pH and influence of the fjord, biota and human activities, appeared to be important factors in determining the observed diatom communities. The high number of unidentified taxa in this study indicates that a profound revision of the Arctic diatom flora is highly desired, not only to improve our fundamental knowledge concerning the diversity, ecology, community associations and biogeography of Arctic diatoms but also to aid applied (palaeo)environmental sciences.

Pseudo-nitzschia arctica sp. nov., a new cold-water cryptic Pseudo-nitzschia species within the P.pseudodelicatissima complex.

A new nontoxic Pseudo-nitzschia species belonging to the P.pseudodelicatissima complex, P.arctica, was isolated from different areas of the Arctic. The erection of P.arctica is mainly supported by molecular data, since the species shares identical ultrastructure with another species in the complex, P.fryxelliana, and represents a new case of crypticity within the genus. Despite their morphological similarity, the two species are not closely related in phylogenies based on LSU, ITS and rbcL. Interestingly, P.arctica is phylogenetically most closely related to P.granii and P.subcurvata, from which the species is, however, morphologically different. P.granii and P.subcurvata lack the central larger interspace which is one of the defining features of the P.pseudodelicatissima complex. The close genetic relationship between P.arctica and the two species P.granii and P.subcurvata is demonstrated by analysis of the secondary structure of ITS2 which revealed no compensatory base changes, two hemi-compensatory base changes, and two deletions in P.arctica with respect to the other two species. These findings emphasize that rates of morphological differentiation, molecular evolution and speciation are often incongruent for Pseudo-nitzschia species, resulting in a restricted phylogenetic value for taxonomic characters used to discriminate species. The description of a new cryptic species, widely distributed in the Arctic and potentially representing an endemic component of the Arctic diatom flora, reinforces the idea of the existence of noncosmopolitan Pseudo-nitzschia species and highlights the need for combined morphological and molecular analyses to assess the distributional patterns of phytoplankton species.

Biogeographical patterns and environmental controls of phytoplankton communities from contrasting hydrographical zones of the Labrador Sea

The Labrador Sea is an important oceanic sink for atmospheric CO2 because of intensive convective mixing during winter and extensive phytoplankton blooms that occur during spring and summer. Therefore, a broad-scale investigation of the responses of phytoplankton community composition to environmental forcing is essential for understanding planktonic food-web organisation and biogeochemical functioning in the Labrador Sea. Here, we investigated the phytoplankton community structure (>4μm) from near surface blooms (<50m) from spring and early summer (2011–2014) in detail, including species composition and environmental controls. Spring blooms (>1.2mgchlam−3) occurred on and near the shelves in May and in offshore waters of the central Labrador Sea in June due to haline- and thermal-stratification, respectively. Sea ice-related (Fragilariopsis cylindrus and F. oceanica) and Arctic diatoms (Fossula arctica, Bacterosira bathyomphala and Thalassiosira hyalina) dominated the relatively cold (<0°C) and fresh (salinity<33) waters over the Labrador shelf (e.g., on the southwestern side of the Labrador Sea), where sea-ice melt and Arctic outflow predominates. On the northeastern side of the Labrador Sea, intense blooms of the colonial prymnesiophyte Phaeocystis pouchetii and diatoms, such as Thalassiosira nordenskioeldii, Pseudo-nitzschia granii and Chaetoceros socialis, occurred in the lower nutrient waters (nitrate<3.6μM) of the West Greenland Current. The central Labrador Sea bloom occurred later in the season (June) and was dominated by Atlantic diatoms, such as Ephemera planamembranacea and Fragilariopsis atlantica. The data presented here demonstrate that the Labrador Sea spring and early summer blooms are composed of contrasting phytoplankton communities, for which taxonomic segregation appears to be controlled by the physical and biogeochemical characteristics of the dominant water masses.

Contribution towards a morphological and molecular taxonomic reference library of benthic marine diatoms from two Arctic fjords on Svalbard (Norway)

Knowledge on benthic diatom taxonomy and hence biodiversity from Arctic locations is rare, though the few existing analyses address morphological characteristics only. Morphological treatment is a time-consuming process and just allows the taxonomic identification of a low sample volume. In contrast, molecular sequencing is a faster and universally applicable approach, and also has the potential for refined analyses. Unfortunately, a lot of molecular information in databases is not trustable, and often existing genetic data do not fit together with morphological aspects. Therefore, the main goal of our study was for the first time to combine both morphological features and DNA information on benthic Arctic diatoms. In a first step, we studied the morphology of 26 strains from the Rostock Culture Collection of Arctic diatoms via light and electron microscopies. Secondly, we addressed their molecular taxonomy by sequencing the DNA barcode 18S V4 and the chloroplast marker rbcL. As a result, we could identify five species. Seven of the altogether twelve taxa could not be attributed to any known species, because listed sequences and morphology did not fit together. From these, three species were described as new to science here. Although the studied strains represent only a minor fraction of the diatom flora of Svalbard, this study is a first milestone in coupling morphological and molecular data of benthic diatoms in such high latitudes. By making these data available online, they contribute to a taxonomic reference library for Arctic benthic diatoms and are available for comparative studies.

Diatom distribution of surface sediment in the Bering Sea and Chukchi Sea

Diatoms from surface sediment samples in the Bering and Chukchi seas were analyzed to reveal the distribution patternsand their relationship with the ocean environment. A low abundance was found to the north of the Arctic Sea Ice Minimum(ASIMin), indicating that diatom growth is strongly inhibited by perennial sea ice. Between the ASIMin and the Arctic Sea IceMaximum (ASIMax) which experiences seasonal sea ice, the sea ice related diatoms (mainly Fragilariopsis oceanica and Fragilariopsiscylindrus ) were dominant, thereby confirming that sea ice is an important influencing factor. The Chaetoceros restingspores were more abundant in the Chukchi Sea, which corresponds well with the active phytoplankton distribution in the watercolumn, and is possibly attributed to the stronger hydrodynamic conditions present in the Bering Sea. The abundances of Chaetoceros resting spores were the lowest on the northeast Bering Shelf, possibly because of lower water depth, stronger coastal currents,river influx, coarser particle sizes and stronger winds and bottom currents. The Arctic Diatom Group (dominated by B acterosirabathyomphala, Thalassiosira antarctica v. borealis and Thalassiosira antarctica resting spores) was more abundant in theBering Basin and the areas central of and to the north of Chukchi Plateau, while the Coastal Benthic Diatoms (including Paraliasulcata and Delphineis surirella ) were mainly found on the northeast Bering Shelf and nearby Cape Lisburne in the Chukchi Sea. Thalassiosira nordenskioeldii was found to be the most abundant around the Bering Strait, while Neodenticula seminae was onlyfound in the Bering Sea and mainly distributed in the Bering Basin, indicating its close correlation with the Pacific waters.

UV radiation and its effects on P-uptake in arctic diatoms

Due to severe alterations of the physical conditions in Arctic ice-covered ecosystems (decrease of sea ice, thinning of the ozone layer), the underwater light climate is changing both with respect to its intensity and spectral composition. It is commonly observed that phytoplankton respond differently to photosynthetically active radiation (PAR, 400-700nm) and ultraviolet radiation (UVR, 280–400nm) in terms of their elemental stoichiometry, where high levels of PAR tend to increase the ratios of carbon to phosphorus (C:P), while UVR has the opposite effect. Since this has importance not only for elemental cycling and P-sequestration, but also for the algal food quality for grazers, it is of considerable interest to reveal the role of different spectral regimes of elemental uptake and stoichiometry. There are ambiguous evidence as to whether UVR stimulates or reduces the uptake of inorganic P, and to test this for three common, arctic marine diatoms; Porosira glacialis, Thalassiosira sp. Synedropsis hyperborean, we performed 33P-uptake assays with exposure to either PAR alone or PAR+UVR. Neither of the species showed strong P-uptake responses to UVR exposure, yet the two former species had a negative trend, while Synedropsis, showed a slight increase in its P-uptake under moderate UVR exposure. The latter species also had a remarkably fast P-uptake kinetics. These mixed results indicate a species-specific complex role of P in the algal response to UVR induced light stress, where not only species affinities but also ambient conditions may yield different outcomes.