Freshwater Diatom Research Articles

Ultrastructure of four Surirella taxa (Bacillariophyceae) from Qinghai Province, northwest China, with reference to the characteristics of portulae

During our investigations on the freshwater diatoms from Qinghai Province, China, four sympatric Surirella taxa belonging to the group Pinnatae were analysed. Based on morphological observations derived from light and scanning electron microscopy, three of these four taxa were identified as: S. minuta, S. lacrimula, and S. brebissonii var. kuetzingii. The fourth taxon is described as Surirella liubingii sp. nov., a new species with a unique combination of characteristics. These include the distribution pattern of its costa-stria bundles (CSBs) alternating with its over-fibula costae (OFCs), which is formed by the distinctly elevated CSBs on the valve face, mostly composed of three costae, and OFCs with a remarkedly lower position. Other characteristics include the presence of robust fibulae extending to the valve midline, the presence of only one portula between two adjacent fibulae, and a “pseudoseptum” extending internally from the valve margin. From the observations on these four Surirella taxa, we described and discussed two types of fibula (one in which the fibula nearly reaches the valve midline and one with short fibula) and two types of portula morphology (one with only one portula between two adjacent fibulae and the other with 2–4 portulae between two adjacent fibulae).

Late-Holocene hydrological variability from the NW Himalaya and southwestern Tibetan Plateau: Paleo-salinity records from Pangong Tso

Variations in lake-levels and hydrology are connected to climatic dynamics over the Trans-Himalaya and Tibetan Plateau (TP). Pangong Tso, a ~140 km long hypersaline lake, is sensitive to changes in air temperature, precipitation and snow-glacier melt over the southwestern TP. The incised tributaries entering the lake expose deltaic sequences constituting topset, foreset and bottomset. Sedimentology, chronology and diatom analysis of delta sediments; and stable isotopic and sclerochronological analysis of gastropods helped delineate the Late-Holocene variation in lake levels, surface temperature and salinity. The elevation of the topsets is considered as representing past lake high-stands from where molluscs were also recovered. Three phased lake level changes are observed during the past 3 ka. The first high-stand (+1.4–3.0 m) was at ~2.8–2.0 ka when lake surface salinity and temperature were 4.67–6.01 ppt (parts per thousand) and 5°C–7°C, respectively, against the modern average values of 7.7 ± 0.09 ppt and 16.1°C ± 2.0°C. Followed by a brief decline, another high-stand (+3.0–3.6 m) is observed at 1.1 ka when the salinity is reduced to 4.03–5.72 ppt and lake surface temperature to 5°C–8°C. A corresponding increase in freshwater diatom concentration is also observed here. The third phase over the past 1 ka witnessed a fall of ~3.6–6 m in lake level that is attributed to an abrupt rise in aridity over the TP. We demonstrate that lake level variation in the region is a function of the variability of the Indian Summer Monsoon (ISM) and the westerlies, however during the high-stand, the hydrology of the lake was dominated by ISM precipitation.

Biomass valorization of agriculture wastewater grown freshwater diatom Nitzschia sp. for metabolites, antibacterial activity, and biofertilizer

The sustainable utilization of agricultural wastewater is a major global challenge. This study evaluated the impact of agricultural fertilizer on the biomass potential of Nitzschia sp. for metabolite production, antibacterial activity, and slow release biofertilizer. Cultivation of Nitzschia sp. in agriculture wastewater (0.5 mg ml−1) exhibited maximum cell density (12×105 cells ml−1), protein content (10.0 mg g−1), and lipid content (14.96%). Carbohydrate and phenol content increases in a dose-dependent manner with 8.27 mg g−1 and 2.05 mg g−1 at a concentration of 2 mg ml−1 respectively. There was a 2.1-fold increment in chrysolaminarin content. Both gram-negative and gram-positive bacteria were susceptible to the antibacterial activity of the biomass. The effects of using diatom biomass as a biofertilizer were evaluated on the growth of periwinkle plants, which showed significant improvements in leaf development, branching at an early stage, flowering, and a marked increase in shoot length. Diatom biorefinery holds immense potential in addressing agriculture wastewater recycling and sustainable generation of high-value compounds.

Are diatom community assembly processes scale invariant in streams?

Abstract Biological communities are structured by deterministic and stochastic factors often regarded to as community assembly processes. Deterministic factors comprise environmental filtering and biotic interactions while stochastic factors include random dispersal and ecological drift. Earlier research suggests that the relative importance of assembly processes may vary with scale. This is because biotic interactions typically occur at fine spatial scales, whereas heterogeneous environment filters species more efficiently at larger scales. The aim of this study was to investigate whether stream diatom community assembly processes varied among three spatial scales, ranging from within‐site scales to across drainage‐system scales in Finland. The smallest spatial scale was a single riffle where a local community was defined as diatoms living on a single stone. The next larger spatial scale was a drainage system while the largest spatial scale comprised the pooled data from the eight drainage systems. To test whether the diatom communities were stochastically or deterministically assembled, the null model approach based on Raup–Crick dissimilarities (βRC) was used. Redundancy analyses were also run to visualise diatom community variation and to investigate the main environmental variables associated with community variation. At the within‐riffle scale, mean and median βRC values were approaching −1, suggesting that environmental filtering shaped the communities. At the within‐drainage system scale, mean and median βRC values were also negative, but notably larger than within riffles. Across drainages, βRC values were at similar level to the within‐drainage system scale. At the riffle scale, water depth and light had the strongest influence on diatom community variation followed by current velocity along the stream. At the drainage scale, conductivity, pH, latitude, total phosphorus, and water colour had the largest influence on diatom community variation. Overall, negative Raup–Crick dissimilarities indicated that environmental filtering dominated diatom community assembly at all scales but especially at small, within‐riffle scales. There was no consistent support for limiting similarity even at the smallest scales and the effect of stochastic, dispersal‐related factors became stronger at larger scales. These results support the idea that chemical conditions in streams are decisive factors for freshwater diatom biodiversity especially at regional scales.

A freshwater diatom perspective on the evolution of the southern westerlies for the past ∼14,000 years in southwestern Patagonia

Conflicting, even opposite interpretations on the evolution of the Southern Westerly Winds (SWW) are evident in paleoenvironmental records from southwestern Patagonia since the last ice age. These divergences call for new approaches utilizing different, ideally independent indicators of paleoenvironmental/paleoclimatic change from sensitive sites in climatically relevant locations. Here we present a multidecadally resolved diatom record from Lago Cipreses (51°S), a small closed-basin lake located in a bedrock depression along the eastern foothills of the southern Patagonian Andes. The hydrological balance evolution of this isolated lake affords a direct tie with SWW intensity in a mountainous sector where zonal wind strength and local precipitation are highly correlated. We detect cold-tolerant diatoms (small fragilarioids) between ∼14-11.9 cal. ka BP followed by a shift to planktonic assemblages (Discostella pseudostelligera, Aulacoseira spp.) under warmer Holocene conditions. Diatom assemblages indicative of stratified water-column conditions (Discostella pseudostelligera, Achnanthidium aff tepidaricola, Achnanthidium sieminskae) reached their maximum stability between ∼9.1-7.4 cal. ka BP. Stronger water-column mixing is evident by an abrupt species turnover to Aulacoseira spp. between ∼7.4-3.1 cal. ka BP, superimposed on centennial-scale alternations between assemblages since ∼6.1 cal. ka BP. Cold-tolerant diatoms resurge at ∼3.1 cal. ka BP and persist until the present. Our record offers assemblage-based evidence we interpret as sub-centennial to multimillennial scale changes in hydroclimate indicative of: (i) strong SWW influence between ∼14-11.9 cal. ka BP, (ii) a transition between ∼11.9-11.3 cal. ka BP to weak SWW influence between ∼11.3-6.5 cal. ka BP, with a SWW minimum between ∼9.1-7.4 cal. ka BP, and (iii) strong SWW influence since ∼6.5 cal. ka BP, with a Holocene SWW maximum since ∼3.1 cal. ka BP. We posit that enhanced hydroclimate variability since ∼6.1 cal. ka BP attests to the onset of Southern Annular Mode-like changes at centennial-to sub-centennial timescales. We detect a remarkably coherent and synchronous response of terrestrial and aquatic ecosystems at local scale since ∼14 cal. ka BP, highlighting the overriding importance of variations in SWW influence in terrestrial and aquatic environments at multiple timescales.

Fossil diatom assemblage changes due to paleoenvironment change, tsunami, and typhoon in southern Japan

AbstractTracking paleoenvironmental change and past event deposits is very important to evaluate the natural hazard spatially. This paper presents how the environment changes and implies the event deposit depending on the diatom assemblage change. To investigate paleoenvironmental change and identify the difference between tsunami and typhoon deposits, we analyze diatoms from the sediments in two coastal lakes in southern Japan where flood deposits have been linked to historical typhoon and tsunami events (Lakes Kawahara and Ryuo). The sediment cores extend from B.C.E. 500 to approximately C.E. 1000 and the downcore variation in diatom assemblages indicates a series of transitions from saline to fresher conditions in both Lake Kawahara and Lake Ryuo between approximately C.E. 500 and 1700. We observe an obvious deviation in diatom assemblages in event deposits previously identified to be either of tsunami or typhoon in origin. For the most prominent event deposit preserved in Lake Ryuo by the Hoei tsunami of C.E. 1707, the deposition of marine diatoms serves as evidence of marine flooding, while the subsequent deposition of soil and freshwater diatoms indicates the mobilization of terrigenous sediment during returning seaward flows. In contrast, the most prominent event deposit in Lake Kawahara is associated with freshwater flooding by the Kamikaze typhoon of C.E. 1281 and contains very low diatom abundances and a peak of freshwater taxa, followed by a peak in diatom counts potentially due to greater biological activity induced by a resultant influx of nutrients and re‐oxygenation during the event.

Озерные палеоархивы изменения природной среды полуострова Песчаный, Японское море (южное Приморье)

The stages of evolution of two barrier lakes and vegetation development of the Peschany Peninsula (South Primorye) have been reconstructed on the base of multi-proxy study of the sediment sequences, connected with hydroclimatic changes over the past 6300 years. Tephra of the caldera-forming eruption of Baitoushan volcano (946/947 AD) is one of the chronological markers. The Middle Holocene paleolake existed on the accumulative landform, and the lake on the isthmus connecting the paleoisland with the mainland, was formed at the beginning of the Late Holocene. Several periods of watering due to precipitation increase are distinguished in its development. Redeposited freshwater diatoms from subbasalt deposits are bioindicators of sheet wash activation during heavy rains associated with typhoons. Findings of valves of marine and brackish diatoms indicate the passage of extreme storms. Overgrowing of the paleolake began at the end of the Holocene thermal optimum (~5410 years ago). The periods of dryness are usually associated with cooling and a decrease in the intensity of the summer monsoon. The Little Ice Age is the exception: the flooding of the swamp with a relict lake on the isthmus increased sharply. A change of Korean pine/broad-leaved forests with the participation of fir, birches and polydominant broad-leaved forests was established. It is shown that since the Holocene Optimum there have been ecotopes with contrasting heat and moisture supply. Human impact to geosystems was recorded. Paleolimnological studies made possible to identify the Middle-Late Holocene short-period climatic rhythm on the coast of the Amur Bay. The manifestation of cold and warm events has been established, their correlation with regional data and global events has been carried out, and the connection with ocean anomalies and the intensity of the summer monsoon has been shown.

The role of metacaspases in the development and death of cells in a laboratory culture of the diatom Ulnaria acus (Kützing) Aboal

The study of the participation of metacaspases and the antioxidant system on the example of the genes UaMC1, UaMC2, UaMC3, aldehyde dehydrogenase and glutathione synthetase under different conditions of cultivation of the freshwater diatom Ulnaria acus (Kützing) Aboal using real-time PCR showed their participation not only in cell death processes, but also expression with different activity during the interphase.

Characterization and in vitro anticancer potential of exopolysaccharide extracted from a freshwater diatom Nitzschia palea (Kütz.) W.Sm. 1856

Diatoms are photoautotrophic microalgae classified under class Bacillariophyceae, engulfed by hard silicate frustules, which give mechanical support and protection from bacterial infections. They exude polysaccharides extracellularly that help them with their gliding motion (locomotion). However, the bioactivity of such compounds was least explored from freshwater diatoms. In the present study, a single species of pennate diatom identified as Nitzschia palea was isolated and molecularly characterized by 18S rRNA smaller subunit gene (partial) sequencing and submitted to GenBank NCBI and accession number retrieved as ON360983. Based on logarithmic growth curve analysis, the exponential phase was obtained from 3rd to 4th day of diatom culture. The exopolysaccharide was extracted by the hot-water extraction method, and characterized by FT-IR. The total yield of exopolysaccharide from Nitzschia palea was estimated as 1.56 mg in 100 mL of culture after 7 days of incubation. The estimated carbohydrate content was 51.35 µg/100 µL. The monosaccharide constituents were determined by acid hydrolysis of exopolysaccharide, silylation (derivatization), followed by GC–MS analysis and tabulated. The extracted exopolysaccharide was evaluated for its anti-cancer potential against the Human Adenocarcinoma lung cancer cell line (A549) and the estimated IC50 value was 62.64 µg/mL. Acridine orange staining assay and DNA fragmentation assay also confirmed the apoptotic activity of exopolysaccharide derived from the diatom Nitzschia palea.

The oldest raphe-bearing diatoms: Evidence from the Upper Cretaceous of western and northern Canada

Diatoms, with their unique siliceous cell walls, are among the most successful eukaryotic lineages on Earth. They inhabit virtually every wet environment and are central organisms in aquatic food webs and biogeochemical cycles. A highly diverse number of species with an impressive array of morphological designs have evolved since the late Mesozoic, and especially since the Late Cretaceous. Pennate diatoms comprise the largest clade, and include those forms that evolved a raphe, an advancement that yielded motility and the ability to move across surfaces. We report multiple raphe-bearing freshwater diatoms from two Cretaceous fossil localities in Canada, the Battle Formation in Alberta (67Ma), and the Wombat locality in the Northwest Territories (82 Ma). A new species of the genus Adlafia, A. wolfeii sp. nov., is reported from the Battle Formation. Multiple species of the genus Eunotia were uncovered from the Wombat locality pushing back the minimum earliest age for this taxon by ∼30 Ma. The discoveries represent two of the oldest known records of raphe-bearing diatoms and support the hypothesis that the raphe evolved in a freshwater environment during the Cretaceous. These new records are placed in context with previous fossil records, and provide new age constraints for use in linking diatom molecular-based phylogenies to the geological record.

Diatom and Dinocyst Production, Composition and Flux from the Annual Cycle Sediment Trap Study in the Barents Sea

This paper presents the diatom and palynomorph data from a sediment trap deployed in the northern part of the East Barents Sea for an annual cycle from August 2017 to August 2018. The average monthly fluxes of diatoms and dinoflagellate cysts in the photic layer of the northeastern part of the Barents Sea varies from 10.4 × 103 to 640.8 × 103 valves m−2 day−1 and from 0.3 × 103 to 90.0 × 103 cysts m−2 day−1, respectively. Their fluxes are related to the low irradiance of the photic layer during the sea-ice cover period, dominance of southward currents, modern climate, and nepheloid layer conditions. Based on redundancy analysis of the relationship between the fluxes of diatoms and dinoflagellate cysts and organic carbon fluxes, sea-ice covers, and the seasonal cycle of light availability we determined the following. First, sea-ice-associated diatoms and dinocysts are exported to the sediment trap from the melting sea ice with a two-week delay. Second, the appearance of freshwater diatoms and green algae in the sinking material accumulating from March 2018 to July 2018 is also related to the melting of sea ice. And third, the presence of Coscinodiscus radiatus, C. perforatus, Shionodiscus oestrupii and Operculodinium centrocarpum in the diatoms and dinocysts species composition throughout the year indicates the advection of Atlantic waters into the Barents Sea up to 80° N.

Diversity and ecology of freshwater diatoms as pollution indicators from the freshwater Ponds of Kanyakumari district, Tamilnadu

The present study explores the diversity and ecology of freshwater diatom in the Kanyakumari district, Tamilnadu. Six freshwater ponds namely, Thuckalay pond, Eraniel pond, Pampanvilai pond, Villukuri pond, Erachakulam and Thottiyodu pond were selected from Kalkulam Taluk in Kanyakumari district. A total of 27 taxa of 17 genera were recorded from these freshwater ponds. Among the freshwater diatom species, Gomphonema montanum was found to be the predominant species in all stations except St- 2. The correlation analysis between diatom density and physico-chemical parameters revealed a significant positive correlation of diatoms (Nitzschia sp., Navicula sp., Cyclotella meneghiniana and Stauroneis sp) with nitrate, phosphate and lead at sampling statins St- 2 and 6. CCA analysis revealed that the concentration of calcium, chloride, pH, alkalinity, silica, and ammonia were found to favor the occurrence of dominant species such as Cymbella turgiduta, Gomphonema sp., Rhopalodia sp., in St-3 and St-4. Among all ponds, certain parameters like total alkalinity, nitrite, phosphate, sodium, cobalt, potassium perhaps reflect the influence of agricultural waste, sewage waste, or anthropogenic activities. The statistical indices in coherence with the dominance of pollution indicator species such as Nitzschia palea, Melosira granulata and Cyclotella meneghiniana revealed that the stations St-2, 4 and 6 were heavily polluted, whereas St- 1, 3 and 5 were less polluted.

Effect of salinity on valves morphology in freshwater diatoms.

Increased salt concentration is one of the most widespread problems affecting freshwater worldwide. Aquatic communities, and in particular periphytic diatoms, react to this alteration in water quality by modifying their structural parameters and physiology at the individual level, which is commonly manifested by the appearance of teratological forms. The present work presents the results of an experimental laboratory study in which a biofilm grown on artificial substrates was subjected to a gradient of water conductivities for 4weeks. The results show an increase in the number of deformed valves over time proportionally to the increase in conductivity for each experimental treatment. These effects are also verified by analyzing the concentration of chlorophyll-a in the experimental biofilms, which demonstrate a metabolic response to the induced osmotic stress. No changes were recorded; however, in species richness or diversity of taxa present in the treatments. Our results, therefore, confirm at the experimental level numerous previous field observations about the harmful effect of salinity on periphytic diatoms, and also their ability to reintegrate with the new stress conditions.

Large-scale spatial patterns of riverine communities: niche versus geographical distance

Freshwater ecosystems are among the most threatened ecosystems on Earth. Effective conservation strategies are essential to reverse this trend and should be based on sound knowledge of biodiversity patterns and the main drivers structuring them. In this study, we investigated the role of environmental and dispersal-connectivity controls on freshwater diatom and fish communities’ variability. We used 441 biological samples obtained from Spanish biomonitoring datasets, which cover a highly variable environmental gradient across the national river network. We compared the taxonomic and trait-based spatial dependency of the two biotic groups using distance-decay relationships and variation partitioning with spatially constrained randomisations. Our findings showed that most of the diatoms and fish biological variation was attributed to pure spatial and spatially structured environmental variation. Compared to diatoms, fish community composition presented a stronger spatial dependency, likely because of their weaker dispersal ability. In addition, broad-scale environmental characteristics showed a higher predictive capacity for fish assemblages’ variation. Trait-based similarities presented lower spatial dependency than taxonomic datasets, indicating that they are less susceptible to dispersal-connectivity effects. These findings contribute to understand the mechanisms underlying river community assembly at large spatial scales (i.e., at and beyond the river network) and point out the importance of dispersal-connectivity processes, which are usually neglected in traditional niche-based biomonitoring programmes but can influence their outcomes (e.g., masking the detection of anthropogenic impacts). Therefore, the integration of the dispersal-connectivity component, as well as information on organisms’ dispersal abilities, are crucial when establishing effective conservation objectives and designing biomonitoring strategies.

Morphological, physiological, and transcriptional responses of the freshwater diatom Fragilaria crotonensis to elevated pH conditions.

Harmful algal blooms (HABs) caused by the toxin-producing cyanobacteria Microcystis spp., can increase water column pH. While the effect(s) of these basified conditions on the bloom formers are a high research priority, how these pH shifts affect other biota remains understudied. Recently, it was shown these high pH levels decrease growth and Si deposition rates in the freshwater diatom Fragilaria crotonensis and natural Lake Erie (Canada-US) diatom populations. However, the physiological mechanisms and transcriptional responses of diatoms associated with these observations remain to be documented. Here, we examined F. crotonensis with a set of morphological, physiological, and transcriptomic tools to identify cellular responses to high pH. We suggest 2 potential mechanisms that may contribute to morphological and physiological pH effects observed in F. crotonensis. Moreover, we identified a significant upregulation of mobile genetic elements in the F. crotonensis genome which appear to be an extreme transcriptional response to this abiotic stress to enhance cellular evolution rates-a process we have termed "genomic roulette." We discuss the ecological and biogeochemical effects high pH conditions impose on fresh waters and suggest a means by which freshwater diatoms such as F. crotonensis may evade high pH stress to survive in a "basified" future.

Relation between Present Freshwater Diatom Assemblage and Grain Size of the Lake: A Comparative Case Study from the Two Swamps of Rani-Garbhanga Reserve Forest, Assam

Relation between Present Freshwater Diatom Assemblage and Grain Size of the Lake: A Comparative Case Study from the Two Swamps of Rani-Garbhanga Reserve Forest, Assam

Fossilized diatoms as indirect indicators of the origin of carbon stored in intertidal flats

Coastal systems store enormous carbon quantities in their sediment, which originates from various autochthonous and allochthonous sources. Carbon fluxes in coastal ecosystems have a strong effect on the recipient food-webs and carbon emission offsets. Yet, the relative importance of autochthonous vs. allochthonous C inputs to coastal carbon budget is still challenging to identify. Here, we combine diatoms preserved in the sediment with geochemical analyses to identify the sources of carbon stored in Africa’s largest intertidal seagrass beds at Banc d’Arguin, Mauritania. The area lies between an active ocean upwelling and the ‘Sahara-dust hotspot’ systems. The extensive seagrass beds of the area are thus expected to receive C from these neighboring systems in addition to producing C in-situ. Three sediment cores (50 cm) were collected at three intertidal sites with different hydrodynamic regimes, and analyzed for diatom composition, total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), and carbon isotopic signatures (δ13C). Diatom taxa are grouped into three guilds: (1) benthic (epiphyte, epipelon, and epipsammon), (2) planktonic, and (3) freshwater. Benthic diatoms are considered to be autochthonous, while typical oceanic and freshwater diatoms are considered to be allochthonous. Benthic diatoms are the most diverse and abundant group, while allochthonous freshwater (i.e., dust imported) and typical upwelling (i.e., tidal imported) taxa ranked last in both abundance and species’ richness. Structure equation modelling shows that variation in the stored carbon is best explained by the total abundance of diatoms and guild composition. We conclude that the C stored in the intertidal seagrass beds of Banc d’Arguin is predominantly autochthonous. Our method provides an effective way to identify historical carbon sources in coastal systems.

The last millennium viewed from a fine-resolution freshwater diatom record from northwestern Patagonia

Little is known about the response of terrestrial and aquatic ecosystems to changes in climatic and human influences during the last millennium in northwestern Patagonia (NWP, 40°-44°S). By virtue of their sensitivity and specificity, diatoms are ideal for examining past changes in aquatic ecosystems and deciphering the ranges of variability under natural and human-induced conditions. To date, however, very few fossils diatom studies have examined in detail the environmental evolution during the last millennium throughout Patagonia. Here we present a fine-resolution diatom record from a lake-sediment core collected from Lago Pichilaguna (41°S), a closed-basin and shallow lake with a small catchment area located in the lowlands of the Chilean Lake District in NWP. The record spans the last millennium with a median time resolution of ∼12 years between samples, and shows abundant small Aulacoseira spp. between ∼1000-900 and ∼600-300 cal yr BP, which alternate in dominance with small fragilarioids and small raphid diatoms between ∼900-600 and ∼300-200 cal yr BP. A rapid shift to planktonic diatoms started at ∼200 cal yr BP and led to their modern dominance. We interpret centennial-scale changes in temperature, precipitation, and lake turbulence, with warm/dry/stratified phases between ∼1000-900 and ∼600-300 cal yr BP related to weak westerly winds, and intervals of cold/wet and mixed water column conditions between ∼900-600 and ∼300-200 cal yr BP, favored by stronger winds. The transition from periphytic to planktonic diatoms that started at ∼200 cal yr BP suggests juxtaposition of the warmest/driest phase of the last millennium and the onset of large-scale disturbance by Chilean/European settlers in NWP. Our results reveal that human disturbance during historical time surpassed the natural ranges of variability and resilience of aquatic and terrestrial ecosystems over the last millennium, generating abrupt changes in biodiversity, species composition, and community structure.

Rapid transcriptomic and physiological changes in the freshwater pennate diatom Mayamaea pseudoterrestris in response to copper exposure.

Diatoms function as major primary producers, accumulating large amounts of biomass in most aquatic environments. Given their rapid responses to changes in environmental conditions, diatoms are used for the biological monitoring of water quality and for performing ecotoxicological tests in aquatic ecosystems. However, the molecular basis for their toxicity to chemical compounds remains largely unknown. Here, we sequenced the genome of a freshwater diatom, Mayamaea pseudoterrestris NIES-4280, which has been proposed as an alternative strain of Navicula pelliculosa UTEX 664 for performing the Organisation for Economic Co-operation and Development ecotoxicological test. This study shows that M. pseudoterrestris has a small genome and carries the lowest number of genes among freshwater diatoms. The gene content of M. pseudoterrestris is similar to that of the model marine diatom, Phaeodactylum tricornutum. Genes related to cell motility, polysaccharide metabolism, oxidative stress alleviation, intracellular calcium signalling, and reactive compound detoxification showed rapid changes in their expression patterns in response to copper exposure. Active gliding motility was observed in response to copper addition, and copper exposure decreased intracellular calcium concentration. These findings enhance our understanding of the environmental adaptation of diatoms, and elucidate the molecular basis of toxicity of chemical compounds in algae.

Geological record of 18th and 19th century tsunamis along the Japan Sea coast of Tsugaru Peninsula, northwestern Japan

Our stratigraphic and paleontological investigations revealed three sandy event deposits (E1, E2, and E3) within peaty clay at the Sotomeyachi site on the Japan Sea coast of Honshu (mainland of Japan), northwestern Japan. Visual examination and X-ray computed tomographic images of samples of the deposits provided evidence of strong currents characterized by rip-up mud clasts, load casts, and erosional lower stratigraphic contacts. Marine, brackish, and freshwater diatom species in the deposits indicate they were derived from various coastal sources. Radiocarbon dating constrained the ages of event deposit E2 to 1764–1902 CE and E3 to 1558–1845 CE, 1699–1918 CE, and 1704–1781 CE. According to historical records, the E2 deposits likely correspond to the tsunami generated by the 1833 Shonai-oki earthquake, and the E3 deposits correspond to a tsunami associated with either a submarine landslide associated with the 1741 Oshima-Oshima eruption or the 1793 Nishi-Tsugaru earthquake. Because our numerical simulation of the 1741 tsunami showed no significant inundation at the study site, it is more likely that the 1793 earthquake was responsible for tsunami deposit E3. The E1 deposit is discontinuous and showed different characteristics (absence of load structures, rip-up mud clasts and mixed diatom assemblages) to those of events E2 and E3, so it is unlikely to be a tsunami deposit.