Material recycling in a closed aquatic ecosystem. I. Nitrogen transformation cycle and preferential utilization of ammonium to nitrate by phytoplankton as an optimal control problem

A mathematical model of the nitrogen transformation cycle in an aquatic environment is studied. Using Pontryagin's maximum principle, a preferential utilization of ammonium to nitrate by phytoplankton is explained and verified by experimental data. A multiparameter bifurcation is given. The model was found to have four types of equilibrium sets. It is shown that a Hopf bifurcation may occur.     

Libri novi

Ohne Zusammenfassung     

Long-term within pond variation of egg deposition sites in the agile frog, <Emphasis Type="Italic">Rana dalmatina</Emphasis>

This study presents long-term fluctuation, within pond distribution and mortality of egg masses in a population of Rana dalmatina reproducing in a permanent semi-natural pond in Romania. The average number of egg masses per year was 410 (SD = 114.56, min.-max. = 265–581). The mean spawning duration (days) was 26.72 (SD = 5.53). The egg mass distribution was not constant during the years. The long-term change in the distribution of egg masses can be related to changes in the reed cover (pond variable) and landscape structure, i.e. the presence of the forest and a pasture between the pond and the forest. There was a reduction in the number of dead egg masses during the study period. We suggest that the maintenance of the connectivity with the forest and the variable amount of reed assure the main aquatic and terrestrial habitats for the agile frogs. This will allow agile frogs to shift breeding habitats in this area according to their ecological needs and find safe terrestrial habitats for summering and wintering.     

Interactions between Carbon and Nitrogen Metabolism in Fibrobacter succinogenes S85: a 1H and 13C Nuclear Magnetic Resonance and Enzymatic Study

The effect of the presence of ammonia on [1-¹³C]glucose metabolism in the rumen fibrolytic bacterium Fibrobacter succinogenes S85 was studied by ¹³C and ¹H nuclear magnetic resonance (NMR). Ammonia halved the level of glycogen storage and increased the rate of glucose conversion into acetate and succinate 2.2-fold and 1.4-fold, respectively, reducing the succinate-to-acetate ratio. The ¹³C enrichment of succinate and acetate was precisely quantified by ¹³C-filtered spin-echo difference ¹H-NMR spectroscopy. The presence of ammonia did not modify the ¹³C enrichment of succinate C-2 (without ammonia, 20.8%, and with ammonia, 21.6%), indicating that the isotopic dilution of metabolites due to utilization of endogenous glycogen was not affected. In contrast, the presence of ammonia markedly decreased the ¹³C enrichment of acetate C-2 (from 40 to 31%), reflecting enhanced reversal of the succinate synthesis pathway. The reversal of glycolysis was unaffected by the presence of ammonia as shown by ¹³C-NMR analysis. Study of cell extracts showed that the main pathways of ammonia assimilation in F. succinogenes were glutamate dehydrogenase and alanine dehydrogenase. Glutamine synthetase activity was not detected. Glutamate dehydrogenase was active with both NAD and NADP as cofactors and was not repressed under ammonia limitation in the culture. Glutamate-pyruvate and glutamate-oxaloacetate transaminase activities were evidenced by spectrophotometry and ¹H NMR. When cells were incubated in vivo with [1-¹³C]glucose, only ¹³C-labeled aspartate, glutamate, alanine, and valine were detected. Their labelings were consistent with the proposed amino acid synthesis pathway and with the reversal of the succinate synthesis pathway.     

p53 serum antibodies as prognostic indicator in head and neck cancer

p53 antibodies are a new serological parameter of unknown potential in patients with malignancies. Their occurrence has been described in various types of cancer patients. The mechanism underlying the immunization process is still unclear. We investigated the incidence of p53 serum antibodies in 143 head and neck cancer patients with an enzyme-linked immunosorbent assay. The post-therapy course of two matched study groups (n = 38 each), one p53-antibody-seropositive and one p53-antibody-seronegative, was followed up for 24 months. Thirty-nine head and neck cancer patients (27.3%) were seropositive for p53 antibodies. During the follow-up, the p53-antibody-seropositive patients accounted for more local tumor recurrences (n = 12 versus n = 8) and more tumor-related deaths (n = 11 versus n = 5) than did seronegative patients, and second primary tumors (n = 9 versus n = 0) occurred exclusively in seropositive patients. In total, therapy failures (recurrences, tumor-related deaths, second primaries) were observed in 17/38 cases (44.7%) in the p53-antibody-seropositive group and in 8/38 cases (21.1%) in the p53-antibody-seronegative group. These results, after a follow-up of 2 years, seem to indicate a prognostic value of p53 serum antibodies for therapy failure in patients with head and neck cancer. Received: 5 December 1996 / Accepted: 4 January 1997