Impact of changes in natural ultraviolet radiation on pigment composition, physiological and morphological characteristics of the Antarctic moss, Grimmia antarctici

The impact of ambient ultraviolet (UV)-B radiation on the endemic bryophyte, Grimmia antarctici, was studied over 14 months in East Antarctica. Over recent decades, Antarctic plants have been exposed to the largest relative increase in UV-B exposure as a result of ozone depletion. We investigated the effect of reduced UV and visible radiation on the pigment concentrations, surface reflectance and physiological and morphological parameters of this moss. Plexiglass screens were used to provide both reduced UV levels (77%) and a 50% decrease in total radiation. The screen combinations were used to separate UV photoprotective from visible photoprotective strategies, because these bryophytes are growing in relatively high light environments compared with many mosses. G. antarctici was affected negatively by ambient levels of UV radiation. Chlorophyll content was significantly lower in plants grown under near-ambient UV, while the relative proportions of photoprotective carotenoids, especially beta-carotene and zeaxanthin, increased. However, no evidence for the accumulation of UV-B-absorbing pigments in response to UV radiation was observed. Although photosynthetic rates were not affected, there was evidence of UV effects on morphology. Plants that were shaded showed fewer treatment responses and these were similar to the natural variation observed between moss growing on exposed microtopographical ridges and in more sheltered valleys within the turf. Given that other Antarctic bryophytes possess UV-B-absorbing pigments which should offer better protection under ambient UV-B radiation, these findings suggest that G. antarctici may be disadvantaged in some settings under a climate with continuing high levels of springtime UV-B radiation.

Magnetosomal matrix: ultrafine structure may template biomineralization of magnetosomes

The organic matrix surrounding bullet-shaped, cubo-octahedral, D-shaped, irregular arrowhead-shaped, and truncated hexa-octahedral magnetosomes was analysed in a variety of uncultured magnetotactic bacteria. The matrix was examined using low- (80 kV) and intermediate- (400 kV) voltage TEM. It encapsulated magnetosomes in dehydrated cells, ultraviolet-B-irradiated dehydrated cells and stained resin-embedded fixed cells, so the apparent structure of the matrix does not appear to be an artefact of specimen preparation. High-resolution images revealed lattice fringes in the matrix surrounding magnetite and greigite magnetosomes that were aligned with lattice fringes in the encapsulated magnetosomes. In all except one case, the lattice fringes had widths equal to or twice the width of the corresponding lattice fringes in the magnetosomes. The lattice fringes in the matrix were aligned with the {311}, {220}, {331}, {111} and {391} related lattice planes of magnetite and the {222} lattice plane of greigite. An unidentified material, possibly an iron hydroxide, was detected in two immature magnetosomes containing magnetite. The unidentified phase had a structure similar to that of the matrix as it contained {311}, {220} and {111} lattice fringes, which indicates that the matrix acts as a template for the spatially controlled biomineralization of the unidentified phase, which itself transforms into magnetite. The unidentified phase was thus called pre-magnetite. The presence of the magnetosomal matrix explains all of the five properties of the biosignature of the magnetosomal chain proposed previously by Friedmann et al. and supports their claim that some of the magnetite particles in the carbonate globules in the Martian meteorite ALH84001 are biogenic. Two new morphologies of magnetite magnetosomes are also reported here (i.e. tooth-shaped and hexa-octahedral magnetosomes). Tooth-shaped magnetite magnetosomes elongated in the [110] direction are reported, and are distinct from arrowhead-shaped and bullet-shaped magnetosomes. Elongation of magnetite magnetosomes in the [110] direction has not been reported previously. A Martian hexa-octahedral magnetite particle was previously characterized by Thomas-Keptra et al. and compared with truncated hexa-octahedral magnetite magnetosomes. Hexa-octahedral magnetite magnetosomes with the same morphology and similar sizes and axial ratios as those reported by Thomas-Keptra et al. are characterized here. These observations support their claim that ALH84001 contains evidence for a past Martian biota.