Benthic Diatom Blooms of Blue Haslea spp. in the Mediterranean Sea

Blue Haslea species are marine benthic pennate diatoms able to synthesize a blue-green water-soluble pigment, like marennine produced by H. ostrearia Simonsen. New species of Haslea synthetizing blue pigments were recently described (H. karadagensis, H. nusantara, H. provincialis and H. silbo). Their marennine-like pigments have allelopathic, antioxidative, antiviral and antibacterial properties, which have been demonstrated in laboratory conditions. Marennine is also responsible for the greening of oysters, for example, in the Marennes Oléron area (France), a phenomenon that has economical and patrimonial values. While blue Haslea spp. blooms have been episodically observed in natural environments (e.g., France, Croatia, USA), their dynamics have only been investigated in oyster ponds. This work is the first description of blue Haslea spp. benthic blooms that develop in open environments on the periphyton, covering turf and some macroalgae-like Padina. Different sites were monitored in the Mediterranean Sea (Corsica, France and Croatia) and two different blue Haslea species involved in these blooms were identified: H. ostrearia and H. provincialis. A non-blue Haslea species was also occasionally encountered. The benthic blooms of blue Haslea followed the phytoplankton spring bloom and occurred in shallow calm waters, possibly indicating a prominent role of light to initiate the blooms. In the absence of very strong winds and water currents that can possibly disaggregate the blue biofilm, the end of blooms coincided with the warming of the upper water masses, which might be profitable for other microorganisms and ultimately lead to a shift in the biofilm community.

Complete mitochondrial genome and draft chloroplastic genome of Haslea ostrearia (Simonsen 1974)

The first completed, circular mitochondrial genome and the first draft, linear chloroplastic genome of the blue diatom Haslea ostrearia (Simonsen 1974, Naviculaceae, Bacillariophyceae) were assembled from Illumina and PacBio sequencing. The mitochondrial genome was composed of 38,696 bases and contained 64 genes, including 31 protein-coding genes (CDS), 2 ribosomal RNA (rRNA) genes and 23 transfer RNA (tRNA) genes. For the chloroplast, the genome was composed of 130,200 bases with 169 genes (131 CDS, 6 rRNA genes, 31 tRNA genes, and 1 transfer messenger RNA gene). Phylogenetic trees, using the maximum-likehood method and partial genes currently available for Haslea ostrearia and other diatom species, suggested the proximity of all the Haslea ostrearia strains/isolates and the possibility of using these genomes as future references.

Theoretical and Experimental Studies on the Evidence of 1,3-β-Glucan in Marennine of Haslea ostrearia

Marennine, a blue pigment produced by the blue diatom Haslea ostrearia, is known to have some biological activities. This pigment is responsible for the greening of oysters on the West Coast of France. Other new species of blue diatom, H. karadagensis, H. silbo sp. inedit., H. provincialis sp. inedit, and H. nusantara, also produce marennine-like pigments with similar biological activities. Aside from being a potential source of natural blue pigments, H. ostrearia-like diatoms present a commercial potential for the aquaculture, food, cosmetics, and health industries. Unfortunately, for a hundred years, the exact molecular structure of this bioactive compound has remained a mystery. A lot of hypotheses regarding the chemical structure of marennine have been proposed. The recent discovery of this structure revealed that it is a macromolecule, mainly carbohydrates, with a complex composition. In this study, some glycoside hydrolases were used to digest marennine, and the products were further analyzed using nuclear magnetic resonance (NMR) and mass spectroscopy (MS). The reducing sugar assay showed that marennine was hydrolyzed only by endo-1,3-β-glucanase. Further insight into the structure of marennine was provided by the spectrum of 1H NMR, MS, a colorimetric assay, and a computational study, which suggest that the chemical structure of marennine contains 1,3-β-glucan.

The nanomolar affinity of C-phycocyanin from virtual screening of microalgal bioactive as potential ACE2 inhibitor for COVID-19 therapy

The global pandemic of COVID-19 caused by SARS-CoV-2 has caused more than 400 million infections with more than 5.7 million deaths worldwide, and the number of validated therapies from natural products for treating coronavirus infections needs to be increased. Therefore, the virtual screening of bioactive compounds from natural products based on computational methods could be an interesting strategy. Among many sources of bioactive natural products, compounds from marine organisms, particularly microalgae and cyanobacteria, can be potential antiviral agents. The present study investigates bioactive antiviral compounds from microalgae and cyanobacteria as a potential inhibitor of SARS-CoV-2 by targeting Angiotensin-Converting Enzyme II (ACE2) using integrated in silico and in vitro approaches. Our in silico analysis demonstrates that C-Phycocyanin (CPC) can potentially inhibit the binding of ACE2 receptor and SARS-CoV-2 with the docking score of -9.7 kcal mol^-1. This score is relatively more favorable than the native ligand on ACE2 receptor. Molecular dynamics simulation also reveals the stability interaction between both CPC and ACE2 receptor with a root mean square deviation (RMSD) value of 1.5 Å. Additionally, our in vitro analysis using the surface plasmon resonance (SPR) method shows that CPC has a high affinity for ACE2 with a binding affinity range from 5 to 125 µM, with K_D 3.37 nM. This study could serve as a reference to design microalgae- or cyanobacteria-based antiviral drugs for prophylaxis in SARS-CoV-2 infections.

The Polysaccharidic Nature of the Skeleton of Marennine as Determined by NMR Spectroscopy

The water-soluble blue-green pigment marennine, produced and partly excreted by the diatom Haslea ostrearia, and known for a long time for its role in the greening of oysters, was isolated from the culture medium, purified, and analyzed by Nuclear Magnetic Resonance (NMR) in order to gain insight into its chemical structure. The spectra show mainly carbohydrates of a complex composition, apparently highly branched, and with a mass in the order of 10 kDa. There are, in addition, some signals of aliphatic and, much weaker, aromatic groups that present aglycons. The latter might be responsible for the color. These carbohydrates are always associated with the blue-green color and cannot be separated from it by most treatments; they are interpreted as constituting the frame of the pigment. NMR after hydrolysis identifies the most abundant monosaccharides in marennine as galactose, xylose, mannose, rhamnose, and fucose.

What Was Old Is New Again: The Pennate Diatom Haslea ostrearia (Gaillon) Simonsen in the Multi-Omic Age

The marine pennate diatom Haslea ostrearia has long been known for its characteristic blue pigment marennine, which is responsible for the greening of invertebrate gills, a natural phenomenon of great importance for the oyster industry. For two centuries, this taxon was considered unique; however, the recent description of a new blue Haslea species revealed unsuspected biodiversity. Marennine-like pigments are natural blue dyes that display various biological activities—e.g., antibacterial, antioxidant and antiproliferative—with a great potential for applications in the food, feed, cosmetic and health industries. Regarding fundamental prospects, researchers use model organisms as standards to study cellular and physiological processes in other organisms, and there is a growing and crucial need for more, new and unconventional model organisms to better correspond to the diversity of the tree of life. The present work, thus, advocates for establishing H. ostrearia as a new model organism by presenting its pros and cons—i.e., the interesting aspects of this peculiar diatom (representative of benthic-epiphytic phytoplankton, with original behavior and chemodiversity, controlled sexual reproduction, fundamental and applied-oriented importance, reference genome, and transcriptome will soon be available); it will also present the difficulties encountered before this becomes a reality as it is for other diatom models (the genetics of the species in its infancy, the transformation feasibility to be explored, the routine methods needed to cryopreserve strains of interest).

Algae as a Source of Natural Flavors in Innovative Foods

As a result of their nutritive values, algae have been used as a food resource for centuries, and there is a growing interest to use them as enrichment ingredients in food products. However, food product acceptance by consumers is strongly linked to their organoleptic properties, especially the aroma, taste, and a combination of the two, flavor. With regard to edible algae, "fresh seashore", "seafood-like", "cucumber green", and "earthy" are descriptors commonly used to define their aromas. Several families of molecules participate in the diversity and peculiarities of algal aromas: pungent sulfur compounds and marine halogenated components but also herbaceous fatty acid derivatives and fruity-floral terpenoids. In both macroalgae (seaweeds) and microalgae, these compounds are studied from a chemistry point of view (identification and quantification) and a sensorial point of view, involving sensorial evaluation by panelists. As a whole food, a food ingredient, or a feed, algae are valued for their nutritional composition and their health benefits. However, because the acceptance of food by consumers is so strongly linked to its sensorial features, studies have been performed to explore the aromas of algae, their impact on food, their evolution through processing, and their ability to produce selected aromas using biotechnology. This review aims at highlighting algal aromas from seaweed and microalgae as well as their use, their handling, and their processing in the food industry.

Electrochromic Properties and Electrochemical Behavior of Marennine, a Bioactive Blue-Green Pigment Produced by the Marine Diatom Haslea ostrearia

Marennine has long been known as the unique peculiar pigment responsible for the natural greening of oysters. It is specifically produced by the marine diatom Haslea ostrearia and it is a natural blue molecule indeed promising for food industry because of the rarity of such non-toxic, blue-colored pigments. In the search for its still not defined molecular structure, investigation of the color changes with the redox state has been carried out combining different approaches. Reducing and oxidizing chemicals have been added to purified marennine solutions and a stable blue-green color has been confirmed for the oxidized state, while a yellow color corresponded to the reduced unstable state. Raman spectroscopy has been used to monitor changes in the Raman spectra corresponding to the different colored states, and cyclic voltammetry has allowed the detection of a redox system in which protons and electrons are exchanged. These findings show that marennine is a suitable stable blue pigment for use in food applications and help in the elucidation of the chromophore structure.

Haslea silbo, A Novel Cosmopolitan Species of Blue Diatoms

Specimens of a new species of blue diatoms from the genus Haslea Simonsen were discovered in geographically distant sampling sites, first in the Canary Archipelago, then North Carolina, Gulf of Naples, the Croatian South Adriatic Sea, and Turkish coast of the Eastern Mediterranean Sea. An exhaustive characterization of these specimens, using a combined morphological and genomic approach led to the conclusion that they belong to a single new to science cosmopolitan species, Haslea silbo sp. nov. A preliminary characterization of its blue pigment shows similarities to marennine produced by Haslea ostrearia, as evidenced by UV-visible spectrophotometry and Raman spectrometry. Life cycle stages including auxosporulation were also observed, providing data on the cardinal points of this species. For the two most geographically distant populations (North Carolina and East Mediterranean), complete mitochondrial and plastid genomes were sequenced. The mitogenomes of both strains share a rare atp6 pseudogene, but the number, nature, and positions of the group II introns inside its cox1 gene differ between the two populations. There are also two pairs of genes fused in single ORFs. The plastid genomes are characterized by large regions of recombination with plasmid DNA, which are in both cases located between the ycf35 and psbA genes, but whose content differs between the strains. The two sequenced strains hosts three plasmids coding for putative serine recombinase protein whose sequences are compared, and four out of six of these plasmids were highly conserved.

Harmful or harmless: Biological effects of marennine on marine organisms

Marennine is a water-soluble blue-green pigment produced by the marine diatom Haslea ostrearia. The diatom and its pigment are well known from oyster farming areas as the source of the greening of oyster gills, a natural process increasing their market value in Western France. Blooms of blue Haslea are also present outside oyster ponds and hence marine organisms can be exposed, periodically and locally, to significant amounts of marennine in natural environments. Due to its demonstrated antibacterial activities against marine pathogenic bacteria (e.g. Vibrio) and possible prophylactic effects toward bivalve larvae, marennine is of special interest for the aquaculture industry, especially bivalve hatcheries. The present study aimed to provide new insights into the effects of marennine on a large spectrum of marine organisms belonging to different phyla, including species of aquaculture interest and organisms frequently employed in standardised ecotoxicological assays. Different active solutions containing marennine were tested: partially purified Extracellular Marennine (EMn), and concentrated solutions of marennine present in H. ostrearia culture supernatant; the Blue Water (BW) and a new process called Concentrated Supernatant (CS). Biological effects were meanwhile demonstrated in invertebrate species for the three marennine-based solutions at the highest concentrations tested (e.g., decrease of fertilization success, delay of embryonic developmental stages or larval mortality). Exposure to low concentrations did not impact larval survival or development and even tended to enhance larval physiological state. Furthermore, no effects of marennine were observed on the fish gill cell line tested. Marennine could be viewed as a Jekyll and Hyde molecule, which possibly affects the earliest stages of development of some organisms but with no direct impacts on adults. Our results emphasize the need to determine dosages that optimize beneficial effects and critical concentrations not to be exceeded before considering the use of marennine in bivalve or fish hatcheries.

Complete mitogenome of Cerithidea obtusa, the red chut-chut snail from the Cần Giờ Mangrove in Vietnam

We sequenced the complete mitogenome of the red chut-chut snail Cerithidea obtusa, from the Cần Giờ mangrove in Vietnam. The mitogenome is 15,708 bp long. It is colinear with the mitogenomes of other members of the superfamily Cerithioidea, and the maximum-likelihood phylogeny obtained with the cox1, cox2 and cox3 genes of several Caenogastropoda associated all Cerithioidea together inside a strongly supported clade.

Mitogenome sequence of a Black Sea isolate of the kinetoplastid Bodo saltans

We obtained the mitogenome sequence of a Black Sea isolate of the kinetoplastid Bodo saltans. This sequence consists of two contigs totaling 24,925 bp and encodes ten protein-coding genes, one conserved ORF and one rRNA gene. Alignment of the Black Sea mitogenome with the limited sequence data currently available in public databases for another strain of B. saltans revealed significant genetic divergence between the two isolates. Maximum likelihood phylogenetic inference clearly resolved the Bodonidae from the Trypanosomatidae.

The complete mitochondrial DNA of the tropical oyster Crassostrea belcheri from the Cần Giò' mangrove in Vietnam

The complete mitochondrial genome of the oyster Crassostrea belcheri from the Cần Giò' mangrove in Vietnam has been sequenced. It consists of a circular DNA molecule of 21020 base pairs (bp), coding for 12 proteins, 20 transfer RNAs, and two ribosomal RNAs. Like the mitogenomes of Crassostrea iredalei and Crassostrea sp. DB1, it contains a non-coding region and two ORFs. The C. belcheri mitogenome provides information that could improve the molecular phylogeny of Asian oysters and be useful to the development of oyster aquaculture in South East Asia.

Antimicrobial Compounds from Eukaryotic Microalgae against Human Pathogens and Diseases in Aquaculture

The search for novel compounds of marine origin has increased in the last decades for their application in various areas such as pharmaceutical, human or animal nutrition, cosmetics or bioenergy. In this context of blue technology development, microalgae are of particular interest due to their immense biodiversity and their relatively simple growth needs. In this review, we discuss about the promising use of microalgae and microalgal compounds as sources of natural antibiotics against human pathogens but also about their potential to limit microbial infections in aquaculture. An alternative to conventional antibiotics is needed as the microbial resistance to these drugs is increasing in humans and animals. Furthermore, using natural antibiotics for livestock could meet the consumer demand to avoid chemicals in food, would support a sustainable aquaculture and present the advantage of being environmentally friendly. Using natural and renewable microalgal compounds is still in its early days, but considering the important research development and rapid improvement in culture, extraction and purification processes, the valorization of microalgae will surely extend in the future.

Bacterial adhesion and growth reduction by novel rubber-derived oligomers

In the medical field, attached bacteria can cause infections associated with catheters, incisions, burns, and medical implants especially in immunocompromised patients. The problem is exacerbated by the fact that attached bacteria are ∼1000 times more resistant to antibiotics than planktonic cells. The rapid spread of antibiotic resistance in these and other organisms has led to a significant need to find new methods for preventing bacterial attachment. The goal of this research was to evaluate the effectiveness of novel polymer coatings to prevent the attachment of three medically relevant bacteria. Tests were conducted with Pseudomonas aeruginosa, Staphylococcus epidermidis, and Staphylococcus aureus for oligomers derived from modifications of natural rubber (cis 1,4-polyisoprene). The different oligomers were: PP04, with no quaternary ammonium (QA); MV067, one QA; PP06, three QA groups. In almost all experiments, cell attachment was inhibited to various extents as long as the oligomers were used. PP06 was the most effective as it decreased the planktonic cell numbers by at least 50% for all bacteria. Differences between species sensitivity were also observed. P. aeruginosa was the most resistant bacteria tested, S. aureus, the most sensitive. Further experiments are required to understand the full extent and mode of the antimicrobial properties of these surfaces.

Antifouling action of polyisoprene-based coatings by inhibition of photosynthesis in microalgae

Previous studies have demonstrated that ionic and non-ionic natural rubber-based coatings inhibit adhesion and growth of marine bacteria, fungi, microalgae, and spores of macroalgae. Nevertheless, the mechanism of action of these coatings on the different micro-organisms is not known. In the current study, antifouling activity of a series of these rubber-based coatings (one ionic and two non-ionic) was studied with respect to impacts on marine microalgal photosynthesis using pulse-amplitude-modulation (PAM) fluorescence. When grown in contact with the three different coatings, an inhibition of photosynthetic rate (relative electron transport rate, rETR) was observed in all of the four species of pennate diatoms involved in microfouling, Cocconeis scutellum, Amphora coffeaeformis, Cylindrotheca closterium, and Navicula jeffreyi. The percentage of inhibition ranged from 44% to 100% of the controls, depending on the species and the coating. The ionic coating was the most efficient antifouling (AF) treatment, and C. scutellum and A. coffeaeformis are the most sensitive and tolerant diatoms tested, respectively. Photosynthetic inhibition was reversible, as almost complete recovery of rETR was observed 48 h post exposure, after detachment of cells from the coatings. Thus, the antifouling activity seemed mostly due to an effect of contact with materials. It is hypothesized that photosynthetic activity was suppressed by coatings due to interference in calcium availability to the microalgal cells; Ca(2+) has been shown to be an essential micro/macro nutrient for photosynthesis, as well as being involved in cell adhesion and motility in pennate diatoms.

Inheritance of mitochondrial DNA in the Pennate diatom Haslea ostrearia (Naviculaceae) during auxosporulation suggests a uniparental transmission

We present the first study examining mtDNA transmission in diatoms, using sexual progeny of the pennate species Haslea ostrearia (Naviculaceae). A fragment of the cytochrome oxidase subunit I gene (cox1) with 7 nucleic substitutions between parental clones was used as a parental tracer in 16 F1 clones obtained from two pairs of mating crosses. Each cross involved a parental clone isolated from France (Bay of Bourgneuf) and Sweden (Kattegat Bay). We determined that all progeny possessed only one cox1 parental haplotype. These results suggest that the mitochondrial DNA transmission in H. ostrearia is uniparental. Implications and new topics of investigation are discussed.

Comparative efficiency of macroalgal extracts and booster biocides as antifouling agents to control growth of three diatom species

The application of 'booster biocides' Diuron, Tolylfluanid and Copper thiocyanate inbantifouling paints, used to prevent development of biofouling, needs to be monitored before assessing their impacts on the environment. An alternative approach aims to propose eco-friendly and effective antifoulants isolated from marine organisms such as seaweeds. In this study, the effects of 'booster biocides' and the ethanol and dichloromethane extracts from a brown (Sargassum muticum) and a red alga (Ceramium botryocarpum) have been compared by algal growth inhibition tests of marine diatoms. The most efficient extracts were ethanol fraction of S. muticum and C. botryocarpum extracts with growth EC(50)=4.74 and 5.3μg mL(-1) respectively, with reversible diatom growth effect. The booster biocides are more efficient EC(50)=0.52μg mL(-1), but are highly toxic. Results validate the use of macroalgal extracts as non toxic antifouling compounds, and they represent valuable environmentally friendly alternatives in comparison with currently used biocides.

Modern stromatolite phototrophic communities: a comparative study of procaryote and eucaryote phototrophs using variable chlorophyll fluorescence

Stromatolites are laminated organosedimentary structures formed by microbial communities, principally cyanobacteria although eucaryote phototrophs may also be involved in the construction of modern stromatolites. In this study, productivity and photophysiology of communities from stromatolites (laminated) and thrombolites (nonlaminated) were analysed using fluorescence imaging. Sub-samples of mats were excised at Highborne Cay, Bahamas, and cross-sectioned to simultaneously analyse surface, near-surface (1-2 mm), and deeper (2-10 mm) communities. Rapid light curve parameters and nonphotochemical downregulation showed distinct differences between phototroph communities, consistent with the reported quasi-succession of classic stromatolite mat types. Greater productivity was shown by cyanobacteria in Type 1 and Type 3 mats (first and final stage of the succession, Schizothrix gebeleinii and Solentia sp. respectively) and lower productivity within Type 2 mats (intermediate mat type). Eucaryote mat types, dominated by stalked (Striatella sp. and Licmophora sp.) and tube-dwelling (e.g. Nitzschia and Navicula spp.) diatoms, showed greater productivity than cyanobacteria communities, with the exception of Striatella (low productivity) and an unidentified coccoid cyanobacterium (high productivity). Findings indicate comparative variability between photosynthetically active procaryote and eucaryote sub-communities within stromatolites, with a pattern logically following the succession of 'classic' mat types, and lower than the productivity of eucaryote dominated 'nonclassic' mat types.

Biological activities of purified marennine, the blue pigment responsible for the greening of oysters

Marennine, the blue pigment produced by the diatom Haslea ostrearia , exists in two different forms, the intra- and extracellular forms. We investigated the antibacterial, antiviral, and antiproliferative properties of both of these forms. Both forms of marennine inhibited the development of marine bacteria, in particular the pathogenic organism Vibrio aesturianus , at concentrations as low as 1 μg/mL, but they did not display any effect on a wide range of pathogenic bacteria that are relevant for food safety. Both forms of the pigment produced by H. ostrearia also exhibited antiviral activity against the HSV1 herpes virus, with intra- and extracellular marennine having EC(50) values of 24.0 and 27.0 μg/mL, respectively. These values are 2 orders of magnitude higher than the value for the reference drug, Zovirax. Moreover, both forms of marennine were effective in slowing or inhibiting the proliferation of cancer cells. This study confirms the potential of marennine as a biologically active organic molecule, which could have a protective effect on bivalves, which filter seawater and fix the pigment on their gills. Moreover, marennine could be used in food engineering and chemistry as a natural blue pigment. However, despite that it is eaten and possibly assimilated by green oyster consumers, it also deserves in depth evaluation before being considered for use as a nutraceutical.

Antifouling activity of macroalgal extracts on Fragilaria pinnata (Bacillariophyceae): a comparison with Diuron

The tributyltin-based products and organic biocides which are incorporated into antifouling paints have had a negative impact on the marine environment, and the ban on tributyltin-based antifouling products has urged the industry to find substitutes to prevent the development of fouling on ship hulls. Natural antifouling agents could be isolated from marine resources, providing an alternative option for the industry. The effects of different marine seaweed extracts from Sargassum muticum and Ceramium botryocarpum on the growth, pigment content and photosynthetic apparatus of the marine diatom Fragilaria pinnata were compared with those of Diuron, a biocide widely used in antifouling paints. The addition of the macroalgal extracts in the culture medium resulted in an inhibition of the growth of F. pinnata, but this inhibition was lower than that obtained with Diuron. After transfer to a biocide-free medium, F. pinnata cells previously exposed to the macroalgal extracts exhibited normal growth, in contrast to Diuron-treated cells, which died, demonstrating that the effects of the natural antifouling agents were reversible. Macroalgal extracts and Diuron-induced modifications in F. pinnata cellular pigment content. Chlorophyll a, fucoxanthin, and the xanthophyll pool, diadinoxanthin and diatoxanthin, were the most affected. Changes in the structure and function of the photosynthetic apparatus were studied by microspectrofluorimetry, and provided a comprehensive evaluation of the inhibition of the diatom Photosystem II (PSII) by the biocides. This study confirms that natural extracts from the macroalgae studied have the potential to be used as a substitute to commercial biocides in antifouling paints.

Light is a key factor in triggering sexual reproduction in the pennate diatom Haslea ostrearia

Sexual reproduction is an obligatory phase in the life cycle of most diatoms, as cell size decreases with successive vegetative divisions and the maximal cell size is only restored by a specialized cell, the auxospore, which follows zygote formation as a result of sexual reproduction. While in pennate diatoms the induction of sexual reproduction depends primarily on cell-cell interactions, the importance of different external factors for the induction of sexual reproduction is less well known. Here, we investigated the effects of light on sexualization in the marine benthic pennate diatom Haslea ostrearia (Gaillon) R. Simonsen. Compatible clones were crossed and exposed to different combinations of light levels, qualities, and photoperiods. Light was found to be a key factor for sexualization, and to a certain extent, to control auxosporulation in H. ostrearia. The light conditions most favorable for sexual reproduction were low irradiances (<50 micromolphotons m(-2) s(-1)) and short photoperiods (6-10 h), conditions that prevail during winter, and to a lesser extent, the higher irradiances and longer photoperiods that correspond to the spring and fall, when blooms of this organism form in the natural environment. Auxospore formation was very rare in continuous light, and maximum in presence of red radiation, while it was never observed in darkness or in radiation other than red.

DETECTION OF DIATOM XANTHOPHYLL CYCLE USING SPECTRAL REFLECTANCE(1)

Analysis of reflectance spectra was used to monitor the conversion of diadinoxanthin (DD) into diatoxanthin (DT) in two benthic diatom species, Amphora coffeaeformis (C. Agardh) Kütz. and Cylindrotheca closterium (Ehrenb.) J. C. Lewin et Reiman, cultured at high light (HL, 400 μmol · m(-2)  · s(-1) PAR) and low light (LL, 25 μmol · m(-2)  · s(-1) PAR). Cultures were exposed to saturating light for 32 min. HL cultures of both species showed higher (DT + DD) content, whereas LL cultures exhibited higher chl a and fucoxanthin content. DD to DT conversion, measured by HPLC, occurred mainly in the first 2 min (LL) or 5 min (HL) after exposure to saturating light. Nonphotochemical quenching (NPQ), measured by PAM fluorescence, showed the same pattern as DT/(DD + DT), resulting in a linear relationship between these parameters. Addition of dithiothreitol (DTT) blocked the conversion of DD into DT and significantly reduced NPQ induction. Reflectance spectra showed no obvious change after light exposure. However, second derivative spectra (δδ) showed a shift in reflectance from 487 to 508 nm, which was not present for DTT-treated samples. Changes in δδ487 were strongly correlated with changes in DD (r = 0.76), while changes in δδ508 were strongly correlated with changes in DT (r = 0.94). The best index to estimate DD to DT conversion was δδ508 /δδ630 (r = 0.87). This index was very sensitive to minute changes that occurred immediately after exposure to light and was species insensitive. Good relationships were observed between indices for xanthophyll cycle activation (DD to DT conversion and NPQ induction) and the second derivative spectra. With further in situ validation, this index may prove to be highly useful for investigation into aquatic global photoregulation mechanisms in diatom-dominated samples.

Dynamics of short-term acclimation to UV radiation in marine diatoms

In order to investigate the dynamics of the acclimation of marine diatoms to ultraviolet radiation (UVR), Amphora coffeaeformis, Odontella aurita and Skeletonema costatum were exposed for 5 h per day to a combination of UVA and UVB (UVBR/UVAR ratio 4.5%) with a total UVR daily dose of 110 kJ m(-2), which is equivalent to that observed in the natural environment. This treatment was applied in the middle of the photoperiod and was repeated on five successive days. During the UVR treatment, chlorophyll fluorescence parameters were monitored, damage and repair constants were calculated from effective quantum yield values (phi(PSII)), and rapid light curves (electron transport rate versus irradiance curves using short light steps of different intensity) were plotted to determine the maximum relative electron transport rate (rETR(max)) and maximum light use efficiency (alpha). In all species the growth rate was lower than control from day 1-3, but increased thereafter, except for S. costatum. The cellular chlorophyll a content increased significantly with repeated daily exposure to UVR for A. coffeaeformis only. In all species, the fluorescence parameters (F(m), the maximum fluorescence level measured in the dark, phi(PSII), rETR(max) and alpha) decreased during UVR exposure, in contrast to F(0) (the minimum fluorescence level measured in the dark). The response to UVR stress was species-specific. S. costatum was very sensitive, and failed to survive for more than three days, whereas A. coffeaeformis and O. aurita were able to acclimate to UVR stress. These two species used different strategies. In A. coffeaeformis, the repair constant was lower than the damage constant, but phi(PSII) values returned to baseline values at the beginning of each experimental day, indicating that an effective active recovery process occurred after stress. In O. aurita, the repair processes took place during the stress, and could account for the UVR tolerance of this species.

Comparison between fluorimetry and oximetry techniques to measure photosynthesis in the diatom Skeletonema costatum cultivated under simulated seasonal conditions

This study reports comparison of two techniques measuring photosynthesis in the ubiquitous diatom Skeletonema costatum, i.e., the classical oximetry and the recent modulated fluorimetry. Microalgae in semi-continuous cultures were exposed to five different environmental conditions simulating a seasonal effect with co-varying temperature, photoperiod and incident light. Photosynthesis was assessed by gross rate of oxygen evolution (P(B)) and the electron transport rate (ETR) measurements. The two techniques were linearly related within seasonal treatments along the course of the P/E curves. The light saturation intensity parameters (Ek and Ek(ETR)), and the maximum electron transport rate increased significantly with the progression of the season while the maximum light utilization efficiency for ETR (alpha(ETR)) was constant. By contrast, the maximum gross oxygen photosynthetic capacity (Pmax(B)) and the maximum light utilization efficiency for P(B) (alpha(B)) increased from December to May treatment but decreased from May to July treatment. Both techniques showed clear photoacclimation in microalgae with the progression of the season, as illustrated by changes in photosynthetic parameters. The relationship between the two techniques changed when high temperature, photoperiod and incident light were combined, possibly due to an overestimation of the PAR--averaged chlorophyll-specific absorption cross-section. Despite this change, our results illustrate the strong suitability of in vivo chlorophyll fluorimetry to estimate primary production in the field.

Acclimation of Haslea ostrearia to light of different spectral qualities – confirmation of `chromatic adaptation' in diatoms

The marine diatom Haslea ostrearia was cultured under light of different qualities, white (WL), blue (BL), green (GL), yellow (YL), red (RL), and far-red (FRL) and at two irradiance levels, low and high (20 and 100 micromolphotonsm(-2)s(-1), respectively). The effects of the different light regimes were studied on growth, pigment content, and photosynthesis, estimated by the modulated fluorescence of chlorophyll, as relative electron transport rate (rETR). For all the light qualities studied, growth rates were higher at high irradiance. Compared to the corresponding WL controls, growth was higher in BL and lower in YL at low irradiance, and lower in YL and GL at high irradiance. Except for YL, almost all the pigment contents of the cells were lower at high irradiance. At low irradiance, cell pigment contents (chlorophyll a and c, fucoxanthin) and pigment ratios (in function of chlorophyll a) were lower in YL, RL, and FRL. Whatever the irradiance level, the maximum PSII quantum efficiency (F(v)/F(m) remained almost constant for WL, BL, and GL. Other fluorescence parameters (photochemical quenching, rETR(max), and alpha, the maximum light utilization coefficient) were lower in GL, YL, RL, and FRL, at low irradiance. Although not statistically significant, BL caused an increase in these fluorescence parameters. These findings are interpreted as evidence that inverse chromatic acclimation occurs in diatoms.