Temporal and spatial trends in heavy metal concentrations in the marine mussel Mytilus edulis from the Western Scheldt estuary (The Netherlands)

Since the first North Sea Conference (1984, Bremen), all countries bordering the North Sea made commitments to reduce discharge of hazardous substances into the North Sea. From Belgium and The Netherlands, large reductions (upto 90) in heavy metal emissions from land-based sources have been reported between 1985 and 2000. Recently, some studies in the Western Scheldt estuary (WS) have shown that total metal concentration in the water, sediments and suspended particles have decreased compared to levels in the 70s. However, data on aquatic organisms is still very limited and it is therefore difficult to confirm whether the reductions in pollution input and generally improving water quality in the WS have a corresponding impact on the levels of heavy metals in aquatic organisms. The current study measured metal concentrations in the soft tissues of mussels, Mytilus edulis (known to be good indicators of environmental metal pollution) during the period 1996–2002. Spatial (salinity and pollution gradients), temporal and seasonal variations were also studied. Results showed a down-stream decreasing trend for the metals studied (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during all sampling campaigns. There was also a significant seasonal effect on tissue metal concentrations, with a peak observed around spring in both WS and the nearby less polluted Eastern Scheldt (ES). On temporal trends, a clear drop of metals in mussels was observed in the early 80s coinciding with the start of the efforts to reduce chemical pollution input into the North Sea. Since those early reductions, metal concentrations in mussels generally remained unchanged upto mid 90s. However, in recent times metal concentration in mussels have increased significantly, for example Cd in 2002 was almost 10 times the values in 1983 and similar to levels seen during the peak in the 70s. Other metals also increased in the 90s also reaching levels seen in the 70s. As there is no indication of recent increase in metal input into the estuary, we suggest that increased metal concentrations observed in mussels in recent years especially in the upper estuary are most likely a result of changes in physical and chemical speciation and metal bioavailability. Such changes may be caused by changes in some water quality parameters in the estuary (i.e. increased dissolved oxygen, concentration of organic matter), resulting in conditions that favour releases of sediment-bound metals into the water column. The relationship between metal content and season showed very similar annual profiles in the polluted WS and less polluted ES. Thus, seasonal variations in metal concentrations appear to be largely controlled by biological processes, while total body burdens are dependent on environmental levels and bioavailability.     

ADP binding to myosin cross-bridges and its effect on the cross-bridge detachment rate constants

We have studied the binding of adenosine diphosphate (ADP) to attached cross-bridges in chemically skinned rabbit psoas muscle fibers and the effect of that binding on the cross-bridge detachment rate constants. Cross-bridges with ADP bound to the active site behave very similarly to cross-bridges without any nucleotide at the active site. First, fiber stiffness is the same as in rigor, which presumably implies that, as in rigor, all the cross-bridges are attached. Second, the cross-bridge detachment rate constants in the presence of ADP, measured from the rate of decay of the force induced by a small stretch, are, over a time scale of minutes, similar to those seen in rigor. Because ADP binding to the active site does not cause an increase in the cross-bridge detachment rate constants, whereas binding of nucleotide analogues such as adenyl-5'-yl imidodiphosphate (AMP-PNP) and pyrophosphate (PPi) do, it was possible, by using ADP as a competitive inhibitor of PPi or AMP-PNP, to measure the competitive inhibition constant and thereby the dissociation constant for ADP binding to attached cross-bridges. We found that adding 175 microM ADP to 4 mM PPi or 4 mM AMP-PNP produces as much of a decrease in the apparent cross-bridge detachment rate constants as reducing the analogue concentration from 4 to 1 mM. This suggests that ADP is binding to attached cross-bridges with a dissociation constant of approximately 60 microM. This value is quite similar to that reported for ADP binding to actomyosin subfragment-1 (acto-S1) in solution, which provides further support for the idea that nucleotides and nucleotide analogues seem to bind about as strongly to attached cross-bridges in fibers as to acto-S1 in solution (Johnson, R.E., and P. H. Adams. 1984. FEBS Letters. 174:11-14; Schoenberg, M., and E. Eisenberg. 1985. Biophysical Journal. 48:863-871; Biosca, J.A., L.E. Greene, and E. Eisenberg. 1986. Journal of Biological Chemistry. 261:9793-9800).     

Adaptation of Phytoplankton-Degrading Microbial Communities to Thermal Reactor Effluent in a New Cooling Reservoir

In water column and sediment inocula from a nuclear reactor cooling reservoir, natural phytoplankton substrate labeled with ¹⁴C was used to determine aerobic and anaerobic mineralization rates for a range of temperatures (25, 40, 55, and 70°C) expected during reactor operation. For experiments that were begun during reactor shutdown, aerobic decomposition occurred at temperatures of <55°C. After 2 months of reactor operation, aerobic rates increased substantially at 55 and 70°C, although maximum rates were observed at temperatures of ≤40°C. The temperature range for which maximum anaerobic mineralization (i.e., the sum of CH₄ and CO₂) was observed was 25 to 40°C when the reactor was off, expanding to 25 to 55°C during reactor operation. Increased rates at 55°C, but not 70°C, correlated with an increase in the ratio of cumulative methane to carbon dioxide produced over 21 days. When reduced reactor power lowered the maximum temperature of the reservoir to 42°C, aerobic decomposition at 70°C was negligible, but remained substantial at 55°C. Selection for thermophilic decomposers occurred rapidly in this system in both aerobic and anaerobic communities and did not require prolonged exposure to elevated temperatures.     

Regulated nuclear polyadenylation of Xenopus albumin pre-mRNA.

Cytoplasmic regulation of the length of poly(A) on mRNA is a well-characterized process involved in translational control during development. In contrast, there is no direct in vivo evidence for regulation of the length of poly(A) added during nuclear pre-mRNA processing in somatic cells. We previously reported that Xenopus serum albumin [Schoenberg et al. (1989) Mol. Endocrinol. 3, 805-815] and transferrin [Pastori et al. (1992) J. Steroid Biochem. Mol. Biol. 42, 649-657], mRNA have exceptionally short poly(A) tails ranging from 12 to 17 residues, whereas vitellogenin mRNA has long poly(A). An RT-PCR protocol was adapted to determine the length of poly(A) added onto pre-mRNA, defined here as that species bearing the terminal intron. Using this assay we show that vitellogenin pre-mRNA has the same long poly(A) tail as mature vitellogenin mRNA. In contrast, albumin pre-mRNA has the same short poly(A) as found on fully-processed albumin mRNA. These results indicate that the short poly(A) tail on albumin mRNA results from regulation of poly(A) addition during nuclear 3' processing.     

Human immunodeficiency virus type 1 Rev is required in vivo for binding of poly(A)-binding protein to Rev-dependent RNAs.

In the absence of Rev or the Rev-responsive element, the Rev-dependent human immunodeficiency virus type 1 (HIV-1) RNAs do not behave as mRNAs; rather, they exhibit nuclear defects in splicing and/or nuclear export and cytoplasmic defects in stability and translation. A translational initiation factor, eIF-5A, has recently been shown to bind specifically to the Rev activation domain. As the binding of poly(A)-binding protein 1 (PAB1) to the poly(A) tail of mRNAs is involved in both the stability and translation of cytoplasmic mRNAs, we investigated whether Rev might influence the association of PAB1 with cytoplasmic HIV-1 RNAs. Antibodies were generated against PAB1. We used these antibodies in an immunoprecipitation assay to detect specific binding of PAB1 to cytoplasmic mRNAs. We found that in the presence of Rev, PAB1 was associated with Rev-dependent and Rev-independent RNAs in the cytoplasm of transfected cells. However, in the absence of functional Rev, we found little or no PAB1 associated with Rev-dependent RNAs. These RNAs were capable of binding PAB1 in vitro. These results demonstrate that HIV-1 RNAs are defective in PAB1 association in the absence of Rev.     

NITROGEN LIMITATION IN ISOCHRYSIS GALBANA (HAPTOPHYCEAE). I. PHOTOSYNTHETIC ENERGY CONVERSION AND GROWTH EFFICIENCIES1

The effect of steady-state nitrogen limitation on photo-synthetic characteristics and growth efficiency was examined in the marine haptophyte Isochrysis galbana Green. Nitrate limited chemostats were maintained at nine dilution rates, ranging from 0.18-0.96 d^?1, under continuous irradiance levels of 175 μmole quanta·m^?2·s^?1, an irradiance level which saturated photosynthesis at all growth rates. Nitrogen limitation led to an overall reduction in pigmentation and a decrease in the cellular concentration of reaction centers; however, the optical absorption cross section, normalized to Chl a, increased. Moreover, Chl c/a ratios were higher in nitrogen-limited cells: the change in Chl c/a ratios were correlated with an increase in the functional size of Photosystem II. Both light saturated photosynthetic rates normalized per cell and specific respiratory losses were positively linearly correlated with growth rate. Light saturated photosynthetic rates normalized to Chl a remained relatively insensitive to the rate of nitrogen supply. The minimum quantum requirement for gross photosynthetic oxygen evolution increased from 12.4 to 17.0 quanta/O₂. At the growth irradiance, the quantum requirement increased 88%, from 19.9 to 37.5 quauta/O₂ Photosynthesis/respiration ratios remained relatively constant at dilution rates greater than 35% of the maximum relative growth rate. Consequently, net growth efficiency, defined as the ratio of the specific growth rate, μ, to specific gross photosynthesis, P, also remained relatively constant over this range of growth rates averaging 85 ± 3%.     

Coordinate estrogen-regulated instability of serum protein-coding messenger RNAs in Xenopus laevis

Estrogen causes the cytoplasmic destabilization of albumin and gamma-fibrinogen mRNA in Xenopus laevis liver. The purpose of the present study was to determine whether mRNA destabilization is a generalized phenomenon in response to estrogen, or whether this process is restricted to a particular class of mRNAs. To address this, we have expanded our bank of serum protein-coding cDNA clones to include transferrin, the second protein of inter-alpha-trypsin inhibitor and clone 12B, for which there is no mammalian homolog. Together with albumin and gamma-fibrinogen, these represent more than 85% of the mRNAs encoding liver secreted proteins. Estrogen administration to male Xenopus or to liver explant cultures causes the generalized disappearance of all of these mRNAs. In contrast, estrogen has no effect on actin, ferritin, or poly(A)-binding protein mRNA, all of which encode intracellular proteins. We have previously demonstrated that albumin mRNA is degraded in both messenger ribonucleoprotein and polysome fractions. Sucrose gradient analysis demonstrates the same pattern for degradation of all other serum protein-coding mRNAs. Estrogen has no effect on the amounts or gradient distribution of actin, ferritin, or poly(A)-binding protein mRNA. We conclude that regulated destabilization of mRNAs encoding secreted proteins is a generalized phenomenon in response to estrogen stimulation of Xenopus liver.