Photosynthetic acclimation of Nannochloropsis oculata investigated by multi-wavelength chlorophyll fluorescence analysis.

Multi-wavelength chlorophyll fluorescence analysis was utilised to examine the photosynthetic efficiency of the biofuel-producing alga Nannochloropsis oculata, grown under two light regimes; low (LL) and high (HL) irradiance levels. Wavelength dependency was evident in the functional absorption cross-section of Photosystem II (σII(λ)), absolute electron transfer rates (ETR(II)), and non-photochemical quenching (NPQ) of chlorophyll fluorescence in both HL and LL cells. While σII(λ) was not significantly different between the two growth conditions, HL cells upregulated ETR(II) 1.6-1.8-fold compared to LL cells, most significantly in the wavelength range of 440-540 nm. This indicates preferential utilisation of blue-green light, a highly relevant spectral region for visible light in algal pond conditions. Under these conditions, the HL cells accumulated saturated fatty acids, whereas polyunsaturated fatty acids were more abundant in LL cells. This knowledge is of importance for the use of N. oculata for fatty acid production in the biofuel industry.

The effect of Diel temperature and light cycles on the growth of nannochloropsis oculata in a photobioreactor matrix.

A matrix of photobioreactors integrated with metabolic sensors was used to examine the combined impact of light and temperature variations on the growth and physiology of the biofuel candidate microalgal species Nannochloropsis oculata. The experiments were performed with algal cultures maintained at a constant 20 °C versus a 15 °C to 25 °C diel temperature cycle, where light intensity also followed a diel cycle with a maximum irradiance of 1920 µmol photons m(-2) s(-1). No differences in algal growth (Chlorophyll a) were found between the two environmental regimes; however, the metabolic processes responded differently throughout the day to the change in environmental conditions. The variable temperature treatment resulted in greater damage to photosystem II due to the combined effect of strong light and high temperature. Cellular functions responded differently to conditions before midday as opposed to the afternoon, leading to strong hysteresis in dissolved oxygen concentration, quantum yield of photosystem II and net photosynthesis. Overnight metabolism performed differently, probably as a result of the temperature impact on respiration. Our photobioreactor matrix has produced novel insights into the physiological response of Nannochloropsis oculata to simulated environmental conditions. This information can be used to predict the effectiveness of deploying Nannochloropsis oculata in similar field conditions for commercial biofuel production.

The determination of activity of the enzyme Rubisco in cell extracts of the dinoflagellate alga Symbiodinium sp. by manganese chemiluminescence and its response to short-term thermal stress of the alga.

The dinoflagellate alga Symbiodinium sp., living in symbiosis with corals, clams and other invertebrates, is a primary producer in coral reefs and other marine ecosystems. The function of the carbon-fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in dinoflagellates is difficult to study because its activity is rapidly lost after extraction from the cell. We report procedures for the extraction of Rubisco from Symbiodinium cells and for stable storage. We describe a continuous assay for Rubisco activity in these crude cell extracts using the Mn(2+) chemiluminescence of Rubisco oxygenase. Chemiluminescence time courses exhibited initial transients resembling bacterial Form II Rubisco, followed by several minutes of linearly decreasing activity. The initial activity was determined from extrapolation of this linear section of the time course. The activity of fast-frozen cell extracts was stable at -80 degrees C and, after thawing and storage on ice, remained stable for up to 1 h before declining non-linearly. Crude cell extracts bound [(14)C] 2-carboxy-D-arabitinol 1,5-bisphosphate to a high molecular mass fraction separable by gel filtration chromatography. After pre-treatment of Symbiodinium cell cultures in darkness at temperatures above 30 degrees C, the extracted Rubisco activities decreased, with almost complete loss of activity above 36 degrees C. The implications for the sensitivity to elevated temperature of Symbiodinium photosynthesis are assessed.

Complete spectra of the far-red chemiluminescence of the oxygenase reaction of Mn2+-activated ribulose-bisphosphate carboxylase/oxygenase establish excited Mn2+ as the source.

Chemiluminescence emitted by Mn(2+)-activated ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) while catalyzing oxygenation was analyzed to clarify the source of the emission. Using dual detectors capturing radiation over a wide range of visible and infrared wavelengths, we tested for radiation from singlet O(2) decay and found it to be essentially absent (less than 0.1% of the total luminescence intensity). Spectra were determined between 647 and 885 nm with a very sensitive, charge-coupled detector-based spectrograph to detect differences in the emission spectra between rubiscos from bacterial and higher plant sources. All Mn(2+)-activated rubiscos emitted a broad, smooth spectrum of chemiluminescence, unchanging as the reaction progressed. The spectra from higher plant rubiscos (spinach and both the wild type and an L335V mutant from tobacco), all exhibited maxima at about 800 nm. However, Mn(2+)-activated rubisco from the bacterium, Rhodospirillum rubrum, emitted at shorter wavelengths (760 nm peak), demonstrating host ligand-field influences arising from aminoacyl residue differences and/or conformational changes caused by the absence of small subunits. The findings provide strong evidence that the chemiluminescence arises from an excited state of the active-site Mn(2+) that is produced during oxygenation. We propose that the Mn(2+) becomes excited by a one-electron exchange mechanism of oxygenation that is not available to Mg(2+)-activated rubisco.

Methylation of arsenic in vitro by cell extracts from bentgrass (Agrostis tenuis): effect of acute exposure of plants to arsenate.

Compared with microorganisms and mammalian tissues, information is scant on the enzymes responsible for arsenic metabolism in plants. This study investigated the arsenic methylation activities extractable from leaves and roots of Agrostis tenuis Sibth. plants grown in complete nutrient media and exposed to arsenate (135-538 M) for 3 d before harvesting. Methylation activity was determined in leaf and root extracts using an in vitro assay based onS-[3H-methyl]adenosyl-L-methionine (3H-SAM) with either arsenite or arsenate as substrate. Arsenite methylation activity was low in leaf extracts from plants not exposed to arsenate, but was greatly enhanced after acute exposure, with the induced methylation activity greatest in extracts from plants exposed to 269 M arsenate. Monomethylarsonate (MMA) was the predominant early product, but over longer assay times dimethylarsinate (DMA) accumulated at the rate of 660 amol mg protein-1 min-1 to levels exceeding MMA. With arsenate as substrate, methylation activity was much lower than with arsenite, implying that arsenite is the preferred substrate for methylation. Root extract assays exhibited no DMA, however small amounts of MMA were formed with arsenite as substrate. In contrast to leaves, the methylation activity did not increase in root extracts from plants exposed to arsenate. These findings suggest that arsenate in the plant growth medium was taken up by the roots and converted to arsenite before methylation proceeded in the leaves, accompanied by induction of arsenic methyltransferase activities.