Centric Diatom Species Research Articles

The conspecificity of Aulacoseira konstantinovii and Praecyclostephanos boengchhmariensis with comments on the genus Praecyclostephanos

The centric diatom species Aulacoseira konstantinovii was described from Lake Tonlé Sap (Cambodia) but was erroneously assigned to the genus Aulacoseira. Subsequently, based on a unique set of morphological characters, the genus Praecyclostephanos with the single species P. boengchmariensis was established based on the material from this lake. Examination of published data has allowed us to compare the morphology of A. konstantinovii and P. boengchmariensis and confirm their conspecificity. A new combination, Praecyclostephanos konstantinovii comb. nov., is therefore proposed.

Enigmatic fossil centric diatom species from Antarctica

In the course of our study of Cenozoic sediments from the Bunger Oasis (Eastern Antarctica), we found specimens of the centric diatom with unclear taxonomic placement. Examination of the material by light and scanning electron microscopy led to the conclusion that this diatom species belonged to the genus Hyalodiscus, but its true identity is unclear. Further investigations were needed to determine the taxonomic position of this taxon.

Diatoms of phytoplankton in rivers in the south of the taiga zone within the catchment area of the Middle Volga

The present research provides information on the taxonomic composition and some indicators of the quantitative development of diatoms in the phytoplankton of the left-bank tributaries of the river Volga basin. It was shown that the flora of the studied rivers is diatom-green-euglenic. In the taxonomic composition of diatoms, the leading genera Navicula, Nitzschia, Pinnularia, Gomphonema etc. were noted. The use of electron microscopy made it possible to expand significantly the list of centric diatom species, some of which were recorded for the first time in the flora of the studied rivers. At present, both planktonic and benthic invasive species of diatoms have been identified.

Main functional groups of the plankton in front of the fishing port of Anconcito, Santa Elena-Ecuador

Coastal marine plankton is an indicator of the link in the pelagic food chains of the first level (phytoplankton) and the second trophic level (zooplankton); therefore, it is of great ecological, fisheries and economic importance. In this context, the objective of this study was to describe the main components of two plankton communities (phytoplankton and zooplankton) and environmental variables (temperature, salinity, oxygen) at three distances (1, 5 and 10 miles) off the fishing port of Anconcito from May 2019 to February 2020, as an indicator of the ecosystem health of a fishing sector. Samples were collected horizontally using plankton nets with 55 and 300 μm mesh eyes for 10 minutes. Temperature, salinity, pH and dissolved oxygen were measured in situ with a multiparameter probe at the surface level. Mean temperature values were recorded among the three study areas of 25°C, salinity of 31 ups, dissolved oxygen of 4.6 mg.L-1 and pH of 8.The results showed that the first trophic level recorded the abundance of centric diatoms, considered indicators of productive zones mainly in August, while their density in the other months was lower. The phytoplankton composition recorded 167 species; only 12 species of centric diatoms and 13 species of dinoflagellates recorded higher algal density (> 2%), with 142 species present/scarce, the same that evidenced slight differentiation in the three zones. Therefore, diatoms and dinoflagellates were the phytoplankton functional groups that contributed a heterogeneous diversity index in the three sites. Twenty-three zooplanktonic groups were identified, with an average of six species of jellyfish, twelve siphonophores, two cladocerans, two appendicularians and various larval forms of different taxa. Copepods presented the highest mean abundance (48639.3 ind. (100m3 )-1 and 33.3 % respectively), followed by invertebrate eggs (48434.3 ind. (100m3 )-1 and 36. The ANOSIM analysis showed that there is no significant difference between the population densities of the three fixed stations, and in the Bonferroni pairwise comparison between stations 1 and 10 miles, there is no significant difference. In contrast, between stations (10 and 5), (5 and 1) miles there are significant differences, through the SIMPER application, it was found that the group that contributed to the dissimilarity was the copepods, with 38.29% between stations of 10 and 5 miles. The principal component analysis reported that the groups of cladocerans, crustaceans, mollusks, quetognathans, jellyfish and ichthyoplankton were associated with temperature. In conclusion, this line of research on phytoplankton and zooplankton recorded nutritional indicators such as diatoms and copepods very important in the trophic web, combined with local ecological and oceanographic aspects, indicate that the state of health of the coastal margin in front of Anconcito Bay is optimal.

Vegetative cell enlargement in selected centric diatom species – an alternative way to propagate an individual genotype

ABSTRACT Vegetative cell enlargement (VCE) and structures involved are documented for the first time using epifluorescence and electron microscopy. We examined three centric diatom species with polar and non-polar valve outlines. VCE progressed in two ways. The first involved liberation of the protoplast, nearly isodiametric expansion, and building of the large frustule in a more or less spherical cell. Protoplast retraction as observed in sexual auxospores before the formation of initial valves was never observed. Cell walls produced a variety of scales, including valve-like scales before the first enlarged epivalve was deposited. VCE wall structures were not similar to auxospore walls of the same species. The morphology of the first enlarged valve, the epivalve, differed from that of typical vegetative valves to a varying degree, depending on the species. The second way to vegetatively enlarge cells involved expansion of the girdle region of the existing small frustule. The circumference of the girdle region grew due to the deposition of irregular, supernumerary copulae. These copulae produced one or more protuberances, resulting in a misshapen cell. This misshapen cell then divided mitotically, and the first enlarged valve was a hypovalve. Hypovalve morphology strongly differed from that of typical vegetative valves. Several subsequent mitotic divisions were required to restore normal, species-specific frustules. Over time, irrespective of the manner of enlargement, the enlarged cells grew well and eventually became more frequent than small cells in our cultures. The scale factors of enlarged cells compared with pre-enlargement size in our experiments (1.1–5.2) were similar to those reported for other species (1.4–7.5). Additionally, we found that size increases from VCE were comparable to those resulting from sexual reproduction of the same species. Therefore, VCE can potentially expand the resident time of the population in the environment to the same degree as would production of sexual initial cells.

Rimocostatus bugojnicus gen.et. sp. nov. (Coscinodiscophyceae, Bacillariophyta) – a new fossil diatom genus from Gračanica, Bugojno palaeolake in Bosnia and Herzegovina

A new freshwater fossil diatom taxon Rimocostatus bugojnicus gen. et sp. nov, from Miocene sediments at the Gračanica site, Bugojno palaeolake in Bosnia and Herzegovina, is described. The main morphological feature of the genus is the presence of marginal rimoportulae with simple, round external openings positioned on interfascicles on the valve mantle and stalked, large, wide internal slits, positioned on every costa. Marginal and valve face fultoportulae are absent. With these characteristics, the species shows similarities to both marine and freshwater species, known from recent, and primarily from fossil, diatom deposits. Thus, Rimocostatus bugojnicus gen. et sp. nov. is an important find for understanding the evolution of centric diatom species.

Three Novel Bacteria Associated with Two Centric Diatom Species from the Mediterranean Sea, Thalassiosira rotula and Skeletonema marinoi.

Diatoms are a successful group of microalgae at the base of the marine food web. For hundreds of millions of years, they have shared common habitats with bacteria, which favored the onset of interactions at different levels, potentially driving the synthesis of biologically active molecules. To unveil their presence, we sequenced the genomes of bacteria associated with the centric diatom Thalassiosira rotula from the Gulf of Naples. Annotation of the metagenome and its analysis allowed the reconstruction of three bacterial genomes that belong to currently undescribed species. Their investigation showed the existence of novel gene clusters coding for new polyketide molecules, antibiotics, antibiotic-resistance genes and an ectoine production pathway. Real-time PCR was used to investigate the association of these bacteria with three different diatom clones and revealed their preference for T. rotula FE80 and Skeletonema marinoi FE7, but not S. marinoi FE60 from the North Adriatic Sea. Additionally, we demonstrate that although all three bacteria could be detected in the culture supernatant (free-living), their number is up to 45 times higher in the cell associated fraction, suggesting a close association between these bacteria and their host. We demonstrate that axenic cultures of T. rotula are unable to grow in medium with low salinity (<28 ppt NaCl) whereas xenic cultures can tolerate up to 40 ppt NaCl with concomitant ectoine production, likely by the associated bacteria.

Secretion of sulfated fucans by diatoms may contribute to marine aggregate formation

AbstractMicroalgae produce copious amounts of structurally diverse polysaccharides, some are bound within cells and cell walls, while others are secreted into the surrounding seawater. A fraction of the secreted polysaccharides assembles into particles promoting aggregation and in turn formation of aggregates increases the export of carbon into the deep ocean via sinking. However, specific polysaccharides contributing to particle formation and carbon export remain unknown. Here, we studied microalgae polysaccharide composition in a system of reduced complexity consisting of lab grown monospecific cultures of the centric diatom species Thalassiosira weissflogii and Chaetoceros socialis. We followed the abundance and dynamics of five specific polysaccharide types in the dissolved and particulate organic matter (DOM and POM) for two weeks. Polysaccharides were detected using monoclonal antibodies (mAbs) specific for β‐1,4‐mannan, β‐1,4‐xylan, arabinogalactan, and two fucose‐containing sulfated polysaccharide (FCSP) epitopes. Additionally, glycan composition of all samples was analyzed by monosaccharide analysis. The time series revealed polysaccharides partition differently between the dissolved and particulate carbon pools. β‐1,4‐xylan and β‐1,4‐mannan were mainly present in POM, possibly as cell wall polymers, while FCSPs were found in both DOM and POM. The data showed that the main glycan component secreted by diatoms was fucose‐containing polysaccharide, which accumulated in DOM over time. Roller tank experiments were used to induce aggregate formation finding FCSP transitioned from DOM to POM under aggregating conditions. These results suggest that diatom‐secreted FCSPs are involved in the formation of aggregates, which promote the formation of particles, and potentially carbon export.

An optimised method for intact nuclei isolation from diatoms

Due to their abundance in the oceans, their extraordinary biodiversity and the increasing use for biotech applications, the study of diatom biology is receiving more and more attention in the recent years. One of the limitations in developing molecular tools for diatoms lies in the peculiar nature of their cell wall, that is made of silica and organic molecules and that hinders the application of standard methods for cell lysis required, for example, to extract organelles. In this study we present a protocol for intact nuclei isolation from diatoms that was successfully applied to three different species: two pennates, Pseudo-nitzschia multistriata and Phaeodactylum tricornutum, and one centric diatom species, Chaetoceros diadema. Intact nuclei were extracted by treatment with acidified NH4F solution combined to low intensity sonication pulses and separated from cell debris via FAC-sorting upon incubation with SYBR Green. Microscopy observations confirmed the integrity of isolated nuclei and high sensitivity DNA electrophoresis showed that genomic DNA extracted from isolated nuclei has low degree of fragmentation. This protocol has proved to be a flexible and versatile method to obtain intact nuclei preparations from different diatom species and it has the potential to speed up applications such as epigenetic explorations as well as single cell (“single nuclei”) genomics, transcriptomics and proteomics in different diatom species.

Diatoms rapidly alter sinking behavior in response to changing nutrient concentrations

AbstractA diatom's sinking speed affects its depth in the water column, which determines its access to light and nutrients. Some large, centric diatom species perform an unsteady sinking behavior in which a cell's sinking speed oscillates over more than an order of magnitude on time scales of seconds. Diatoms are known to decrease mean sinking speeds and the magnitude of unsteady sinking following exposure to nutrient replete conditions over hours to days. Here we show that on shorter time scales of minutes to hours, nutrient deprived Coscinodiscus wailesii cells increase the mean and unsteadiness of their sinking when exposed to increased nutrient concentrations. Cultures exposed to nitrate or silicate‐depleted media followed by a spike of the missing nutrient showed a sinking speed increase within the first 2 h that declined over the next 22 h. Phosphate deprived cultures did not respond similarly to a phosphate spike. In an experiment with an artificial nutricline in which cells encountered a sharp increase in nutrient concentrations over a distance of 10 cm, mean sinking speeds increased eight fold, and sinking unsteadiness increased significantly; these sinking speed changes occurred over 33 min. The contrasting short and long‐term sinking behavior responses seen in this study demonstrates the importance of examining sinking behavior over multiple time scales. Initial fast and unsteady sinking upon encountering increasing nutrient concentrations may help diatoms take advantage of patchy nutrient distributions. Longer term, slow and steady sinking may be beneficial for maximizing light exposure and minimizing energy costs from unsteady sinking.

Re-examination of Cyclotella lacunarum Hustedt (Bacillariophyta) from lakes in the Pamir Mountains, western China, and description of two similar Lindavia taxa collected from Tajikistan and Nepal

Originally described from a sample collected in the Pamir Mountains at the end of the nineteenth century, Cyclotella lacunarum Hustedt is a centric diatom species that is not-well defined and seldom reported in the scientific literature. Our investigations revealed that the type locality of this taxon, now referred to as Lindavia lacunarum (Hustedt) Nakov et al., is actually in western China and not in Tajikistan, as wrongly assumed previously. Light microscope (LM) and scanning electron microscope (SEM) observations of the type material and of modern material collected at the type locality allowed us to emend the description of this species. Its most remarkable character is that it possesses central fultoportulae with only two satellite pores instead of three as generally reported for similar species of Lindavia. Lindavia lacunarum was then compared to two similar populations of Lindavia collected from high elevation lakes in Tajikistan and Nepal. These two populations were originally identified as Cyclotella/Lindavia lacunarum and represent the only reported occurrences of this taxon outside its type locality. The population from Tajikistan differs from L. lacunarum by having cells of smaller size, with an almost flat valve face and has been described as a new variety Lindavia lacunarum var. karakulensis var. nov. The Nepalese population is more clearly differentiated from L. lacunarum by having a different type of colliculate pattern in the central area and significantly lower density of central fultoportulae and is therefore described as a new species, Lindavia nepalensis sp. nov.

Application of nuclear magnetic resonance for analyzing metabolic characteristics of winter diatom blooms

Abstract We compared two metabolome profiles of a small centric diatom species, Stephanodiscus hantzschii Grun., grown under conditions with enriched nutrients but different temperatures. This species proliferates in eutrophic rivers during winter. We investigated the population dynamics and internal metabolite changes of Stephanodiscus by performing a simple culture experiment at different temperatures (5 and 15°C). We applied the 1H nuclear magnetic resonance (NMR) technique to fully grown cells to obtain the metabolite profiles of S. hantzschii. Growth rates were significantly different at different temperature conditions (0.99 ± 0.11 day−1 at 15°C and 0.21 ± 0.12 day−1 at 5°C, n = 10). Characterized metabolites included saturated and unsaturated fatty acids, AXP (including AMP, ADP and ATP), and UDP-glucose and UDP-galactose, all of which are important for energy metabolism. These metabolites were abundant within S. hantzschii cells grown at 15°C but were not prolific in those grown at 5°C. Furthermore, other 1H NMR spectrum uncovered very little amounts of metabolites. Based on these observations of cell growth rate, although required nutrients were supplied, colder temperatures suppressed population growth through the deactivation of various internal metabolisms. Thus, winter proliferation of this species is opportunistic, implying that survival success led to dominance in freshwater ecosystems with neither resource competition nor grazing pressure.

NEW METHOD OF ESTIMATION OF THE RELATIVE AREA OF PERFORATIONS ON VALVES OF CENTRIC DIATOMS USING SEM IMAGES ON THE EXAMPLE OF MINIDISUS VODYANITSKIYI LYAKH ET BEDOSHVILI

The diatoms interact with the environment through the siliceous frustule. The total area of frustule perforations determines the ability of diatom to exchange nutrients, gases and other matters. The aim of the present study was to estimate the area of perforations on the valve surface of a centric diatom. In the paper we describe a method for the estimation of the area of perforations on a diatom valve using SEM images. The method is tested on valves of centric diatom Minidiscus vodyanitskiyi Lyakh et Bedoshvili. The results show that the total area of cribral pores is less than 5% of the total valve area. This value is consistent with the relative perforation of land plants leaves, which is less than 3%. We hypothesize that such small valve area occupied by perforations is usual for many other centric diatom species. To verify this hypothesis additional researches are necessary.

DISTRIBUSI DAN KEMELIMPAHAN PLANKTON DI WET DUNE SLACKS GUMUK PASIR PARANGTRITIS, BANTUL, DIY.

Komunitas plankton memiliki peran penting pada proses rantai makanan, siklus nutrien dan proses suksesi awal wet dune slacks. Kawasan wet dune slacks merupakan cekungan yang berada di antara punggung gumuk dan kondisi permukaan air tanah sangat bervariasi tergantung musim. Habitat unik yang telah dilindungi di benua Eropa ini justru belum banyak dipelajari di Indonesia. Tujuan penelitian ini adalah untuk mempelajari distribusi dan kemelimpahan plankton di wet dune slacks serta faktor fisiko-kimia yang paling mempengaruhi distribusi dan kemelimpahannya. Pencuplikan sampel dilakukan pada bulan Februari 2018 di Gumuk Pasir Parangtritis, Bantul, Daerah Istimewa Yogyakarta. Sampel air dicuplik menggunakan ember sebanyak 10 liter dengan 3 ulangan pada 5 wet dune slacks yang berbeda. Sampel air kemudian disaring menggunakan plankton-net ukuran 120 mesh serta diukur parameter fisiko-kimianya. Hasil dari pengamatan sampel plankton menunjukkan terdapat 37 spesies diatom pennate, 1 spesies diatom sentrik, 23 spesies alga unisel, 28 spesies alga koloni, 18 spesies alga filamen, 3 spesies dinoflagellata, 6 spesies protozoa berklorofil, 22 spesies rotifera, 6 spesies cladocera, 11 spesies ostracoda, 5 spesies copepoda, dan 5 spesies protozoa. Spesies plankton yang berhasil mengsuksesi kawasan wet dune slacks merupakan spesies yang memiliki strategi bertahan di habitat kekeringan dengan berubah ke fase istirahat, cyst, spora, dan resting eggs. Spesies fitoplankton yang mendominasi adalah Cyclotella meneghiniana sedangkan untuk zooplankton adalah Chlamydotheca flexilis dan Alona rectangula. Parameter fisiko-kimia kecepatan angin, C-organik, alkalinitas, sulfat, temperatur air, temperatur udara, intensitas cahaya,fosfat, pH dan oksigen terlarut menjadi faktor yang paling mempengaruhi distribusi dan kemelimpahan fitoplankton. Sedangkan untuk faktor fisiko-kimia yang paling mempengaruhi distribusi dan kemelimpahan zooplankton adalah kadar oksigen terlarut, CO2 terlarut, pH, temperatur air, temperatur udara, jeluk, jeluk Secchi, kecepatan angin, intensitas cahaya, C-organik, sulfat, fosfat.&#x0D; &#x0D;

Seasonality and latitudinal variability in the diatom-cyanobacteria symbiotic relationships in the coastal waters of the Red Sea, Saudi Arabia

The seasonal and spatial dynamics of diatom-cyanobacteria associations were observed in the coastal waters of the Gulf of Aqaba as well as in the northern and central Red Sea, Saudi Arabia. However, there was an absence of such associated observations in the southern Red Sea. The heterocystous endosymbiotic cyanobacteria Richelia intracellularis J. Schmidt, 1901 was found living in association with five centric diatom species, including Rhizosolenia styliformis T. Brightwell, 1858, Rhizosolenia hebetata Bailey, 1856, Rhizosolenia imbricata Brightwell, 1858, Guinardia cylindrus (Cleve) Hasle, 1996 and Proboscia alata (Brightwell) Sundstrom, 1986. All five diatom species were observed with R. intracellularis in the central Red Sea region, while R. intracellularis was not found with R. imbricata and P. alata in the other Red Sea regions. The density of symbiotic diatom cells showed seasonal variations in the northern and central Red Sea, and the densities were the most conspicuous in these regions compared to the Gulf of Aqaba region. There was seasonal variation in the percent occurrence of the symbiotic diatoms compared to the nonsymbiotic diatoms. Symbiotic associations between the diatoms and R. intracellularis were only evident within the central Red Sea region from April to October. Multiple R. intracellularis trichomes inside the diatom host cells were common in the central Red Sea, while the other Red Sea regions showed only single trichomes. The size of the individual trichomes of R. intracellularis growing inside the host cells was entirely dependent on the size of the respective diatom species (r = 0.781, P < 0.01). The complete absence of such specific relations in the southern Red Sea reveals the possibility of different ecological conditions in the region.

Aulacoseira pseudomuzzanensis sp. nov. and other centric diatoms from post iron ore mining reservoirs in Poland

Post iron ore mining reservoirs in Łęczyca (Poland) can be considered extreme habitats because they are characterized by high pH, conductivity, and sulfate, sodium and potassium concentrations. We observed high abundances in these water bodies of several centric diatom species that are otherwise known from less extreme environments. These included Thalassiosira duostra, Cyclostephanos dubius, Stephanodiscus binatus, Stephanodiscus invisitatus, Stephanodiscus parvus, Stephanodiscus hantzschii, Lindavia aff. balatonis, Discostella woltereckii, Discostella pseudostelligera, and Cyclotella meneghiniana, revealing the eurytopic nature of these species. Morphological differences between populations from extreme habitats and their counterparts from less extreme environments may indicate phenotypic responses of eurytopic taxa to the differing environmental regimes, or the presence of distinct, more stenotopic taxa. In both cases, this could be relevant for biomonitoring. For this reason, we carefully observed the morphological variability of our target taxa in the Łęczyca reservoir samples compared with literature accounts from less extreme habitats, and in some cases, with type material of the species concerned. Specimens similar to Aulacoseira muzzanensis α-chains were observed. Following comparison with the type material of Melosira muzzanensis Meister (≡A. muzzanensis), we concluded that the Łęczyca populations are sufficiently different to be recognized as a separate species, here described as Aulacoseira pseudomuzzanensis sp. nov. The importance of identifying centric diatoms to species level for water management surveys is emphasized, even for those taxa that are difficult to identify using only LM.

Centric Diatoms (Centrophyceae) of Water Bodies of Urban Agglomerations

On the whole, 17 species of small-sized centric diatoms were revealed using the methods of light and scanning electron microscopy. Among them, five species (Aulacoseira subarctica, Cyclotella ocellata, Handmannia comta, Stephanodiscus minutulus, and Conticribra weissflogii) were found in water bodies of various urban agglomerations (Kiev and Zhitomir) for the first time. Their electron microphotographs, brief morphological description, and ecological and geographic characteristics are given in the paper. It has been found that the number of new species of Centrophyceae did not depend on the type of urban agglomeration. The presence of small-sized centric diatoms, and also of brackishwater species of the genus Thalassiosira Cl., in plankton of urban water bodies is one of the mechanisms of phytoplankton adaptation to anthropogenic load.

New records of centric diatoms from Yakutia (Bolshoe Toko Lake): SEM morphology, ecology and distribution

As a result of a SEM study of phytoplankton, the first data on centric diatom species composition in Bolshoye Toko Lake, Yakutia, were obtained. Ten species (Aulacoseira — 5, Cyclotella — 1, Discostella — 1, Handmannia — 1, Pliocaenicus — 1) were found, and one taxon from the genus Pliocaenicus was identified to the genus level. Of these, nine taxa are reported for the first time in the water bodies of the Aldan River basin and Discostella guslyakovyi in Yakutia.

Optical and biochemical properties of a southwest Florida whiting event

“Whiting” in oceanography is a term used to describe a sharply defined patch of water that contains high levels of suspended, fine-grained calcium carbonate (CaCO3). Whitings have been reported in many oceanic and lake environments, and recently have been reported in southwest Florida coastal waters. Here, field and laboratory measurements were used to study optical, biological, and chemical properties of whiting waters off southwest Florida. No significant difference was found in chlorophyll a concentrations between whiting and outside waters (non-whiting water), but average particle backscattering coefficients in whiting waters were double those in outside waters, and remote sensing reflectance in whiting waters was higher at all wavelengths (400–700 nm). While other potential causes cannot be completely ruled out, particle composition and biochemical differences between sampled whiting water, contiguous water, and outside water indicate a biologically precipitated mode of whiting formation. Taxonomic examination of marine phytoplankton samples collected during a whiting event revealed a community dominated by autotrophic picoplankton and a small (<10 μm), centric diatom species, identified as Thalassiosira sp. through the use of scanning electron microscopy. Amorphous to fully formed crystals of CaCO3 were observed along the girdle bands of Thalassiosira sp. cells and autotrophic picoplankton cells. Although carbonate parameters differed from whiting and contiguous to outside water, more sampling is needed to determine if these results are statistically significant.

Dynamic sinking behaviour in marine phytoplankton: rapid changes in buoyancy may aid in nutrient uptake.

Phytoplankton sinking is an important property that can determine community composition in the photic zone and material loss to the deep ocean. To date, studies of diatom suspension have relied on bulk measurements with assumptions that bulk rates adequately capture the essential characteristics of diatom sinking. However, recent work has illustrated that individual diatom sinking rates vary considerably from the mean bulk rate. In this study, we apply high-resolution optical techniques, individual-based observations of diatom sinking and a recently developed method of flow visualization around freely sinking cells. The results show that in both field samples and laboratory cultures, some large species of centric diatoms are capable of a novel behaviour, whereby cells undergo bursts of rapid sinking that alternate with near-zero sinking rates on the timescales of seconds. We also demonstrate that this behaviour is under direct metabolic control of the cell. We discuss these results in the context of implications for nutrient flux to the cell surface. While nutrient flux in large diatoms increases during fast sinking, current mass transport models cannot incorporate the unsteady sinking behaviour observed in this study. However, large diatoms appear capable of benefiting from the enhanced nutrient flux to their surface during rapid sinking even during brief intervening periods of near-zero sinking rates.