Manganese cycling in an acidic Adirondack lake

There is considerable interest in the chemistry of Mn in acidic waters because of its role in the generation of acid neutralizing capacity during reduction processes, as an adsorbent in element cycling, and as a potential toxicant to aquatic organisms. Temporal and spatial variations in the concentration of Mn were evident in acidic Dart's Lake (1.0–2.3 mol l^–1), located in the Adirondack Region of New York. Seasonal changes in pH and dissolved oxygen concentration had subtle effects on the chemistry and transport of Mn. Despite oversaturation with respect to the solubility of manganite during periods of stratification, vertical deposition of Mn was minimal. The conservative nature of Mn appears to be due to the acidic conditions in Dart's Lake.     

Ionic balance,proton efflux,nitrate reductase activity and growth ofHippophaë rhamnoides L. ssp.rhamnoides as influenced by combined-N nutrition or N2 fixation

Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N₂ fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH ₄ ⁺ , a mixture of NH ₄ ⁺ and NO ₃ ^? producing the highest yields. Growth was severely reduced when solely dependent on N₂ fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N₂-fixing plants. Thein vivo nitrate reductase activity (NRA) of NO ₃ ^? dependent plants was almost entirely located in the roots. However, when grown with a combination of NO ₃ ^? and NH ₄ ⁺ , root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH ₄ ⁺ , NO ₃ ^? contributing only for approximately 20 per cent to the total N assimilation. Specific rates of N acquisition and ion uptake were generally highest in NO ₃ ^? +NH ₄ ⁺ plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH ₄ ⁺ plants than in the NO ₃ ^? plants. Measured extrusions of H⁺ or OH^? (HCO ₃ ^? ) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N₂ fixation amounted to 0.45–0.55 meq H⁺. (mmol N_org)^?1. Without acidity control and in the presence of NH ₄ ⁺ , specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N₂-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance. The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites.     

Calcium-activated protease in the giant algaChara australis

Summary There is a protease, which is activated by Ca²⁺ (about 100 M), works at neutral pH and exists in the cytoplasm inChara australis. This protease may correspond to calpain, the calcium-activated neutral protease, which has been studied in animal cells. This is the first report showing the existence of a calcium-activated protease in plant cells.     

Environmental factors influencing the vertical migration of planktonic rotifers in a hypereutrophic tarn

The diel vertical migration of planktonic rotifers in a small, hypereutrophic tarn was investigated on four occasions in 1983. When the tarn was isothermal the rotifers were distributed throughout the water column. After stratification, the rotifers were confined to the top 1–2 m of oxygenated water. On all four dates the rotifers were aggregated at specific depths in the water column. On some occasions, the pattern of aggregation changed as the animals performed distinct diurnal migrations. Keratella cochlearis, K. quadrata and Polyarthra vulgaris usually followed the reverse migrations of the phytoplankton. In contrast, the movements of Anuraeopsis fissa were less pronounced and were associated with variations in the depth of the oxycline.     

Breeding system ofDaphniopsis ephemeralis: adaptations to a transient environment

Populations of the vernal cladoceranDaphniopsis ephemeralis are found in woodland ponds throughout southern Ontario. The species reproduces by cyclic parthenogenesis, and genotype frequencies at allozyme loci are ordinarily in good agreement with Hardy-Weinberg expectations. Occasional heterozygote deficiencies are apparently the consequence of admixture of ephippial hatchlings produced in temporally separated bouts of sexual reproduction. Considerable heterogeneity in genotypic frequencies exists among local populations in southwestern Ontario, indicating that gene flow among populations is restricted. Inbreeding coefficients suggest that populations receive an average of 0.3 migrants per generation. The completion of a sexual life cycle is made possible despite the brief persistence of populations by the emergence of males from ephippial eggs and by the production of equal numbers of male and female progeny in the first parthenogenetic brood.     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

Ion selectivity of epithelial Na channels

Epithelial Na channels are apparently pore-forming membrane proteins which conduct Na much better than any other biologically abundant ion. The conductance to Na can be 100 to 1000 times higher than that to K. The only other ions that can readily get through this channel are protons and Li. Small organic cations cannot pass through the channel, and water may also be impermeant. The selectivity properties of epithelial Na channels appear to be determined by at least three factors: A high field-strength anionic site, most likely a carboxyl residue of glutamic or aspartic acid residues on the channel protein, probably accounts for the high conductance through these channels of Na and Li and to the low conductance of K, Rb and Cs. A restriction in the size of the pore at its narrowest point probably accounts for the low conductance of organic cations as well as the possible exclusion of water molecules. The outer mouth of the channel appears to be negatively charged and may control access to the region of highest selectivity and may serve as a preliminary selectivity filter, attracting cations over anions. These conclusions are illustrated by the cartoon of the channel in Fig. 3. This picture is obviously both fanciful and simplified, but its general points will hopefully be testable. It leaves open a number of important questions, including: does amiloride block the channel by binding within the outer mouth? what does the inner mouth of the channel look like, and does this part of the channel contribute to selectivity? and what, if any, are the interactions between the features of the channel that impart selectivity and those that control the regulation of the channel by hormonal and other factors?     

Fast charge translocations associated with partial reactions of the Na,K-pump: II. Microscopic analysis of transient currents

Summary Nonstationary pump currents which have been observed in K⁺-free Na⁺ media after activation of the Na,K-ATPase by an ATP-concentration jump (see the preceding paper) are analyzed on the basis of microscopic reaction models. It is shown that the behavior of the current signal at short times is governed by electrically silent reactions preceding phosphorylation of the protein; accordingly, the main information on charge-translocating processes is contained in the declining phase of the pump current. The experimental results support the Albers-Post reaction scheme of the Na,K-pump, in which the translocation of Na⁺ precedes translocation of K⁺. The transient pump current is represented as the sum of contributions of the individual transitions in the reaction cycle. Each term in the sum is the product of a net transition rate times a dielectric coefficient describing the amount of charge translocated in a given reaction step. Charge translocation may result from the motion of ion-binding sites in the course of conformational changes, as well as from movement of ions in access channels connecting the binding sites to the aqueous media. A likely interpretation of the observed nonstationary currents consists in the assumption that the principal electrogenic step is the E₁-P/P-E₂ conformational transition of the protein, followed by a release of Na⁺ to the extracellular side. This conclusion is supported by kinetic data from the literature, as well as on the finding that chymotrypsin treatment which is known to block the E₁-P/P-E₂ transition abolishes the current transient. By numerical simulation of the Albers-Post reaction cycle, the proposed mechanism of charge translocation has been shown to reproduce the experimentally observed time behavior of pump currents.     

Modification of Ca2+-activated K+ channels in cultured medullary thick ascending limb cells by N-bromoacetamide

Summary Ca²⁺-activated K⁺ channels were studied in cultured medullary thick ascending limb (MTAL) cells using the patch-clamp technique in the inside-out configuration. The Ca²⁺ activation site was modified using N-bromoacetamide (NBA). 1mm NBA in the bath solution, at 2.5 m Ca²⁺ reduces the open probability,P _o , of the channel to <0.01, without an effect on single-channel conductance. NBA-modified channels are still Ca²⁺-sensitive, requiring 25mm Ca²⁺ to raiseP _o to 0.2. Both before and after NBA modification channel openings display at least two distributions, indicative of more than one open state. High Ca²⁺ (1mm) protects the channels from modification. Also presented is a second class of Ca²⁺-activated K⁺ channels which are normally present in MTAL cells which open infrequently at 10 m Ca²⁺ (P _o =0.01) but have aP _o of 0.08 at 1mm Ca²⁺. We can conclude (i) that NBA modifies the channel by shifting Ca²⁺-sensitivity to very high Ca²⁺, (ii) that NBA acts on a site involved in Ca²⁺ gating, and (iii) that a low affinity channel is present in the apical cell membrane with characteristics similar to those of normal channels modified with NBA.