Chromite oxidation by manganese oxides in subseafloor basalts and the presence of putative fossilized microorganisms

Chromite is a mineral with low solubility and is thus resistant to dissolution. The exception is when manganese oxides are available, since they are the only known naturally occurring oxidants for chromite. In the presence of Mn(IV) oxides, Cr(III) will oxidise to Cr(VI), which is more soluble than Cr(III), and thus easier to be removed. Here we report of chromite phenocrysts that are replaced by rhodochrosite (Mn(II) carbonate) in subseafloor basalts from the Koko Seamount, Pacific Ocean, that were drilled and collected during the Ocean Drilling Program (ODP) Leg 197. The mineral succession chromite-rhodochrosite-saponite in the phenocrysts is interpreted as the result of chromite oxidation by manganese oxides. Putative fossilized microorganisms are abundant in the rhodochrosite and we suggest that the oxidation of chromite has been mediated by microbial activity. It has previously been shown in soils and in laboratory experiments that chromium oxidation is indirectly mediated by microbial formation of manganese oxides. Here we suggest a similar process in subseafloor basalts

A PKC-Dependent Recruitment of MMP-2 Controls Semaphorin-3A Growth-Promoting Effect in Cortical Dendrites

There is increasing evidence for a crucial role of proteases and metalloproteinases during axon growth and guidance. In this context, we recently described a functional link between the chemoattractive Sema3C and Matrix metalloproteinase 3 (MMP3). Here, we provide data demonstrating the involvement of MMP-2 to trigger the growth-promoting effect of Sema3A in cortical dendrites. The in situ analysis of MMP-2 expression and activity is consistent with a functional growth assay demonstrating in vitro that the pharmacological inhibition of MMP-2 reduces the growth of cortical dendrites in response to Sema3A. Hence, our results suggest that the selective recruitment and activation of MMP-2 in response to Sema3A requires a PKC alpha dependent mechanism. Altogether, we provide a second set of data supporting MMPs as effectors of the growth-promoting effects of semaphorins, and we identify the potential signalling pathway involved

Integration of observational and analytical methodolgoies to characterize organic matter in early Archean rocks: distinguishing biological from abiotically synthesized carbonaceous matter structures

Transmission Electron Microscopy (TEM) was applied to observe and characterize carbonaceous materials (CM) extracted from black cherts and argillite in drill core from the Warrawoona Group of the Pilbara Craton, Western Australia. The black chert came from a 'white smoker type' seafloor deposit in the ca. 3.49 Ga Dresser Formation, whereas the black argillites were obtained from the 3.46 Ga Apex Basalt. The samples were observed and analyzed in TEM combined with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and C-isotope geochemistry. The TEM and HRTEM observations revealed significant morphological and structural differences between the carbonaceous materials of the Dresser and Apex samples enabling interpretations in terms of primary and secondary origins, as well as metamorphic history. Organic petrology using reflected light microscopy was used on whole rock-samples to observe mineral-organic matter relationship and CM structure relative to host rock texture. The results support an in situ, syn-depositional origin for the Dresser Formation CM. Reflectance % (Ro) of CM determined on polished whole rock-samples and polished resin-embedded CM-concentrates enabled the reconstruction of thermal history. Several Ro populations were identified in the Dresser Formation samples: probable microbial cells preserved in fluid inclusions within quartz crystals, thermally degraded CM originally belonging to microbial cells, CM coating mineral grains and reworked CM particles. In contrast, the Apex Basalt samples yielded-consistently very high Ro values corresponding to graphite stage organic-metamorphism. The weak optical anisotropy of the Apex graphite is inconsistent with formation during regional metamorphism. Two main graphite forms were identified, namely platy and tubular varieties. In HRTEM the tubular form showed nano-tubes and fullerenes within mono-layered spheres. Furthermore, TEM and HRTEM show that the void enclosed mono-layered carbon nano-spheres are more often detached from tubular graphite, forming clusters outside the nano-tubes. These forms are a key to the distinction between biologically and abiotically synthesized CM bodies, both by their small size, perfect outline and especially their resistance to thermal degradation. Dresser Formation samples are isotopically light in the range of-32.1‰ to-38.2‰ consistent with a biological source. Although TEM indicated four distinct types of CM, C-isotope analysis was undertaken on mixed CM concentrates. Nevertheless, the isotopically lighter samples contain a notable input from less thermally degraded low Ro material. On the other hand the isotopically heavier samples contain-predominantly thermally degraded high Ro CM. C-isotope compositions of the Apex CM are generally heavier than Dresser samples, between-22.5‰ and-28.6‰ consistent with high thermal stress. The samples show a C-isotope trend in which CM at 143 m depth is isotopically lighter, whereas above and below this level CM becomes increasingly and consistently isotopically heavier. The upper part of the section is dominated by platy graphite with rare nano-tubes. Predominantly tubular graphite and fullerenes characterize sample SAL-13 at 142 m depth coinciding with the isotopically lightest values. Below 143 m CM is less well preserved and predominantly of the fragmented platy graphite type, becoming increasingly isotopically heavier signifying close association with peridotite intrusion