Decalcification at the Mantle-Shell Interface in Molluscs

Decalcification at the mantle-shell interface in Mercenariatnercenaria was studied through the changes in the chemicalcomposition of the extrapallial fluid, and by the measurementof Ca⁴⁵-deposition and solution. Measurements of O₂-tensiondemonstrated that the clam was anaerobic soon after the valveswere closed. Measurements of calcium, carbon dioxide, and hydrogenion concentration showed that all of these components of theextrapallial fluid increase with increasing time of closure.These measurements, and measurements of calcium and succinicacid in the tissues and fluids of the clam, indicated thatsuccinicacid produced by the anaerobic metabolism of the clam was neutralizedby the dissolution of previously deposited shell.     

Inter‐regional comparison of land‐use effects on stream metabolism

1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.     

Ionotropic Nucleation of Calcium Carbonate by Molluscan Matrix

The hydrophilic, sulfated fraction of the organic matrix foundin molluscan shells appears to be involved in crystal nucleation.It is located primarily at the sites of initial nucleation.The hydrophilic fraction favors in vitro formation of calcifieddeposits, when it is fixed in place on the hydrophobic fraction.Calcium is bound by the hydrophillic fraction with high affinityand selectivity. Enzymatic desulfation reduces the calcium binding.However, the binding stoichiometry of one calcium for everytwo ester sulfates is not altered. The calcium binding induceslocal anion binding, which induces secondary calcium binding.This coordinated ion binding is known as ionotropy. The resultantlocal high concentration of ions is thought to bring about nucleation.