The demonstrated modified spectrophotometric method makes use of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and its specific absorbance properties. The absorbance decreases when the radical is reduced by antioxidants. In contrast to other investigations, the absorbance was measured at a wavelength of 550 nm. This wavelength enabled the measurements of the stable free DPPH radical without interference from microalgal pigments. This approach was applied to methanolic microalgae extracts for two different DPPH concentrations. The changes in absorbance measured vs. the concentration of the methanolic extract resulted in curves with a linear decrease ending in a saturation region. Linear regression analysis of the linear part of DPPH reduction versus extract concentration enabled the determination of the microalgae's methanolic extracts antioxidative potentials which was independent to the employed DPPH concentrations. The resulting slopes showed significant differences (6 - 34 μmol DPPH g−1 extract concentration) between the single different species of microalgae (Anabaena sp., Isochrysis galbana, Phaeodactylum tricornutum, Porphyridium purpureum, Synechocystis sp. PCC6803) in their ability to reduce the DPPH radical. The independency of the signal on the DPPH concentration is a valuable advantage over the determination of the EC50 value.

The influence of different nitrate concentrations in combination with three cultivation temperatures on the total fatty acids (TFA) and eicosapentaenoic acid (EPA) content of Nannochloropsis salina was investigated. This was done by virtue of turbidostatic controlled cultures. This control mode enables the cultivation of microalgae under defined conditions and, therefore, the influence of single parameters on the fatty acid synthesis of Nannochloropsis salina can be investigated. Generally, growth rates decreased under low nitrate concentrations. This effect was reinforced when cells were exposed to lower temperatures (from 26 °C down to 17 °C). Considering the cellular TFA concentration, nitrate provoked an increase of TFA under nitrate limitation up to 70% of the biological dry mass (BDM). In contrast to this finding, the EPA content decreased under low nitrate concentrations. Nevertheless, both TFA and EPA contents increased under a low culture temperature (17 °C) compared to moderate temperatures of 21 °C and 26 °C. In terms of biotechnological production, the growth rate has to be taken into account. Therefore, for both TFA and EPA production, a temperature of 17 °C and a nitrate concentration of 1800 μmol L−1 afforded the highest productivities. Temperatures of 21 °C and 26 °C in combination with 1800 μmol L−1 nitrate showed slightly lower TFA and EPA productivities.

Turbidostat cultures of Dunaliella salina (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) were grown at fluctuating concentrations of nitrate, phosphate and silicate. In-line measurements of PAM fluorescence were used to monitor the effects of fluctuating nutrient supply on the photochemical efficiency of photosystem II reaction centres of light-adapted cells (∆F\F h m ). Besides the maximal photochemical efficiency of photosystem II reaction centres of dark-adapted cells (F v \F m ), chlorophyll a, particulate organic carbon, nitrogen and phosphorus, and the cell number were measured frequently during the experiments. Following nutrient-replete growth, the cells were supplied with medium from which either nitrate, phosphate or silicate was omitted. When significant effects of nutrient starvation were indicated by the fluorescence parameters, a pulse of the deficient nutrient was added to the cultures. Our experimental set-up for in-line fluorescence measurements provided sensitive and reproducible detection of the various fluorescence signals, revealing strong influences of nutrient supply on the photochemical efficiency of photosystem II. In general the fluorescence values changed substantially within 1-30 min after re-addition of the deficient nutrient. Addition of phosphate and silicate induced an immediate characteristic decrease in fluorescence, whereas nitrate addition was characterized by a strong, delayed increase in fluorescence. Complete recovery to pre-starvation fluorescence values took about 48 h in all experiments. The physiological background of nutrient uptake is used to explain the observed tight couplings between fluorescence responses and nutrient re-addition. Our study clearly demonstrates that in-line fluorescence measurements provide a new tool for the investigation of phytoplankton reactions to fluctuating nutrients and offer the possibility to detect nutrient starvation in the field.

BackgroundSurface waters of aquatic environments have been shown to both evolve and consume hydrogen and the ocean is estimated to be the principal natural source. In some marine habitats, H2 evolution and uptake are clearly due to biological activity, while contributions of abiotic sources must be considered in others. Until now the only known biological process involved in H2 metabolism in marine environments is nitrogen fixation.Principal FindingsWe analyzed marine and freshwater environments for the presence and distribution of genes of all known hydrogenases, the enzymes involved in biological hydrogen turnover. The total genomes and the available marine metagenome datasets were searched for hydrogenase sequences. Furthermore, we isolated DNA from samples from the North Atlantic, Mediterranean Sea, North Sea, Baltic Sea, and two fresh water lakes and amplified and sequenced part of the gene encoding the bidirectional NAD(P)-linked hydrogenase. In 21% of all marine heterotrophic bacterial genomes from surface waters, one or several hydrogenase genes were found, with the membrane-bound H2 uptake hydrogenase being the most widespread. A clear bias of hydrogenases to environments with terrestrial influence was found. This is exemplified by the cyanobacterial bidirectional NAD(P)-linked hydrogenase that was found in freshwater and coastal areas but not in the open ocean.SignificanceThis study shows that hydrogenases are surprisingly abundant in marine environments. Due to its ecological distribution the primary function of the bidirectional NAD(P)-linked hydrogenase seems to be fermentative hydrogen evolution. Moreover, our data suggests that marine surface waters could be an interesting source of oxygen-resistant uptake hydrogenases. The respective genes occur in coastal as well as open ocean habitats and we presume that they are used as additional energy scavenging devices in otherwise nutrient limited environments. The membrane-bound H2-evolving hydrogenases might be useful as marker for bacteria living inside of marine snow particles.

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The Earth's atmosphere and the Earth's magnetic field protects local life by shielding us

Summary. Chlorophyll fluorescence, plasmalemma potential and resistance were measured simultaneously and subjected to a kinetic analysis. It was found that the light-induced changes of all three signals have two time constants in common. The faster one (r4 = ca. 20 sec) was assigned to the action of light-induced proton uptake across the thylakoid membrane on the plasmalemma H + pump. The slower one (rs~, = 40 sec) is related to the light action of an unknown photosynthetic process on the potassium channel. The action on the K + channel was revealed from the reversal potential of the related effect on membrane potential. The comparison of the data with findings of other authors led to the hypothesis that the unknown photosynthetic mechanism is the depletion of NADP +, which stimulates the uptake of Ca > from the cytosol, which is required for the NAD-kinase. The resulting change in cytosolic Ca 2+ modulates the number of open K + channels.

A bioreactor system was developed for the cultivation of the microalgae Synechocystis sp. PCC6803 under controlled physiological conditions. The determination of the actual physiological state of the microalgae was provided by inline recording of chlorophyll fluorescence parameters. A feed-back loop was employed to keep the microalgae in a defined physiological state. For the construction of this feed-back loop, the temporal behaviour of the system was investigated using changes in light conditions (as caused by modulated UVB radiation) as input signal and chlorophyll fluorescence as output signal. The reproducibility of the responses was high. Kinetic analysis based on curve fitting revealed two time constants in the UVB-induced responses. The knowledge of these time constants was utilised for the development of an efficient feed-back loop which allows the cultivation of the microalgae in a defined physiological state. This new process strategy (called physiostat) was successfully tested. The performance in a culture of growing microalgae is shown.