3 research outputs found
Bullet-Shaped Magnetite Biomineralization Within a Magnetotactic Deltaproteobacterium: Implications for Magnetofossil Identification
Magnetite produced by magnetotactic bacteria (MTB) provides stable paleomagnetic signals because it occurs as natural singleâdomain magnetic nanocrystals. MTB can also provide useful paleoenvironmental information because their crystal morphologies are associated with particular bacterial groups and the environments in which they live. However, identification of the fossil remains of MTB (i.e., magnetofossils) from ancient sediments or rocks is challenging because of their generally small sizes and because the growth, morphology, and chain assembly of magnetite within MTB are not well understood. Nanoscale characterization is, therefore, needed to understand magnetite biomineralization and to develop magnetofossils as biogeochemical proxies for paleoenvironmental reconstructions. Using advanced transmission electron microscopy, we investigated magnetite growth and chain arrangements within magnetotactic Deltaproteobacteria strain WYHRâ1, which reveals how the magnetite grows to form elongated, bulletâshaped nanocrystals. Three crystal growth stages are recognized: (i) initial isotropic growth to produce nearly round ~20 nm particles, (ii) subsequent anisotropic growth along the [001] crystallographic direction to ~75 nm lengths and ~30-40 nm widths, and (iii) unidirectional growth along the [001] direction to ~180 nm lengths, with some growing to ~280 nm. Crystal growth and habit differ from that of magnetite produced by other known MTB strains, which indicates speciesâspecific biomineralization. These findings suggest that magnetite biomineralization might be much more diverse among MTB than previously thought. When characterized adequately at species level, magnetofossil crystallography, and apomorphic features are, therefore, likely to become useful proxies for ancient MTB taxonomic groups or species and for interpreting the environments in which they lived.This study was
supported financially by the National
Natural Science Foundation of China
(grants no. 41920104009, 41890843, and
41621004), The Senior User Project of
RVKEXUE2019GZ06 (Center for
Ocean Me MegaâScience, Chinese
Academy of Sciences), and the
Australian Research Council (grant
DP160100805
Mars: new insights and unresolved questions
Mars exploration motivates the search for extraterrestrial life, the development of space technologies, and the design of human missions and habitations. Here, we seek new insights and pose unresolved questions relating to the natural history of Mars, habitability, robotic and human exploration, planetary protection, and the impacts on human society. Key observations
and findings include:
â high escape rates of early Marsâ atmosphere, including loss of water, impact present-day habitability;
â putative fossils on Mars will likely be ambiguous biomarkers for life;
â microbial contamination resulting from human habitation is unavoidable; and
â based on Marsâ current planetary protection category, robotic payload(s) should characterize the local martian environment for any life-forms prior to human habitation.Some of the outstanding questions are:â which interpretation of the hemispheric dichotomy of the planet is correct;
â to what degree did deep-penetrating faults transport subsurface liquids to Marsâ surface;
â in what abundance are carbonates formed by atmospheric processes;
â what properties of martian meteorites could be used to constrain their source locations;
â the origin(s) of organic macromolecules;
â was/is Mars inhabited;
â how can missions designed to uncover microbial activity in the subsurface eliminate potential false positives caused by microbial contaminants from Earth;
â how can we ensure that humans and microbes form a stable and benign biosphere; and
â should humans relate to putative extraterrestrial life from a biocentric viewpoint (preservation of all biology), or anthropocentric viewpoint of expanding habitation of space?Studies of Marsâ evolution can shed light on the habitability of extrasolar planets. In addition,
Mars exploration can drive future policy developments and confirm (or put into question) the
feasibility and/or extent of human habitability of space
Surges in volcanic activity on the Moon about two billion years ago
Abstract The history of mare volcanism critically informs the thermal evolution of the Moon. However, young volcanic eruptions are poorly constrained by remote observations and limited samples, hindering an understanding of mare eruption flux over time. The Changâe-5 mission returned the youngest lunar basalts thus far, offering a window into the Moonâs late-stage evolution. Here, we investigate the mineralogy and geochemistry of 42 olivine and pyroxene crystals from the Changâe-5 basalts. We find that almost all of them are normally zoned, suggesting limited magma recharge or shallow-level assimilation. Most olivine grains record a short timescale of cooling. Thermal modeling used to estimate the thickness and volume of the volcanism sampled by Changâe-5 reveals enhanced magmatic flux ~2 billion years ago, suggesting that while overall lunar volcanic activity may decrease over time, episodic eruptions at the final stage could exhibit above average eruptive fluxes, thus revising models of lunar thermal evolution