Light-induced turnover of chloroplast cytochrome b-559 in the presence of N-methylphenazonium methosulphate.

In the presence of 0.1-5 muM N-methylphenazonium methosulphate approx. 50-70% oxidation of cytochrome b-559 can be induced by far-red light. The oxidation is best observed with long wavelength far-red light (732 nm) of moderate intensities (approx. 10(4) ergs/cm2 per s) and is reversed by subsequent illumination with red light. Concentrations of N-methylphenazonium methosulphate above 5 muM are inhibitory probably due to cyclic electron flow. The far-red oxidation is inhibited by low concentrations of the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, while 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits red light reduction and increases the amplitude of far-red oxidation. The effect of N-methylphenazonium methosulphate is mimicked by N-methyl-phenazonium ethosulphate, but not by pyocyanine or diaminodurene. Low concentrations (2-3 muM) of N-methylphenazonium methosulphate also stimulate a 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone-inhibitable red light reduction of cytochrome f.     

Light-induced acceleration of retina development in a mouth-brooding teleost.

Rearing animals in the dark had been shown to be either without effect on the development of the retina or to result in a reduction or a delay of retinal maturation. In the present study, the influence of light on a retina which normally develops under conditions of very dim light has been investigated. When 3-day-old embryos of the mouth-brooding teleost Tilapia leucosticta are placed into a continuously lighted environment, from day 6 on, optic tract diameter and inner plexiform layer thickness increase up to day 10 or 12. In the dark-reared retina, this increase occurs only after day 10. Similarly, synaptic junctions of the inner plexiform layer appear at about day 6 in the light-reared retina and increase continuously on following days, whereas in the retina of embryos reared in darkness, they appear at day 7 and do not increase essentially before day 11. These and other data suggest that continuous light induces a precocious growth of retinal structures. The first differences between light- and dark-reared retinae appear synchronously with the beginning of receptor cell development and prior to the first synaptic junctions. A non-neurally mediated effect of light on the retinal ganglion cells is consequently assumed.     

Linkages between N turnover and plant community structure in a tundra landscape

The spatial distribution of organic soil nitrogen (N) in alpine tundra was studied along a natural environmental gradient, covering five plant communities, at the Latnjajaure Field Station, northern Swedish Lapland. The five communities (mesic meadow, meadow snowbed, dry heath, mesic heath, and heath snowbed) are the dominant types in this region and are differentiated by soil pH. Net N mineralization, net ammonification, and net nitrification were measured using 40-day laboratory incubations based on extractable NH₄⁺ and NO₃⁻. Nitrification enzyme activity (NEA), denitrification enzyme activity (DEA), amino acid concentrations, and microbial respiration were measured for soils from each plant community. The results show that net N mineralization rates were more than three times higher in the meadow ecosystems (mesic meadow 0.7 μg N g⁻¹ OM day⁻¹ and meadow snowbed 0.6 μg N g⁻¹ OM day⁻¹) than the heath ecosystems (dry heath 0.2 μg N g⁻¹ OM day⁻¹, mesic heath 0.1 μg N g⁻¹ OM day⁻¹ and heath snowbed 0.2 μg N g⁻¹ OM day⁻¹). The net N mineralization rates were negatively correlated to organic soil C/N ratio (r = −0.652, P < 0.001) and positively correlated to soil pH (r = 0.701, P < 0.001). Net nitrification, inorganic N concentrations, and NEA rates also differed between plant communities; the values for the mesic meadow were at least four times higher than the other plant communities, and the snowbeds formed an intermediate group. Moreover, the results show a different pattern of distribution for individual amino acids across the plant communities, with snowbeds tending to have the highest amino acid N concentrations. The differences between plant communities along this natural gradient also illustrate variations between the dominant mycorrhizal associations in facilitating N capture by the characteristic functional groups of plants. Responsible Editor: Bernard Nicolardot     

Biophysical aspects of resource acquisition and competition in algal mixotrophs

Mixotrophic organisms combine autotrophic and heterotrophic nutrition and are abundant in both freshwater and marine environments. Recent observations indicate that mixotrophs constitute a large fraction of the biomass, bacterivory, and primary production in oligotrophic environments. While mixotrophy allows greater flexibility in terms of resource acquisition, any advantage must be traded off against an associated increase in metabolic costs, which appear to make mixotrophs uncompetitive relative to obligate autotrophs and heterotrophs. Using an idealized model of cell physiology and community competition, we identify one mechanism by which mixotrophs can effectively outcompete specialists for nutrient elements. At low resource concentrations, when the uptake of nutrients is limited by diffusion toward the cell, the investment in cell membrane transporters can be minimized. In this situation, mixotrophs can acquire limiting elements in both organic and inorganic forms, outcompeting their specialist competitors that can utilize only one of these forms. This advantage can be enough to offset as much as a twofold increase in additional metabolic costs incurred by mixotrophs. This mechanism is particularly relevant for the maintenance of mixotrophic populations and productivity in the highly oligotrophic subtropical oceans.     

Stochastic species turnover and stable coexistence in a species-rich, fire-prone plant community

Understanding the mechanisms that maintain diversity is important for managing ecosystems for species persistence. Here we used a long-term data set to understand mechanisms of coexistence at the local and regional scales in the Cape Floristic Region, a global hotspot of plant diversity. We used a dataset comprising 81 monitoring sites, sampled in 1966 and again in 1996, and containing 422 species for which growth form, regeneration mode, dispersal distance and abundances at both the local (site) and meta-community scales are known. We found that species presence and abundance were stable at the meta-community scale over the 30 year period but highly unstable at the local scale, and were not influenced by species' biological attributes. Moreover, rare species were no more likely to go extinct at the local scale than common species, and that alpha diversity in local communities was strongly influenced by habitat. We conclude that stochastic environmental fluctuations associated with recurrent fire buffer populations from extinction, thereby ensuring stable coexistence at the meta-community scale by creating a "neutral-like" pattern maintained by niche-differentiation.     

Light-induced turnover of chloroplast cytochrome b-559 in the presence of N-methylphenazonium methosulphate

In the presence of 0.1–5 μM N-methylphenazonium methosulphate approx. 50–70% oxidation of cytochrome b-559 can be induced by far-red light. The oxidation is best observed with long wavelength far-red light (732 nm) of moderate intensities (approx. 10⁴ ergs/cm² per s) and is reversed by subsequent illumination with red light. Concentrations of N-methylphenazonium methosulphate above 5 μM are inhibitory probably due to cyclic electron flow. The far-red oxidation is inhibited by low concentrations of the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, while 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits red light reduction and increases the amplitude of far-red oxidation. The effect of N-methylphenazonium methosulphate is mimicked by N-methyl-phenazonium ethosulphate, but not by pyocyanine or diaminodurene. Low concentrations (2–3 μM) of N-methylphenazonium methosulphate also stimulate a 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone-inhibitable red light reduction of cytochrome f.     

Light-induced electron transfer in porphyrin-calixarene conjugates.

The fluorescence from a set of porphyrin-calixarene complexes is quenched upon addition of benzo-1,4-quinone (BQ) in fluid solution. In N,N-dimethylformamide solution, fluorescence quenching involves both static and dynamic interactions but there are no obvious differences between porphyrins with or without the appended calixarene. Under such conditions, the static quenching behaviour is attributed to pi-complexation between the reactants and it is concluded that the calixarene cavity does not bind BQ. An additional static component is apparent in dichloromethane solution. This latter effect involves partial fluorescence quenching, for which the intramolecular rate constant can be obtained by time-resolved fluorescence spectroscopy. The derived rate constants depend on molecular structure in a manner consistent with fluorescence quenching being due to electron transfer. In all cases, however, the dominant quenching step involves diffusional contact between the porphyrin nucleus and a non-bound molecule of BQ.     

Light-induced rotation of a transmembrane alpha-helix in bacteriorhodopsin

Spin labeling EPR spectroscopy has been used to characterize light-induced conformational changes of bacteriorhodopsin (bR). Pairs of nitroxide spin labels were attached to engineered cysteine residues at strategic positions near the cytoplasmic ends of transmembrane alpha-helices B, F, and G in order to monitor distance changes upon light activation. The EPR analysis of six doubly labeled bR mutants indicates that the cytoplasmic end of helix F not only tilts outwards, but also rotates counter-clockwise during the photocycle. The direction of the rotation of helix F is the opposite of the clockwise rotation previously reported for bovine rhodopsin. The opposite chirality of the F helix rotation in the two systems is perhaps related to the differences in the cis-trans photoisomerization of the retinal in the two proteins. Using time-resolved EPR, we monitored the rotation of helix F also in real time, and found that the signal from the rotation arises concurrently with the reprotonation of the retinal Schiff base.     

Light-induced inhibition of sporulation in Bacillus licheniformis.

Sporulation of Bacillus licheniformis is inhibited by broad-spectrum light. This phenomenon is intensity dependent and is a near-ultraviolet and blue light effect.     

Mammalian turnover and community structure in the Paleocene of North America

The effects of faunal turnover on mammalian community structure are evaluated for 17 faunal zones of the North American Paleocene through early Eocene land mammal ages (Puercan through early Wasatchian). Generic disappearances were significantly high at the end of the Puercan, Torrejonian, and Tiffanian land mammal ages, but appearances were significantly high only during the early Puercan. Generic richness rose rapidly in the early Puercan, remained stable throughout most of the Paleocene, and increased from the late Paleocene into the early Eocene. The null hypothesis that generic turnover clustered preferentially according to dentally defined trophic or body size categories could be rejected or attributed to sampling problems for all but the early (Pu0) and late Paleocene (Ti5‐Cf2). Early Paleocene change in community structure most probably represented endemic radiation of mammals into previously unoccupied niches. Community restructuring in the late Paleocene reflected a complex of causes, including climatic wanning, intercontinental dispersal, and competition.     

Microbiology of thiobacilli and other sulphur-oxidizing autotrophs, mixotrophs and heterotrophs

Recent studies on the ecophysiology of the obligate chemolithotroph Thiobacillus neapolitanus have given better insight into its specialization for an autotrophic mode of life. This appears not only from its high constitutive levels of autotrophic enzymes, but also from its possession of carboxysomes, which seem to be specialized organelles for CO2 fixation and concentrating reducing power. At the same time, these organisms are metabolically versatile with respect to nitrogen assimilation pathways, and during starvation are able to utilize endogenous resources such as polyglucose for carbon and energy. Studies on the facultative chemolithotrophs such as Thiobacillus novellus and Thiobacillus A2 have shown that they can grow mixotrophically on mixtures of inorganic and organic substrates, i.e. they can utilize these compounds simultaneously provided that they are growth limiting. Thiobacillus A2 displays a remarkable flexibility not only with respect to the organic substrates that it can utilize but, for example, also in the choice of various pathways for glucose metabolism. Competition experiments carried out between specialized and versatile thiobacilli strongly indicate that the ecological advantage of the versatile thiobacilli may lie not so much in their short-term flexibility, but rather in their ability to grow mixotrophically. Studies on most heterotrophic chemolithotrophs are still in their infancy. Promising progress has been made in the study of the physiology of Beggiatoa species. Renewed interest in the sulphur-oxidizing bacteria stems from recent findings about their role in food chains, and their possible application in industry.     

Light-induced conductivity changes in purple membrane suspensions.

Small light-induced changes in the conductivity of light-adapted purple membrane suspended in strong electrolyte solutions were detected. The method used involved modulated light and a phase sensitive detector and it allowed us to detect accurately changes as small as 0.0001% in the conductivity of the suspension. The light-induced conductivity changes turned out to be composed of at least two different event: a small fast increase in conductivity (tau approximately 2 ms) followed by a slower and larger decrease in this parameter (tau = 70 ms-80 ms). The effects of pH and temperature on these changes were studied. Both events reached maximal values around neutral pH and approached zero at both high and low pH's. Heating the suspension decreased the photoconductivity change and Arrhenius plots of the data showed breaks around 31 degrees C. It is suggested that the conductivity changes reflect changes in the surface charge of the membrane and can be used to follow the kinetics of the conformational changes occurring in the system.     

Light-induced electron transfer in a cryptochrome blue-light photoreceptor

Cryptochromes are flavoproteins implicated in multiple blue light-dependent signaling pathways regulating, for example, photomorphogenesis in plants or circadian clocks in animals. Using transient absorption spectroscopy, it is demonstrated that the primary light reactions in isolated Arabidopsis thaliana cryptochrome-1 involve intraprotein electron transfer from tryptophan and tyrosine residues to the excited flavin adenine dinucleotide cofactor.     

Coexistence of mixotrophs, autotrophs, and heterotrophs in planktonic microbial communities

We examine what circumstances allow the coexistence of microorganisms following different nutritional strategies, using a mathematical model. This model incorporates four nutritional types commonly found in planktonic ecosystems: (1) heterotrophic bacteria that consume dissolved organic matter and are prey to some of the other organisms; (2) heterotrophic zooflagellates that depend entirely on bacterial prey; (3) phototrophic algae that depend only on light and inorganic nutrients, and (4) mixotrophs that photosynthesize, take up inorganic nutrients, and consume bacterial prey. Mixotrophs are characterized by a parameter representing proportional mixing of phototrophic and heterotrophic nutritional strategies. Varying this parameter, a range of mixotrophic strategies was examined in hypothetical habitats differing in supplies of light, dissolved organic carbon, and dissolved inorganic phosphorous. Mixotrophs expressing a wide range of mixotrophic strategies persisted in model habitats with low phosphorus supply, but only those with a strategy that is mostly autotrophic persisted with high nutrient supply, and then only when light supply was also high. Organisms representing all four nutritional strategies were predicted to coexist in habitats with high phosphorus and light supplies. Coexistence involves predation by zooflagellates and mixotrophs balancing the high competitive ability of bacteria for phosphorus, the partitioning of partially overlapping resources between all populations, and possibly nonequlibrium dynamics. In most habitats, the strategy predicted to maximize the abundance of mixotrophs is to be mostly photosynthetic and supplement nutritional needs by consuming bacteria.     

Light-induced Changes in Allophycocyanin

Several lines of evidence indicate that allophycocyanin is the previously unidentified “phycochrome” observed in extracts of blue-green algae.     

Light-induced inhibition of papain by a {Mn-NO}6 nitrosyl: identification of papain-SNO adduct by mass spectrometry

Modification of Cys25 at the active site of the cysteine protease papain by S-nitrosylation inhibits its hydrolytic ability. Previous studies have demonstrated that NO donors N-nitrosoanilines inhibit papain activity via formation of S-NO bond formation at the active site while NO donors such as S-nitroso-N-acetyl-penicillamine (SNAP), N-nitrosoaniline derivatives, and S-nitroso-glutathione (GSNO) inhibit the enzyme via S-thiolation by thiyl radicals generated from the S-nitrosothiols. In this study, we report papain inactivation by a photosensitive {Mn-NO}(6) nitrosyl [(PaPy(3))Mn(NO)](ClO(4)) (1) where PaPy(3)(-) is the anion of the designed ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide. This nitrosyl releases NO upon exposure to visible light of low intensity (50W tungsten lamp). With N(alpha)-benzoyl-l-arginine-p-nitroanilide (l-BApNA) as the substrate, the dissociation constant for the breakdown of the enzyme-inactivator complex (K(I)) and the overall inactivation rate constant (k(i)) were calculated to be 2.46mM and 64.8min(-1), respectively. The papainS-NO adduct has been identified using electrospray mass spectrometry (ESI-MS). The results demonstrate that controlled inactivation of papain can be achieved with the {Mn-NO}(6) nitrosyl 1 and light. The reaction is clean and the extent of inactivation is directly proportional to the exposure time.     

Light-induced leucine transport in Halobacterium halobium envelope vesicles: a chemiosmotic system.

Halabacterium halobium cell envelope vesicles accumulate L-[14-C]leucine during illumination, against a large concentration gradient. Leucine uptake requires Na-+ and is optimal in KCl-loaded vesicles resuspended in KCl-NaCl solution (1.5 M:1.5 M). Half-maximal transport is seen at 1 X 10-minus 6 M leucine. In the dark the accumulated leucine is rapidly and exponentially lost from the vesicles. The action spectrum and the light-intensity dependence indicate that the transport is related to the extrusion of protons, mediated by bacteriorhodopsin. Since light gives rise to both a pH gradient and an opposing transmembrane potential (interior negative), it wass responsible for providing the energy for leucine transport. The following results were obtained under illumination: (1) membrane-permeant cations and valinomycin or gramicidin greatly inhibited leucine transport without altering the pH gradient; (2) buffering both inside and outside the vesicles eliminated the pH gradient while enhancing leucine transport; (3) dicyclohexylcarbodiimide increased the pH gradient without affecting leucine transport; (4) arsenate did not inhibit leucine uptake. A diffusion potential, established by adding valinomycin to KCl-loaded vesicles, caused leucine influx in the dark. These results suggest that the leucine transport system is not coupled to ATP hydrolysis, and responds to the membrane potential rather than to the pH gradient. The Na-+ dependence of the transport and the observation that a small NaCl pulse causes transient leucine influx in the dark in KCl-loaded vesicles, resuspended in KCl, even in the presence of p-trifluoromethoxycarbonylcyanide phenylhydrazone or with buffering, suggest that the translocation of leucine is facilitated by symport with Na-+.