Redox-Controlled Ammonium Storage and Overturn in Ediacaran Oceans

As a key nutrient, nitrogen can limit primary productivity and carbon cycle dynamics, but also evolutionary progress. Given strong redox-dependency of its molecular speciation, environmental conditions can control nitrogen localization and bioavailability. This particularly applies to periods in Earth history with strong and frequent redox fluctuations, such as the Neoproterozoic. We here report on chlorophyll-derived porphyrins and maleimides in Ediacaran sediments from Oman. Exceptionally light δ15N values (< –10‰) in maleimides derived from anoxygenic phototrophs point towards ammonium assimilation at the chemocline, whereas the isotopic offset between kerogens and chlorophyll-derivatives indicates a variable regime of cyanobacterial and eukaryotic primary production in surface waters. Biomarker and maleimide mass balance considerations imply shallow euxinia during the terminal Ediacaran and a stronger contribution of anoxygenic phototrophs to primary productivity, possibly as a consequence of nutrient ‘lockup’ in a large anoxic ammonium reservoir. Synchronous δ13C and δ15N anomalies at the Ediacaran–Cambrian boundary may reflect one in a series of overturn events, mixing ammonium and isotopically-light DIC into oxic surface waters. By modulating access to nitrogen, environmental redox conditions may have periodically affected Ediacaran primary productivity, carbon cycle perturbations, and possibly played a role in the timing of the metazoan radiation across the terminal Ediacaran and early Cambrian.

Permeation characteristics of tetracyclines in parallel artificial membrane permeation assay II: Effect of divalent metal ions and mucin

<p class="ADMETabstracttext">The bioavailability of tetracyclines is markedly decreased when co-administered with antacids, milk, or food containing Ca²⁺. Previously, it was suggested that the effective intestinal permeation of tetracycline (TC) was decreased due to Ca²⁺ linked mucin binding in the mucosal side. In the present study, we investigated the effect of Ca²⁺, Mg²⁺, and mucin on the membrane permeation of six tetracyclines (TC, oxytetracycline (OTC), minocycline (MINO), doxycycline (DOXY), demeclocycline (DMCTC), and chlortetracycline (CTC)). The membrane permeability values (P_e) of tetracyclines were measured by the parallel artificial membrane permeation assay (PAMPA) using soybean lecithin – decane (SL–PAMPA) and octanol (OCT–PAMPA) membranes. In SL–PAMPA, Ca²⁺ markedly decreased the P_e values of all tetracyclines. In OCT–PAMPA, Ca²⁺ increased the P_e values of TC, CTC, and DMCTC, but not DOXY, OTC, and MINO. Mg²⁺ decreased the P_e values of all tetracyclines in both SL–PAMPA and OCT–PAMPA (except for CTC in OCT–PAMPA). The addition of mucin had little or no effect in all cases. In contrast to the previously suggested mechanism, the results of the present study suggested that Ca²⁺ chelate formation decreased the membrane permeation of tetracyclines, irrespective of Ca²⁺ linked mucin binding. Molecular speciation analysis suggested that the permeation of TC – metal chelates was negligibly small in SL-PAMPA.</p>