Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean

Marine fixation of atmospheric nitrogen is believed to be an important source of biologically useful nitrogen to ocean surface waters, stimulating productivity of phytoplankton and so influencing the global carbon cycle. The majority of nitrogen fixation in tropical waters is carried out by the marine cyanobacterium Trichodesmium, which supplies more than half of the new nitrogen used for primary production. Although the factors controlling marine nitrogen fixation remain poorly understood, it has been thought that nitrogen fixation is limited by iron availability in the ocean. This was inferred from the high iron requirement estimated for growth of nitrogen fixing organisms and the higher apparent densities of Trichodesmium where aeolian iron inputs are plentiful. Here we report that nitrogen fixation rates in the central Atlantic appear to be independent of both dissolved iron levels in sea water and iron content in Trichodesmium colonies. Nitrogen fixation was, instead, highly correlated to the phosphorus content of Trichodesmium and was enhanced at higher irradiance. Furthermore, our calculations suggest that the structural iron requirement for the growth of nitrogen-fixing organisms is much lower than previously calculated. Although iron deficiency could still potentially limit growth of nitrogen-fixing organisms in regions of low iron availability-for example, in the subtropical North Pacific Ocean-our observations suggest that marine nitrogen fixation is not solely regulated by iron supply.     

Self-assembly of mesoscopically ordered chromatic polydiacetylene/silica nanocomposites

Nature abounds with intricate composite architectures composed of hard and soft materials synergistically intertwined to provide both useful functionality and mechanical integrity. Recent synthetic efforts to mimic such natural designs have focused on nanocomposites, prepared mainly by slow procedures like monomer or polymer infiltration of inorganic nanostructures or sequential deposition. Here we report the self-assembly of conjugated polymer/silica nanocomposite films with hexagonal, cubic or lamellar mesoscopic order using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic monomers uniformly within a highly ordered, inorganic environment. Polymerization results in polydiacetylene/silica nanocomposites that are optically transparent and mechanically robust. Compared to ordered diacetylene-containing films prepared as Langmuir monolayers or by Langmuir-Blodgett deposition, the nanostructured inorganic host alters the diacetylene polymerization behaviour, and the resulting nanocomposite exhibits unusual chromatic changes in response to thermal, mechanical and chemical stimuli. The inorganic framework serves to protect, stabilize, and orient the polymer, and to mediate its function. The nanocomposite architecture also provides sufficient mechanical integrity to enable integration into devices and microsystems.     

Differential inhibition by erythro-9-[3-(2-hydroxynonyl)]adenine of flagella-like and cilia-like movement of Leishmania promastigotes

Erythro-9-[3-(2-hydroxynonyl)]adenine (EHNA) inhibits axonemal dynein ATPase activity and hence the beating of sea urchin and mammalian flagella. We have found that EHNA has an unusual effect on the flagella of Leishmania promastigotes in that it alters both the waveform and polarity of the beat. We report here results which suggest that in Leishmania promastigotes there are either distinct EHNA-sensitive dyneins or different conformational states of a single dynein involved in the cilia-like and flagella-like waveforms and in the propagation of flagellar waves from tip-to-base and from base-to-tip.     

Profound hypothermia in golden hamsters (Mesocricetus auratus) induced by serotonergic potentiating and noradrenergic inhibiting drugs

Summary Profound hypothermia (6°C) was induced in cold exposed golden hamsters (Mesocricetus auratus) by a combination of drugs that potentiate brain serotonergic activity (fluoxetine and pargyline) and inhibit noradrenergic activity (alpha-methyl-p-tyrosine). Individual drugs and combinations of 2 were ineffective.Acknowledgements. We acknowledge the support of a research grant PCM74-05158 A01 to A.H.M. from the National Science Foundation (USA) and thank Lilly Research Laboratories, Indianapolis, Indiana, for the gift of fluoxetine (Lilly 110140).     

Hyperion's sponge-like appearance

Hyperion is Saturn's largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation. Previous work has identified Hyperion's surface as distinct from other small icy objects but left the causes unsettled. Here we report high-resolution images that reveal a unique sponge-like appearance at scales of a few kilometres. Mapping shows a high surface density of relatively well-preserved craters two to ten kilometres across. We have also determined Hyperion's size and mass, and calculated the mean density as 544 +/- 50 kg m(-3), which indicates a porosity of >40 per cent. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained, and accordingly may be the crucial factor in crafting this unusual surface.     

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago

Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate, but the absolute changes in solar intensity over the range of decades to millennia are small and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone, whose northward migration stops near the southern shoreline of Arabia in the present climate. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th-U-dated stalagmite from Oman. The delta18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the delta18O record with the Delta14C record from tree rings, which largely reflects changes in solar activity. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.