International audienceThe study investigated seasonal changes in the photo-regulatory mechanisms of microphytobenthosfound in intertidal mudflats (Aiguillon Bay, the Atlantic, France) and non-tidal sandy coastal shallows(Puck Bay, the Baltic, Poland) based on photosynthetic pigment characteristics and the estimates ofphotosynthetic parameters obtained through oxygen evolution measurements. The intertidal communitiesconsisted of motile diatom species typical of epipelon. The non-tidal microphytobenthos wascomposed of epipsammic species mostly belonging to four taxonomic groups chiefly contributing to theassemblage biomass, namely cyanobacteria, euglenophytes, green algae and diatoms (comprising mainlysmall-sized species). The epipelon was low light acclimated as shown by the lower values of photoprotective/photosynthetic (PPC/PSC) carotenoids and diatoxanthin/diadinoxanthin (Dt/Dd) ratios. Incontrast, the epipsammon exhibited features of high light acclimation (high PPC/PSC and Dt/Dd ratios). Inboth microphytobenthos types, the photosynthetic capacity (Pm) showed the same seasonal variationpattern and there were no statistically significant differences between the investigated sites in correspondingseasons (P > 0.05). In both assemblage types, the photosynthetic efficiency at limiting irradiance(a) decreased over time. The epipelon had higher a compared to the epipsammon. Seasonal changesof the photoacclimation index (Ek) estimated for the epipelic communities reflected variations observedin Pm, whereas in the epipsammon an increasing trend in Ek values was observed. Ek was always higherfor the epipsammon when comparing analogous seasons, which further corroborated low and high lightacclimation in the epipelic and epipsammic communities, respectively. The presence of the photoinhibitionparameter (b) in the epipelon and the lack of it in the epipsammon suggested that the latterwas resistant to high irradiance and the physiological mechanisms were sufficient to protect it fromphotoinhibition. In the epipelon, a downturn in photosynthetic rates showed that it was susceptible tohigh light intensities, suggesting that physiological photoprotective mechanisms must be supported bybehavioural photoacclimation in order to avoid damaging light influence
The impact of environmental factors and mode of action of allelopathic compounds produced by picocyanobacteria on microalgae remains unknown. Therefore, it is essential to characterize allelopathic interactions under different experimental conditions. In this study, the influence of allelopathic activity on the diatom Navicula perminuta was investigated by the addition of cell-free filtrate of picocyanobacterium Synechococcus sp. cultures grown under different light, temperature, and salinity conditions. Temperature affected the most the donor cyanobacterium by increasing its allelopathic activity. The highest decline in growth, fluorescence parameter F v /F m and maximal photosynthesis P m were observed after the addition of cell-free filtrate obtained from Synechococcus sp. grown at 190 mmol photons Á m À2 Á s À1 , 25°C and 8 PSU. These studies indicate for the first time that the picocyanobacterium Synechococcus sp. affect coexisting diatom N. perminuta negatively. It was examined that the Synechococcus sp. reveals allelopathic activity on the photosynthesis and chlorophyll fluorescence, which results in the inhibition of growth of analyzed microalgae. These findings indicate that the production of allelopathic substances by picocyanobacterium is regulated by the availability of environmental factors such as light, temperature, and salinity. Moreover, these results demonstrated that variation in light intensity, water temperature, and salinity should be considered when estimating the potential effects of cyanobacterial allelopathy in aquatic environments.
Eighteen strains of diatoms isolated from different phytoplankton and microphytobenthic assemblages in the Gulf of Gdańsk (Baltic Sea, Poland) and maintained at the Culture Collection of Baltic Algae (CCBA) were investigated for their species identifications. The latter were tested by phylogenetic analyses of nearly full 18S rDNA sequences as well as through sequence comparisons of 5.8S+ITS2 rDNA fragments for use as DNA barcode. The planktonic species were readily identified as a member of the Cyclotella meneghiniana species complex and Skeletonema marinoi, respectively, by both 18S phylogenies and DNA barcoding. In contrast, for the pennate diatom strains the suitability of DNA sequence comparisons for species identification DOI 10.2478/v10009-010-0031-7Original research paper appeared to be still rather limited. Only one strain could be identified to the species level (Navicula gregaria) using DNA barcodes, and no closest relative barcode sequences were available for the other strains. The 18S rDNA phylogenies supported species identification for only one strain (Bacillaria cf. paxillifera). In all other pennate strains identification was only possible to the genus level. Because for most species identified by morphology no closest neighbouring 18S rDNA sequences were available, the CCBA strains may serve as references to represent certain species which have been well characterized by morphology, 18S rDNA sequence analyses and DNA barcodes. Relatively high 18S rDNA differences between pairs of strains of Fistulifera saprophila and Navicula perminuta indicated that each species may represent a complex of several cryptic species. In contrast, the five isolates of Nitzschia microcephala had identical sequences as well as the two isolates of Nitzschia cf. fonticola. With the addition of new species to the 18S rDNA phylogeny of pennate raphid diatoms, the monophyly of the genera Fistulifera, Haslea and Navicula s.str. were confirmed, while Nitzschia appeared paraphyletic. The traditional family Naviculaceae was paraphyletic, in agreement with previous phylogenetic studies.
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