Search

Plants have the potential to accumulate toxic amounts of cadmium (Cd), and understanding how and where Cd is stored in plants is important for ensuring food safety. Previous experiments have determined that a greater amount of Cd is translocated into the leaves of lettuce (Lactuca sativa) as compared to barley leaves (Hordeum vulgare). Preferential retention of Cd in root of barley would explain this difference. Hence, the purpose of this study was to determine the localization and coordination environment of Cd (i.e., the ligands to which Cd was bound) in the different root tissues of lettuce and barley using histochemical staining, electron microscopy and micro X-ray spectroscopy. Retention of Cd in barley roots could be explained by accumulation of Cd at the endodermis, comparatively higher amounts of Cd sequestered in the symplast of cortical cells and binding to xylem cell walls. Increased translocation of Cd to lettuce shoots seemed to be due to a less effective barrier at the endodermis and less sequestration of Cd in the cortex. Regardless of the tissue type, most of the Cd2+ was bound to S ligands in the roots of barley, possibly reflecting accumulation of Cd–phytochelatin and Cd–S molecules in the vacuoles. In lettuce roots, Cd was more evenly distributed among ligands containing S, O and NO3 groups, which is indicative of proportionately more Cd binding to the cell walls, relative to barley. These results will be useful in uncovering the mechanisms of differential Cd-tolerance and sequestration in lettuce and barley.

Calcite microbialites in Pavilion Lake, British Columbia, exhibit a diverse range in macro-morphology, biomass abundance, porosity, and mineral content. To evaluate the role of microorganisms in their formation, samples collected from a range of depths were examined by scanning electron microscopy (SEM) and synchrotron radiation-based micro-X-ray fluorescence (μ-XRF) spectroscopy to characterize both their outer surfaces as well as internal structures. Observed trends in both surface colonization as well as microbialite framework with increasing lake depth include decreasing microbial abundance on outer surfaces as well as increasing ratios of carbonate:biomass in the microbialites. Microscopic investigations of the interiors show bacteria and algae entrapped within calcite, with this calcite exhibiting micropores and casts similar in size and shape to microorganisms. Based on these observations, it is hypothesized that microbialite development in Pavilion Lake initiates calcite precipitation in phototrophic microbial mats, i.e., combined phototrophy and heterotrophy, followed by heterotrophic oxidation of organic matter leading to eventual carbonate infilling of the microbial–mineral matrix. In addition, an observed shift from cyanobacteria to algae with increasing lake depth suggests variability in contemporary conditions controlling microbialite growth and diagenesis. High photosynthetic growth rates at shallower depths result in significant porosity and friability due to biomass accumulation outpacing carbonate precipitation. At intermediate depths, lower light levels and slower growth rates of phototrophs lead to a greater proportion of the microbialite matrix being in-filled by carbonate. Carbonates precipitate initially within the bacteria-EPS matrix, with abundant uncalcified algae maintaining microbialite porosity. In the deepest waters, the presence of only sparse algal colonization as well as fine-grained, laminated metal-rich sediments covering microbialites suggests that present-day insolation levels are too low to support the development of photosynthetic microbial mats. As a consequence, heterotrophic carbonate precipitation has progressively in-filled these microbialite interiors to create lithified calcite fabrics that exhibit minimal porosity but preserve the casts of microorganisms as biosignatures. While the origin of microbialites in Pavilion Lake remains unknown, current observations provide valuable information in evaluating how environmental conditions influence microbialite growth in a freshwater, lacustrine environment.

The influence of microbial activity on carbonate precipitation was investigated within micro-stromatolitic nodules associated with modern, freshwater microbialites located in Pavilion Lake, B.C. Observed carbonate δ13C values enriched by up to + 3.6‰ as compared to predicted abiotic carbonate δ13C values from measured dissolved inorganic carbon (mean − 1.2‰, n = 13) were consistent with microbial photosynthetic influence on in situ precipitation within the nodule microenvironment. Estimated carbonate precipitation temperatures within the nodules based on δ18O were consistent with recorded summertime temperatures, indicative of precipitation during the period of highest levels of photosynthetic activity. Low δ13C values of organic matter within the nodules (− 30.6 to − 21.1‰) and an average inorganic to organic carbon Δδ13C value of 26.8‰ reflected the preferential uptake of 12C during non-CO2 limited photosynthesis, supporting the generation of 13C-enriched DIC. Microelectrode profiles through the nodules showed oxygen supersaturation of up to ∼ 275%, elevated pH compared to ambient water and a lack of any observable dissolved sulphide, Mn or Fe further indicated that photosynthetic activity was the predominant metabolic process within the nodule during light exposure. Microbial phospholipid fatty acid profiles of the nodule communities were indicative of bacteria rather than eukaryotes and PLFA δ13C values were depleted relative to the bulk cell by 2.6–6.6‰, consistent with a predominance of photosynthetic microbes. Scanning electron microscopy images of the relationship between carbonate minerals and filaments indicated that carbonate precipitation had occurred in situ due to microbial influences on the geochemistry within the nodule microenvironment rather than due to cell surface effects or trapping and binding. The observation of photosynthetically induced 13C-enrichment of in situ precipitated carbonate within the nodule microenvironment is thus a biosignature of the activity of these surface communities and is consistent with the hypothesized role of biology in the formation of microbialites.

The Canadian High Arctic contains several of the highest fidelity Mars analogue sites in the world. Situated at nearly 80° north, Expedition Fjord on Axel Heiberg Island is located within a polar desert climate, with the surrounding landscape and conditions providing an invaluable opportunity to examine terrestrial processes in a cold, dry environment. Through the Canadian Space Agency's Analogue Research Network program, scientific activities based out of the McGill Arctic Research Station (M.A.R.S.) are extremely broad in scope, representing physical, biological, and technological investigations. Some of the most unique hydrogeologic features under investigation near M.A.R.S. are a series of cold saline springs that maintain liquid-state flow year round regardless of air temperature. Previous studies have examined their geomorphic relation to discharge-related formations, water chemistry, temperature monitoring, discharge rates, and combined flow/thermal modeling. Recent investigations have identified microbial communities and characterized biological activity within the springs and within permafrost sections, having direct relevance to astrobiological analogue research goals. Another main thrust of research activities based at M.A.R.S. pertains to the detection, mapping, and quantification of subsurface ice deposits. A long-term study is presently underway examining polygonal terrain, comparing surficial patterns found in the region with those identified on Mars, and using surface morphology to estimate ice wedge volumes through a combination of aerial photography interpretation and ground-based geophysical techniques. Other technological developments include the use of in situ microscopy for the detection of biomarkers and improved permafrost drilling techniques. This paper presents an overview of previous studies undertaken at M.A.R.S. over the past decades and will describe in detail both present and upcoming work.

—Spring discharge at Expedition Fiord (Pollard et al., 1999) on Axel Heiberg Island in the Canadian High Arctic produces a variety of travertine forms in addition to a diverse collection of mineral precipitates. This paper focuses on clusters of thermally unstable crystals believed to be the mineral ikaite (CaCO3 · 6H2O) growing seasonally along two spring outflows at Colour Peak. This form of calcium carbonate mineral occurs along small sections of discharge outflow as white euhedral crystals up to 0.5 cm in length. Difficulty in sampling, storage and transport of the samples for analysis has hampered attempts to confirm the presence of ikaite by X-ray diffraction. However, various field observations and the remarkable instability of these crystals at normal ambient temperatures strengthens our argument. This paper provides a description of these particular CaCO3 · 6H2O crystals and their environmental surroundings, and attempts to determine the validity of ikaite precipitation at this site by theoretical geochemical modeling: these results are compared with other reported observations of ikaite to both understand their occurrence and help delineate their geochemical characteristics. It is believed that the restrictive combination of spring water chemistry and long periods of low temperatures characteristic of arctic climates are necessary for ikaite growth at this site. The fact that ikaite is not forming at a second group of saline springs 11 km away allows us to more specifically outline conditions controlling its presence.