Driving Forces for Bicarbonate Transport in the Cyanobacterium Synechococcus R-2 (PCC 7942)

Air-grown Synechococcus R-2 (PCC 7942) cultures grown in BG-11 medium are very alkaline (outside pH is 10.0) and use HCO3- as their inorganic carbon source. The cells showed a dependence on Na+ for photosynthesis, but low Na+ conditions (1 mol m-3) were sufficient to support saturating photosynthesis. The intracellular dissolved inorganic carbon in the light was greater than 20 mol m-3 in both low-Na+ conditions and in BG-11 medium containing the usual [Na+] (24 mol m-3, designated high-Na+ conditions). The electrochemical potential for HCO3- in the light was in excess of 25 kJ mol-1, even in high-Na+ conditions. The Na+-motive force was greater than -12 kJ mol-1 under both Na+ conditions. On thermodynamic grounds, an Na+-driven co-port process would need to have a stoichiometry of 2 or greater ([greater than or equal to]2Na+ in/HCO3-1 in), but we show that Na+ or K+ fluxes cannot be linked to HCO3- transport. Na+ and K+ fluxes were unaffected by the presence or absence of dissolved inorganic carbon. In low-Na+ conditions, Na+ fluxes are too low to support the observed net 14C-carbon fixation rate. Active transport of HCO3- hyperpolarizes (not depolarizes) the membrane potential.     

Billingen (Lower Arenig/Lower Ordovician) acritarchs from the East European Platform and their palaeobiogeographic significance

Billingen (Lower Arenig/Lower Ordovician) sediments of the St. Petersburg region, northwest Russia and the Leba area, northern Poland of the East European Craton yield acritarch assemblages, which are largely homogenous though displaying minor compositional differences that probably reflect a gradient from inner to outer shelf environments. Comparison with coeval acritarch microflora from the Yangtze Platform, South China, shows an overall similarity between Baltoscandian and South Chinese phytoplankton. The widespread uniformity in the fossil microphytoplankton may be related to the extensive global 'evae' sea-level transgression, which characterized the Billingen time. This suggests that during the Tremadoc through early Arenig times, acritarch assemblages displayed essentially an undifferentiated cold-water and oceanic character along the whole margin of Perigondwana in the South, as well as on the South Chinese and Baltic platforms, at middle latitudes (Mediterranean oceanic Realm). Despite this overall similarity, however, some typical taxa of the high-latitude Mediterranean Province (Arbusculidium, Coryphidium and Striatotheca) occur in South China, but are absent in Baltica. This discrepancy is explained as caused by differences in climatic and physiographic conditions that prevailed at the two palaeocontinents at this time. The inferred pattern of oceanic circulation during the Lower Ordovician is consistent with the palynological evidence of a prevailing warmer climate in Baltica than in South China, although the two palaeocontinents occupied the same palaeolatitudinal position.     

CALCIFICATION IN THE GREEN ALGA HALIMEDA. I. AN ULTRASTRUCTURE STUDY OF THALLUS DEVELOPMENT1

The ultrastructure of 4 species of the calcareous, siphonaceous alga Halimeda (H. cylindracea Decaisne, H. discoidea Decaisne, H. macroloba Decaisne and H. tuna (Ellis & Solander) Lamour) has been studied, and the observed changes during growth and development are related to changes in the degree of calcification. A distinct gradient in the types and quantities of cell organelles exists in a growing apical filament. As these filaments grow, branch, and eventually develop into a mature segment, changes in the organization of organelles such as mitochondria and chloroplasts are observed. Calcification begins when the chloroplasts reach structural maturity and when the peripheral utricles adhere (fuse). This adhesion of the peripheral utricles isolates the intercellular space (ICS) in which calcification occurs from the external seawater. Calcification begins in the outermost (pilose) cell wall layer of the walls facing into the ICS. The cell walls at the thallus exterior undergo extensive changes after utricular fusion; the pilose layer is lost, the cuticles of adjacent utricles fuse forming a ridge at their junction, and multiple cuticles are formed. The aragonite (CaCO₃) crystals which are initially precipitated within the pilose wall layer, rapidly increase in size and number, eventually filling much of the ICS. Only the initial nucleation of aragonite is associated with the pilose wall layer, the later precipitation of aragonite is totally independent of the pilose layer. In older segments secondary deposition of CaCO₃ also occurs around existing aragonite needles.     

Interactions of photosynthesis with genome size and function

Photolithotrophs are divided between those that use water as their electron donor (Cyanobacteria and the photosynthetic eukaryotes) and those that use a different electron donor (the anoxygenic photolithotrophs, all of them Bacteria). Photolithotrophs with the most reduced genomes have more genes than do the corresponding chemoorganotrophs, and the fastest-growing photolithotrophs have significantly lower specific growth rates than the fastest-growing chemoorganotrophs. Slower growth results from diversion of resources into the photosynthetic apparatus, which accounts for about half of the cell protein. There are inherent dangers in (especially oxygenic) photosynthesis, including the formation of reactive oxygen species (ROS) and blue light sensitivity of the water spitting apparatus. The extent to which photolithotrophs incur greater DNA damage and repair, and faster protein turnover with increased rRNA requirement, needs further investigation. A related source of environmental damage is ultraviolet B (UVB) radiation (280–320 nm), whose flux at the Earth''s surface decreased as oxygen (and ozone) increased in the atmosphere. This oxygenation led to the requirements of defence against ROS, and decreasing availability to organisms of combined (non-dinitrogen) nitrogen and ferrous iron, and (indirectly) phosphorus, in the oxygenated biosphere. Differential codon usage in the genome and, especially, the proteome can lead to economies in the use of potentially growth-limiting elements     

Early Archean origin of Photosystem II

Photosystem II is a photochemical reaction center that catalyzes the light‐driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.     

Functional fluorescent Ca2+ indicator proteins in transgenic mice under TET control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.     

Uptake of Thallium, a Toxic Heavy-Metal, in the Cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. Leopoliensis) PCC 7942

Uptake of the toxic heavy-metal, thallium, was studied in thecyanobacterium Synechococcus R-2 (PCC 7942) using clinicallyavailable ²⁰¹Tl ⁺. Thallium was found to distribute across theplasmalemma passively, and so the accumulation ratio of theion ([Tl⁺]_i/[Tl⁺]_o) could be used to calculate the apparentmembrane potential (­_i,o) of the cells (^ETI⁺_i,o = ­_i,o).The permeability of the plasmalemma to TI⁺ (^PTI⁺ 1 to 5 nms^–1)is higher than that of K⁺. Valinomycin does not increase thepermeability of TI⁺. Transient changes in the ­_i,o of cells,because of electrogenic transport of ions, could be detectedfrom its effects upon the uptake rate of TI⁺. HCO^–₃ hyperpolarizedSynechococcus cells, whereas NH⁺₄, CH₃NH⁺, and K⁺ led to depolarization.The use of TI⁺ as a reporter of ­_i,o has some inherent limitations.Tl⁺ is toxic at very low concentrations (inhibitory effectsare apparent after about 6 h at concentrations as low as 1 mmolm^–3). The rate of equilibration is slow (t_1/25 to 20 min).Equilibration of TI⁺ takes about 2 h, which limits its valueas a membrane potential probe. Large amounts of TI⁺ bind tothe surface of the cells making the method impracticable formeasuring accumulation ratios of less than about 10 (­_i,o)values smaller than about –60 mV). Cultures continuouslyexposed to Tl⁺ (10 mmol m^–3) eventually become TI⁺ resistantby actively extruding TI⁺ (µTI⁺_i,o= –3±0.2kJ mol^–1) and so thallium cannot be used as a ­_i,oprobe in such cells. (Received October 28, 1997; Accepted August 31, 1998)     

The molecular structure of the IsiA-Photosystem I supercomplex, modelled from high-resolution, crystal structures of Photosystem I and the CP43 protein

We present the molecular structure of the IsiA-Photosystem I (PSI) supercomplex, inferred from high-resolution, crystal structures of PSI and the CP43 protein. The structure of iron-stress-induced A protein (IsiA) is similar to that of CP43, albeit with the difference that IsiA is associated with 15 chlorophylls (Chls), one more than previously assumed. The membrane-spanning helices of IsiA contain hydrophilic residues many of which bind Chl. The optimal structure of the IsiA-PSI supercomplex was inferred by systematically rearranging the IsiA monomers and PSI trimer in relation to each other. For each of the 6,969,600 structural configurations considered, we counted the number of optimal Chl-Chl connections (i.e., cases where Chl-bound Mg atoms are ≤ 25 Å apart). Fifty of these configurations were found to have optimal energy-transfer potential. The 50 configurations could be divided into three variants; one of these, comprising 36 similar configurations, was found to be superior to the other configurations in terms of its potential to transfer excitation energy to the reaction centres under low-light conditions and its potential to dissipate excess energy under high-light conditions. Compared to the assumed model [Biochemistry 42 (2003) 3180-3188], the new Chl increases by 7% the ability of IsiA to harvest sunlight while the rearrangement of the constituent components of the IsiA-PSI supercomplex increases by 228% the energy-transfer potential. In conclusion, our model allows us to explain how the IsiA-PSI supercomplex may act as an efficient light-harvesting structure under low-light conditions and as an efficient dissipater of excess energy under high-light conditions.     

Cyrtocrinids (Echinodermata,Crinoidea) from Upper Jurassic Štramberk‐type limestones in southern Poland

Abstract: A systematic account of highly diverse cyrtocrinid faunules from Upper Jurassic strata of ?tramberk type (Oxfordian–Tithonian) in southern Poland (Polish Carpathians) is presented. Fourteen taxa (Phyllocrinus malbosianus, Ph. stellaris, Ph. sp., Psalidocrinus armatus, Sclerocrinus compressus, S. polonicus sp. nov., Hemicrinus aff. kabanovi, Ancepsicrinus parvus gen. et sp. nov., Tetracrinus baumilleri sp. nov., Eugeniacrinites alexandrowiczi, E. cf. moravicus, E. sp., Eudesicrinus gluchowskii sp. nov. and Hemibrachiocrinus tithonicus sp. nov. are described and illustrated. Representatives of the genus Eudesicrinus, previously recorded only from the Lower Jurassic, are here shown to extend into the uppermost Jurassic. Other cyrtocrinids considered are common in Jurassic/Cretaceous strata across Europe. In the present faunules, isocrinid (Isocrinida), comatulid (Comatulida) and roveacrinid (Roveacrinida sensu Rasmussen, inclusive of Saccocoma) crinoids are associated.