Cyclotella Cryptica Research Articles

Morphological and transcriptomic evidence for ammonium induction of sexual reproduction in Thalassiosira pseudonana and other centric diatoms.

The reproductive strategy of diatoms includes asexual and sexual phases, but in many species, including the model centric diatom Thalassiosira pseudonana, sexual reproduction has never been observed. Furthermore, the environmental factors that trigger sexual reproduction in diatoms are not understood. Although genome sequences of a few diatoms are available, little is known about the molecular basis for sexual reproduction. Here we show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica. RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium. Some of the induced genes are linked to meiosis or encode flagellar structures of heterokont and cryptophyte algae. The identification of ammonium as an environmental trigger suggests an unexpected link between diatom bloom dynamics and strategies for enhancing population genetic diversity.

Genome and methylome of the oleaginous diatom Cyclotella cryptica reveal genetic flexibility toward a high lipid phenotype.

BackgroundImprovement in the performance of eukaryotic microalgae for biofuel and bioproduct production is largely dependent on characterization of metabolic mechanisms within the cell. The marine diatom Cyclotella cryptica, which was originally identified in the Aquatic Species Program, is a promising strain of microalgae for large-scale production of biofuel and bioproducts, such as omega-3 fatty acids.ResultsWe sequenced the nuclear genome and methylome of this oleaginous diatom to identify the genetic traits that enable substantial accumulation of triacylglycerol. The genome is comprised of highly methylated repetitive sequence, which does not significantly change under silicon starved lipid induction, and data further suggests the primary role of DNA methylation is to suppress DNA transposition. Annotation of pivotal glycolytic, lipid metabolism, and carbohydrate degradation processes reveal an expanded enzyme repertoire in C. cryptica that would allow for an increased metabolic capacity toward triacylglycerol production. Identification of previously unidentified genes, including those involved in carbon transport and chitin metabolism, provide potential targets for genetic manipulation of carbon flux to further increase its lipid phenotype. New genetic tools were developed, bringing this organism on a par with other microalgae in terms of genetic manipulation and characterization approaches.ConclusionsFunctional annotation and detailed cross-species comparison of key carbon rich processes in C. cryptica highlights the importance of enzymatic subcellular compartmentation for regulation of carbon flux, which is often overlooked in photosynthetic microeukaryotes. The availability of the genome sequence, as well as advanced genetic manipulation tools enable further development of this organism for deployment in large-scale production systems.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0670-3) contains supplementary material, which is available to authorized users.

Screening of Diatom Strains and Characterization of Cyclotella cryptica as A Potential Fucoxanthin Producer.

Fucoxanthin has been receiving ever-increasing interest due to its broad health beneficial effects. Currently, seaweeds are the predominant source of natural fucoxanthin. However, the disappointingly low fucoxanthin content has impeded their use, driving the exploration of alternative fucoxanthin producers. In the present study, thirteen diatom strains were evaluated with respect to growth and fucoxanthin production potential. Cyclotella cryptica (CCMP 333), which grew well for fucoxanthin production under both photoautotrophic and heterotrophic growth conditions, was selected for further investigation. The supply of nitrate and light individually or in combination were all found to promote growth and fucoxanthin accumulation. When transferring heterotrophic cultures to light, fucoxanthin responded differentially to light intensities and was impaired by higher light intensity with a concomitant increase in diadinoxanthin and diatoxanthin, indicative of the modulation of Diadinoxanthin Cycle to cope with the light stress. Taken together, we, for the first time, performed the screening of diatom strains for fucoxanthin production potential and investigated in detail the effect of nutritional and environmental factors on C. cryptica growth and fucoxanthin accumulation. These results provide valuable implications into future engineering of C. cryptica culture parameters for improved fucoxanthin production and C. cryptica may emerge as a promising microalgal source of fucoxanthin.

Molecular and morphological analyses of solitary forms of brackish Thalassiosiroid diatoms (Coscinodiscophyceae), with emphasis on their phenotypic plasticity

Blooms of centric diatoms are a common feature in the Bilbao estuary during summer when river flow is at its lowest and water temperature is above 20ºC. To gain insight into the specific composition of these diatom blooms, net samples and cultures of estuarine isolates were analysed under the scanning electron microscope (SEM) and by molecular analyses of the Internal Transcribed Spacers 1 and 2 plus the coding region 5.8S (ITS region) and the 28S rRNA gene. Seven species of solitary centric diatoms belonging to four genera were found in the estuary including: Conticribra weissflogii, Cyclotella atomus var. atomus, Cyclotella cryptica, Cyclotella marina, Cyclotella meneghiniana, Discostella pseudostelligera and Thalassiosira pseudonana. Dominant species during blooms were C. meneghiniana and Co. weissflogii in the upper estuary and D. pseudostelligera and T. pseudonana in the middle estuary. The morphological traits used to differentiate between species pairs of similar morphology (C. meneghiniana/C. cryptica or D. pseudostelligera/D. woltereckii) were observed to vary with environmental conditions, denoting a great deal of phenotypic plasticity which would hinder accurate identification of the species when using morphological approaches alone.

Unlocking nature's treasure-chest: screening for oleaginous algae.

Micro-algae synthesize high levels of lipids, carbohydrates and proteins photoautotrophically, thus attracting considerable interest for the biotechnological production of fuels, environmental remediation, functional foods and nutraceuticals. Currently, only a few micro-algae species are grown commercially at large-scale, primarily for “health-foods” and pigments. For a range of potential products (fuel to pharma), high lipid productivity strains are required to mitigate the economic costs of mass culture. Here we present a screen concentrating on marine micro-algal strains, which if suitable for scale-up would minimise competition with agriculture for water. Mass-Spectrophotometric analysis (MS) of nitrogen (N) and carbon (C) was subsequently validated by measurement of total fatty acids (TFA) by Gas-Chromatography (GC). This identified a rapid and accurate screening strategy based on elemental analysis. The screen identified Nannochloropsis oceanica CCAP 849/10 and a marine isolate of Chlorella vulgaris CCAP 211/21A as the best lipid producers. Analysis of C, N, protein, carbohydrate and Fatty Acid (FA) composition identified a suite of strains for further biotechnological applications e.g. Dunaliella polymorpha CCAP 19/14, significantly the most productive for carbohydrates, and Cyclotella cryptica CCAP 1070/2, with utility for EPA production and N-assimilation.

Potential of lipid metabolism in marine diatoms for biofuel production.

BackgroundDiatoms are an ecologically relevant group of microalgae that are not commonly considered for bio-oil production even if they are responsible for massive blooms at sea. Seventeen diatom species were screened for their capacity to produce biomass and lipids, in relation to their growth rate. Triglyceride levels were also assessed as a preferential source of biofuels.ResultsUsing statistical analysis, two centric diatoms, Thalassiosira weissflogii and Cyclotella cryptica, were selected as good candidates for oil production. Lipid levels significantly increased when the two diatoms were cultivated in a two-stage process under nitrogen limitation. The effect was less pronounced in cultures where silicon was reduced to 20% of the standard supply. Nitrogen limitation did not affect growth rates but led to lipid remodeling and de novo synthesis of triacylglycerols.ConclusionsTriacylglycerols in T. weissflogii and C. cryptica can account for up to 82% and 88% of total glycerolipids, thereby suggesting that the two species are promising candidates for large-scale experimentation for biofuel production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0212-4) contains supplementary material, which is available to authorized users.

A droplet-based screen for wavelength-dependent lipid production in algae

We report a digital microfluidic system designed for droplet-based long-term culture and analysis of algae. The system includes unique innovations relative to standard devices including an active reservoir structure to maintain homogeneous cell density, a customized device layout capable of controlling a wide range of different droplet volumes, vertical interconnects to collect spent reagents, detection zones compatible with parallel-scale optical measurements using a standard multiwell plate reader, and optimized features for droplet dispensing in parallel. The method allows for automated, multiplexed analysis with significant reductions in human intervention, representing a decrease from 600 pipette steps (for a conventional screen in multiwell plates) to fewer than 20 (for the new microfluidic technique). The system was applied to screen conditions favourable for lipid generation in the widely used algal model for biofuel production, Cyclotella cryptica. A dependence on illumination wavelength was observed, with the best conditions (representing a four-fold increase relative to control) comprising an alternation between yellow (∼580 nm) and blue (∼450 nm) illumination wavelengths. These effects were observed for both micro- and macro-scale cultures, and are consistent with a putative mechanism involving photooxidative stress. We propose that the microfluidic system described here is an attractive new screening tool with potential advantages for applications in renewable energy, biotechnology, materials science, and beyond.

Composition and Quantitation of Microalgal Lipids by ERETIC 1H NMR Method

Accurate characterization of biomass constituents is a crucial aspect of research in the biotechnological application of natural products. Here we report an efficient, fast and reproducible method for the identification and quantitation of fatty acids and complex lipids (triacylglycerols, glycolipids, phospholipids) in microalgae under investigation for the development of functional health products (probiotics, food ingredients, drugs, etc.) or third generation biofuels. The procedure consists of extraction of the biological matrix by modified Folch method and direct analysis of the resulting material by proton nuclear magnetic resonance (1H NMR). The protocol uses a reference electronic signal as external standard (ERETIC method) and allows assessment of total lipid content, saturation degree and class distribution in both high throughput screening of algal collection and metabolic analysis during genetic or culturing studies. As proof of concept, the methodology was applied to the analysis of three microalgal species (Thalassiosira weissflogii, Cyclotella cryptica and Nannochloropsis salina) which drastically differ for the qualitative and quantitative composition of their fatty acid-based lipids.

High throughput imaging to the diatom Cyclotella cryptica demonstrates substantial cell-to-cell variability in the rate and extent of triacylglycerol accumulation

In microalgal cultures, most analyses of cellular processes are done in bulk, on the entire population of cells. Information gained from this is representative of the mean; however, it obscures the richness of cell-to-cell variation, which is a well-documented phenomenon. Using imaging flow cytometry, we evaluate changes in triacylglycerol (TAG) content and chlorophyll resulting from silicon or nitrogen deprivation in the diatom Cyclotella cryptica. This approach allows detailed interrogation of large numbers of individual cells and reveals cell-to-cell variation. This study demonstrates several previously undescribed phenomena related to TAG accumulation in microalgae. First, the rate of TAG accumulation varies over time, with a faster rate occurring at the latter stage of the process, resulting in hyperaccumulation in which the majority of the cell volume is comprised of lipid droplets. In C. cryptica and other diatoms this hyperaccumulation occurs strictly under autotrophic conditions. Second, there are distinct responses to silicon or nitrogen limitation, and variation within a given type of limitation treatment. Under most conditions there is a large spread in the population when measuring either chlorophyll or TAG. Heterogeneity within the total population indicates that caution should be taken in interpreting bulk measurements for a variety of variables (TAG, transcript, protein, metabolites, etc.) related to cellular responses. However, a potential means to couple subpopulation-level responses with bulk analysis approaches is described, which could take advantage of the nuances observed during the TAG accumulation process.

Heterotrophic growth and nutritional aspects of the diatom Cyclotella cryptica (Bacillariophyceae): effect of nitrogen source and concentration

To investigate the nutritional value of the diatom Cyclotella cryptica Reimann, Lewin, and Guillard as an alternative feed for the use in the aquaculture industry, the heterotrophic growth characteristics, total fatty acids, and the resultant fatty acid profile of the microalga were studied when cultivated with sodium nitrate, ammonium chloride, or urea. All three nitrogen sources supported growth under heterotrophic conditions, and their uptake affected the pH of the cultivation medium, even when buffered. The use of sodium nitrate or urea resulted in a significant increase in the pH of the cultivation media, whereas the use of ammonium chloride caused a minor decrease in the pH of the cultivation media. The maximum specific growth rate was highest when urea and ammonium chloride were supplied at a low concentration; however, the total fatty acid content was not significantly affected (P = 0.101) by the nitrogen source when supplied at 10.7 mM nitrogen. The total fatty acid content and fatty acid profile of C. cryptica was more affected by the growth phase (predominately influenced by the initial nitrogen concentration) than by the source of nitrogen.

Heterotrophic growth and nutritional aspects of the diatom Cyclotella cryptica (Bacillariophyceae): Effect of some environmental factors

To investigate the nutritional value of the diatom Cyclotella cryptica (Reimann, Lewin, and Guillard) as an alternative feed for use in the aquaculture industry, the heterotrophic growth characteristics and resultant fatty acid profile of the microalga were studied when cultivated under a variety of controlled salinity and temperature conditions. In addition, the effects of pH on the growth characteristics were also studied. The maximum specific growth rate was affected by initial pH and cultivation temperature, but not by salinity. The optimal pH and temperature ranges for growth were 7.2 to 8.1 and 22.5 to 25.0 degrees C, respectively. Lipid accumulation and the fatty acid composition were also affected by cultivation temperature and salinity. The optimal temperature range and salinity level for lipid accumulation were 18.0 to 25.0 degrees C and 11.2 psu, respectively. In all cases the fatty acid distribution was similar, with the most abundant fatty acids being palmitic acid (16:0), palmitoleic acid (16:1 n-7), stearidonic acid (18:4 n-3, SDA), eicosapentaenoic acid (20:5 n-3, EPA), and decosahexaenoic acid (22:6 n-3, DHA).

Dynamics of silica cell wall morphogenesis in the diatom Cyclotella cryptica: Substructure formation and the role of microfilaments

Diatoms are unicellular algae that make cell walls out of silica with highly ornate features on the nano- to microscale. The complexity and variety of diatom cell wall structures exceeds those possible with synthetic materials chemistry approaches. Understanding the design and assembly processes involved in diatom silicification should provide insight into patterning on the unicellular level, and information for biomimetic approaches for materials synthesis. In this report we examine the formation of distinct cell wall structures (valves and girdle bands) in the diatom Cyclotella cryptica by high resolution imaging using SEM, AFM, and fluorescence microscopy. Special attention was paid to imaging structural intermediates, which provided insight into the underlying design and assembly principles involved. Distinct stages in valve formation were identified, indicating a transition from a fractally organized structure to a dynamic pathway-dependent process. Substructures in the valves appeared to be pre-positioned prior to complete silicification, suggesting that organics responsible for these structures were pre-assembled and put in place. Microtubules and microfilamentous actin play significant roles in the positioning process, and actin is also important in the pathway-dependent expansion of the front of silicification. Our results indicate that even though all silica structures in C. cryptica are made of assemblies of nanoparticulate silica, control of meso- and microscale structure occurs on a higher order. It is apparent that diatoms integrate bottom up and top down control and synthesis mechanisms to form the diversity of structures possible.

Growth dynamics and the proximate biochemical composition and fatty acid profile of the heterotrophically grown diatom Cyclotella cryptica

To investigate the nutritional value of the diatom Cyclotella cryptica as an alternative feed for aquaculture, its heterotrophic growth characteristics were studied. First, the proximate biochemical composition and fatty acid profiles were studied under a controlled heterotrophic growth condition. The approximate total ash, carbohydrate, lipid, and protein content were 245 mg g−1 (dry weight), 360 mg g−1, 165 mg g−1 and 260 mg g−1, respectively. Polyunsaturated fatty acids accounted for 24.5, 31.3, 45.1 and 17.3% of the total lipids in the phospholipid, sterol, free fatty acid and triglyceride classes. Secondly, the effect of aeration and agitation rates on the specific growth rate of C. cryptica under heterotrophic conditions was studied. The maximum specific growth rate was not significantly affected (P > 0.05) by the rate of agitation within the range of 100 to 160 rpm, but it was significantly affected (P > 0.05) by the rate of aeration. Optimal growth occurred when the aeration rate was within the range of 0.44 to 1.07 v/v/min. Viability measurements throughout the growth period showed that the C. cryptica cells remained viable in spite of the varied cultivation conditions. Hydrodynamic forces are an important parameter within biological systems, and optimisation is crucial for the successful scale-up of microalgal cultivation systems. Whilst the investigation was preliminary in nature, the information gained in this study will be useful for the continual development of an alternative and cost-effective feed for bivalve spat rations.

Methodologies for transferring DNA into eukaryotic microalgae: a review

Agitation with glass beads, electroporation and microparticle bombardment are all used to transfer exogenous genes into unicellular eukaryotic algae (microalgae). For nuclear transformation most researchers use glass beads techniques or, to a lesser extent, electroporation, while for chloroplast transformation bombardment is often used. Glass bead agitation and electroporation require the removal of the cell wall while bombardment can be performed with intact microalgae. Chlamydomonas reinhardtii has been the microalga most commonly transformed, but success has also been reported with Volvox carteri, Chlorella spp., Dunaliella salina, Haematococcus pluvialis, Euglena gracilis, diatoms (Phaeodactylum tricornutum, Navicula saprophila, Cyclotella cryptica and Thalassiosira weissflogii), dinoflagellates (Amphidinium klebsii and Symbiodinium microadriaticum) and red algae (Porphyridium spp and Cyanidioschyzon merolae). The first C. reinhardtii transformants produced by making use of Agrobacterium tumefaciens have also been reported. A comparison of the above methods might help provide the information necessary to successfully transform further microalgal species.

Association of Fucoxanthin Chlorophyll a/c-binding Polypeptides with Photosystems and Phosphorylation in the Centric Diatom Cyclotella cryptica

Solubilization of thylakoid membranes of Cyclotella cryptica with dodecyl-beta maltoside followed by sucrose density gradient centrifugation or deriphate polyacrylamide gel electrophoresis resulted in the isolation of pigment protein complexes. These complexes were characterized by absorption and fluorescence spectroscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western immunoblotting using antisera against fucoxanthin chlorophyll a/c-binding proteins and the reaction center protein D2 of photosystem II. Sucrose density gradient centrifugation yielded four bands. Band 1 consisted of free pigments with minor amounts of fucoxanthin chlorophyll a/c-binding proteins. Bands 2, 3, and 4 represented a major fucoxanthin chlorophyll a/c-binding protein fraction, photosystem II, and photosystem I, respectively. Deriphate polyacrylamide gel electrophoresis gave rise to five bands, representing photosystem I, photosystem II, two fucoxanthin chlorophyll a/c-binding protein complexes, and a band mostly consisting of free pigments. In the Western immunoblotting experiments, the specific association of two fucoxanthin chlorophyll a/c-binding proteins, Fcp2 and Fcp4, to the photosystems could be demonstrated. In vivo experiments using antibodies against phosphothreonine residues and in vitro studies using [gamma-32P]ATP showed that fucoxanthin chlorophyll a/c binding-proteins of 22 kDa became phosphorylated.

Immuno-electron microscopic quantification of the fucoxanthin chlorophyll a/c binding polypeptides Fcp2, Fcp4, and Fcp6 of Cyclotella cryptica grown under low- and high-light intensities.

The diatom Cyclotella cryptica was grown under low- and high-intensity white light of 50 and 500 micromol photons m-2 s-1, respectively. Western immunoblotting showed that the diatom adapted its light-harvesting apparatus, giving rise to different amounts of distinct fucoxanthin chlorophyll a/c binding polypeptides (Fcp). The amount of Fcp2 was approximately two-fold higher under low-light than under high-light conditions, whereas the amount of Fcp6 increased four- to five-fold under high-light conditions. For Fcp4, no significant differences were detected in response to either light regime. Cells of Cyclotella grown under high- and low-light intensity were subjected to immunoelectron microscopy. Quantification of the gold label, expressed as gold particles per microm2, confirmed the results obtained by Western immunoblotting. Exposure to low light resulted in the detection of approximately six times more Fcp2-bound gold particles per microm2 in thylakoid membranes, whereas in cells grown under high light the number of Fcp6-bound gold particles increased ten-fold. For Fcp4, similar amounts of gold particles per microm2 were counted under the two light regimes. These immunocytochemical results confirmed molecular data derived from phylogenetic analyses of the sequences of genes encoding fucoxanthin chlorophyll a/c binding polypeptides (fcp genes) and from measurements of steady-state fcp mRNA concentrations. The results show that Fcp2 and Fcp6 accumulate under low- and high-light intensity, respectively, whereas Fcp4 seems to be constitutively synthesized.

The Rate-equation for Biogenic Silica Dissolution in Seawater – New Hypotheses

This paper investigates the kinetics of biogenic silica dissolution in seawater, through batch dissolution, where the reaction is observed as the increase in dissolved silicic acid concentration with time. It utilises new data from dissolution of the marine diatom Cyclotella cryptica, and the freshwater diatom C. meneghiniana, as well as literature results. The sum of exponentials form: \(C_{\rm t}=A_{\infty}(1-e^{-k_at})+B_{\infty}(1-e^{-k_bt})\), is hypothesised as the most general rate equation, with the single exponential form occurring in a minority of cases. The consistency of this behaviour with a near-exponential decay of surface area with time, an appropriate mathematical integration, and surface heterogeneity, is discussed. (Serious errors in some existing integrations are identified.) The rate of dissolution at constant surface area is shown to decrease non-linearly as the ambient concentration of silicic acid increases. A fractional order with respect to silicic acid in the back reaction, close to 0.5, leads to a mechanism in which an intermediate is formed from the surface and an, as yet, unidentified molecule, probably water. Good preliminary fits are found between the model and literature results found using entirely different methods. A parallel treatment of hydrogen ion dependency is suggested. The likely distortion of full reaction curves from exponential behaviour imposed by the back reaction, is considered in detail.

Localisation of fucoxanthin chlorophyll a/c-binding polypeptides of the centric diatom Cyclotella cryptica by immuno-electron microscopy

Antisera were raised against the C termini of three fucoxanthin chlorophyll a/c-binding polypeptides, Fcp2, Fcp4, and Fcp6, of the centric diatom Cyclotella cryptica. Immunogold electron microscopy of ultrathin-sectioned cells indicated that Fcp2 and Fcp4 are present in almost the same amounts, whereas approximately 8- to 10-fold less gold label was registered for Fcp6. Immunogold electron microscopy of freeze-fracture replicas of thylakoid membranes showed that the C termini of at least Fcp2 and Fcp4 were located in the thylakoid lumen, thus demonstrating a 3-dimensional structure similar to that already described for the chlorophyll a/b-binding light-harvesting polypeptides of higher plants.

In vitro, batch-dissolution of biogenic silica in seawater – the application of recent modelling to real data

Both for straight-forward oceanographic needs as well as application to climate-change amelioration, there is a need for quantitative description of the spatial and temporal variability in the recycled flux of silicic acid in the ocean water column in terms of governing processes. As part of that, the need for a fuller chemical kinetics treatment of biogenic silica dissolution, involving laboratory rate measurements, rate equations and mechanisms, is stressed. Advantage is taken of recent modelling of batch reactor dissolutions, which relies upon simple and sum of exponentials models. This is applied to archived data, and to some new data for Cyclotella cryptica, to stiffen an earlier four-point classification of dissolution behaviour which relies upon plots of ln{( C ∞ − C t )/ C ∞} versus time. The models are also used to demonstrate the potential for serious errors in the earlier rate constants derived from the batch approach. By extending recent modelling of non-linearity in the back reaction of the O’Connor and Greenberg equation, it is shown how simple exponential behaviour during dissolution might still be approximated, even though a net reaction is involved. The earlier approach is further extended to introduce non-linearity in the forward term of the O’Connor and Greenberg equation, caused by adsorption of silicic acid on the silica surface. The study of marine biogenic silica dissolution is reviewed in the wider context of general mineral dissolution, with an explanation of the relative merits of Transition State Theory used generally in mineral dissolution, and the more pragmatic approach to kinetics used here. The relative strength of chemo-stat (flow-through) and batch reactors is discussed.

Iodate reduction by Isochrysis galbana is relatively insensitive to de-activation of nitrate reductase activity—are phytoplankton really responsible for iodate reduction in seawater?

The reduction of iodate to iodide in laboratory cultures of the marine microalga Isochrysis galbana was found to be essentially independent on the presence or absence of nitrate reductase activity. This places serious doubt on the hypothesis that iodate is inadvertently reduced by NR, when it is mistaken for nitrate. NR activity was de-activated by growing the algae with tungsten instead of molybdenum, and supplying ammonium-N. An alternative mechanism is proposed linking iodate to photo-inhibition in phytoplankton. When observed, iodide production was found to be close to linear with time. A preliminary fit of the Michaelis–Menten model to the graph of iodide production versus iodate substrate concentration is promising, yielding V max and K m (with 95% confidence limits) of 62 (±21) nM day −1 and 87 (±80) μM, respectively. Iodide production was only observed when the iodate concentration was higher than that found naturally. This was also found to be the case with cultures of Skelotenema costatum. Further, no significant iodide production was observed from 0.5 μM iodate in cultures of Cyclotella cryptica and Synechococcus sp. At the higher concentrations of iodate and after rationalisation to the prevailing concentrations of chlorophyll a and iodate, iodide production was typically 0.003 nM-I − μg chlorophyll a −1 day −1 μM −1-IO 3 −. At most, these dense cultures were only able to reduce 0.3% of the prevailing iodate, and it is argued that this is too low to explain the 0.2 μM reduction commonly experienced across the temperate continental shelf or within the photic zone of the tropical water column.