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 440-540nm. 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. © 2014 Elsevier Ltd

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 μM) for 3 d before harvesting. Methylation activity was determined in leaf and root extracts using an in vitro assay based on S-[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 μM 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

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 Mn2+-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 O2 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 Mn2+-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, Mn2+-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 Mn2+ that is produced during oxygenation. We propose that the Mn2+ becomes excited by a one-electron exchange mechanism of oxygenation that is not available to Mg2+-activated rubisco

The characteristics of rhizosphere microbes associated with plants in arsenic-contaminated soils from cattle dip sites

Soil microorganisms and plants were studied in samples of arsenic-contaminated soil from two cattle dip sites. The aim was to delineate the parameters that will determine the feasibility of future remediation by growing arsenic-accumulating plants, including the identity and characteristics of some rhizosphere soil microbes. The soil samples contained high total, but low soluble arsenic concentrations which, together with other properties, resembled the previously reported characteristics of dip-site soils from this region of rural Australia. A glasshouse trial demonstrated that dip-site rhizosphere microbes promoted arsenic accumulation by the grass Agrostis tenuis on contaminated dip-site soil without inhibition of growth. The arsenic content of the shoots was increased by 45%. We studied the colonization of roots of dip-site plants by mycorrhizal fungi and tentatively identified six genera of other fungi present in the soil samples. Two plant species growing at the sites, Kikuyu grass (the most abundant plant) and Rainbow fern, exhibited mixed infections of their roots by endomycorrhizal fungi (tentatively identified as Acaulospora and Gigaspora) and by soil-born pathogens. Five rhizosphere bacteria were identified to genus level and we determined the effect of arsenic on their growth. The two most prevalent strains differed greatly in their growth sensitivity to arsenate; Arthrobacter sp. being the most sensitive while Ochrobactrum sp. exhibited exceptional resistance to arsenate. Of the other, less prevalent strains, two were Bacillus spp. and the last, Serratia sp., was the most resistant to arsenite. These findings show the importance of understanding plant-soil microbe interactions for developing future strategies aimed at a phytoremediation-based approach to removing arsenic from soil at dip sites. © 2007 Elsevier B.V. All rights reserved