Abiotic versus Biotic Drivers of Ocean pH Variation under Fast Sea Ice in McMurdo Sound,Antarctica

Ocean acidification is expected to have a major effect on the marine carbonate system over the next century, particularly in high latitude seas. Less appreciated is natural environmental variation within these systems, particularly in terms of pH, and how this natural variation may inform laboratory experiments. In this study, we deployed sensor-equipped moorings at 20 m depths at three locations in McMurdo Sound, comprising deep (bottom depth>200 m: Hut Point Peninsula) and shallow environments (bottom depth ∼25 m: Cape Evans and New Harbor). Our sensors recorded high-frequency variation in pH (Hut Point and Cape Evans only), tide (Cape Evans and New Harbor), and water mass properties (temperature and salinity) during spring and early summer 2011. These collective observations showed that (1) pH differed spatially both in terms of mean pH (Cape Evans: 8.009±0.015; Hut Point: 8.020±0.007) and range of pH (Cape Evans: 0.090; Hut Point: 0.036), and (2) pH was not related to the mixing of two water masses, suggesting that the observed pH variation is likely not driven by this abiotic process. Given the large daily fluctuation in pH at Cape Evans, we developed a simple mechanistic model to explore the potential for biotic processes – in this case algal photosynthesis – to increase pH by fixing carbon from the water column. For this model, we incorporated published photosynthetic parameters for the three dominant algal functional groups found at Cape Evans (benthic fleshy red macroalgae, crustose coralline algae, and sea ice algal communities) to estimate oxygen produced/carbon fixed from the water column underneath fast sea ice and the resulting pH change. These results suggest that biotic processes may be a primary driver of pH variation observed under fast sea ice at Cape Evans and potentially at other shallow sites in McMurdo Sound.     

Analysis of co-transfected expression vectors amplified by methotrexate selection in rat-1 cells.

In an earlier investigation of the influence of high level expression of p21H-ras, rat-1 cells were co-transfected with a selectable vector (pSV2Neo), an amplifiable vector (encoding dihydrofolate reductase; DHFR) and an H-ras expression vector. In this study we have analyzed the gene dose and expression levels of the three co-transfected plasmid vectors in cell lines that had been selected and isolated at different methotrexate concentrations. Growth of the cells in the absence of selection and Southern blot analyses indicate that the transfected vectors are stably co-integrated into the host genome. High expression levels from all three co-transfected vectors were evident at both the mRNA and protein levels, indicating that they are tightly linked in the host genome. The presence of a large amount of unspliced H-ras mRNA in cells expressing high levels of H-ras p21 indicates that processing of mRNA may be rate-limiting. Comparison of the gene dose and expression levels shows that the resistance of cells to increased methotrexate concentrations can occur by different mechanisms. It is concluded that co-transfection of individual plasmid vectors into rat-1 cells, followed by methotrexate selection, is an effective manner of achieving high level expression of proteins in cultured cells.     

Influence of soil thickness on stand characteristics in a Sierra Nevada mixed-conifer forest

Soil thickness can be an important factor influencing vegetation, yet few spatially-explicit studies have examined soil horizon thickness and vegetation composition in summer-drought forests. We compared seismic and soil penetration measurements of combined A + C and Cr horizon thickness, soil moisture and temperature, and stand variables in a contiguous 4-ha mixed-conifer stand of the Sierra Nevada. Thickness of A + C and Cr horizons were highly variable but were not correlated to each other. Total basal area and canopy cover were positively related with A + C horizon thickness, and shrub cover was positively related with Cr horizon thickness. Basal area of white fir [Abies concolor (Gord and Glend) Lindl.] and incense-cedar [Calocedrus decurrens (Torrey) Florin] were positively correlated with A + C horizon thickness, but there was no relationship between A + C or Cr horizon thickness and basal area of Jeffrey pine (Pinus jeffreyi Grev. and Balf.), sugar pine (P. lambertiana Douglas), or red fir (A. magnifica A. Murray). Both white and red fir seedlings were associated with decreased soil temperature, but only white fir seedlings were positively associated with soil moisture. Soil penetration estimates of soil thickness were similar to seismic estimates for shallow soils (<50 cm depth) but were poorly related on deeper soils. Visual surface conditions and tile probe estimates of soil thickness can be highly misleading because ‘shallow’ areas may have a thick layer of weathered bedrock that can serve as a potential rooting medium for deep-rooted trees and shrubs. In our study only the refraction seismic method had the potential to measure total soil depth that included A + C and Cr horizon thickness.     

Drought-induced changes in soil contact and hydraulic conductivity for roots of Opuntia ficus-indica with and without rhizosheaths

Water movement between roots and soil can be limited by incomplete root–soil contact, such as that caused by air gaps due to root shrinkage, and can also be influenced by rhizosheaths, composed of soil particles bound together by root exudates and root hairs. The possible occurrence of air gaps between the roots and the soil and their consequences for the hydraulic conductivity of the root–soil pathway were therefore investigated for the cactus t Opuntia ficus-indica, which has two distinct root regions: a younger, distal region where rhizosheaths occur, and an older, proximal region where roots are bare. Resin-embedded sections of roots in soil were examined microscopically to determine root–soil contact for container-grown plants kept moist for 21 days, kept moist and vibrated to eliminate air gaps, droughted for 21 days, or droughted and vibrated. During drought, roots shrank radially by 30% and root–soil contact in the bare root region of nonvibrated containers was reduced from 81% to 31%. For the sheathed region, the hydraulic conductivity of the rhizosheath was the least limiting factor and the root hydraulic conductivity was the most limiting; for the bare root region, the hydraulic conductivity of the soil was the least limiting factor and the hydraulic conductivity of the root–soil air gap was the most limiting. The rhizosheath, by virtually eliminating root–soil air gaps, facilitated water uptake in moist soil. In the bare root region, the extremely low hydraulic conductivity of the root–soil air gap during drought helped limit water loss from roots to a drier soil.     

Purification and initial characterization of AhrC: the regulator of arginine metabolism genes in Bacillus subtilis

The arginine-dependent repressor-activator from Bacillus subtilis, AhrC, has been overexpressed in Escherichia coli and purified to homogeneity. AhrC, expressed in E. coli, is able to repress a Bacillus promoter (argCp), which lies upstream of the argC gene. The purified protein is a hexamer with a subunit molecular mass of 16.7 kDa. Its ability to recognize DNA has been examined in vitro using argCp in both DNase I and hydroxyl radical protection assays. AhrC binds at two distinct sites within the argCp fragment. One site, argCo1, with the highest affinity for protein, is located within the 5' promoter sequences, whilst the other, argCo2, is within the coding region of argC. The data are consistent with the binding of a single hexamer of AhrC to argCo1 via four of its subunits, possibly allowing the remaining two subunits to bind at argCo2 in vivo forming a repression loop similar to those observed for the E. coli Lac repressor.