Trophic interactions in the pelagic

Physiological and nutritional factors that influence the population dynamics of North Sea copepods were identified. Five digestive enzymes were measured in individual copepods. Short-term starvation elicited complex enzyme reactions. Protease isozyme patterns did not change. Seston C:N and POP:TPP correlated with reproductive success. Grazing behaviour and digestive system mediated the effect. Copepods took up N:P in ratios matching their body ratios. C:N and POP ingestion followed the seston pattern. Compensatory feeding was observed. Food quality and stable isotope signatures changed over time (Helgoland, 2004-2005) and between GLOBEC-stations (southern North Sea). Seston d15N and d13C revealed major composition shifts in spring and a strong heterotroph component in summer. A. clausi and T. longicornis utilised different carbon sources despite similar high trophic levels. A low seston d13C (spring) was favourable. Chitobiase activity indicated C assimilation, the other enzymes N assimilation. A low body d13C may denote high turnover rates. Combining methods explains zooplankton- phytoplankton interactions better than only one approach

Biochemische Zusammensetzung und katalytisches Potenzial einzelner Nordsee-Copepoden

Biochemical composition and catalytic potential of individual North Sea copepodsCopepods belong to the most important primary and secondary consumers in marine pelagic systems. They are capable of efficiently utilizing different feeds like phytoplankton or other microzooplankton species. In turn, copepods form the principal food source for animals from higher trophic levels such as larvae of fishes. Estimations about transfer rates of matter and energy between trophic levels as well as the classification of food quality requires detailed knowledge of biochemical parameters on an individual level. Therefore, we adapted and optimized analytical methods to determine biochemical composition and enzyme activities of individual North Sea copepods.The dry mass of individual females ranged between 4.9 and 6.6 µg (Acartia clausii, Acartia tonsa, Corycaeus anglicus, Paracalanus parvus) and between 26 and 38 µg (Centropages hamatus, Centropages typicus, Temora longicornis). The relative amount of protein was higher in the smaller species (30 to 40 % of dry mass) than in the larger ones (16 to 19 % of dry mass). Total lipids were determined in groups of 15 to 30 animals with a microplate-assay which was based on the sulphophosphovanillin-method. Total lipid concentrations ranged between 2.4 and 6 % of dry mass. Low lipid amounts were confirmed by low atomic C:N-ratios of 4:1 to 5:1. The overall energy content as calculated from carbon values amounted from 18.3 to 21.1 J/mg dry mass.The activities of a set of digestive enzyme (chitobiase, β-glucosidase, lipase/esterase, and peptidases) were determined fluorometrically. Enzymes showed different levels of activities between species suggesting different traits of enzymatic food utilization. Zymograms of endopeptidases were determined in individual animals. Each species expressed individual and characteristic patterns of enzymes. Intraspecific variation was insignificant.Both Acartia species as well as Corycaeus anglicus had highest relative protein amounts. Acartia clausii, Paracalanus parvus and Temora longicornis showed the highest lipid contents per dry mass. Five species had relative energy values of 20 to 21 J/mg dry mass. Only Acartia tonsa and Corycaeus anglicus had lower energy values of about 18 J/mg. All species were characterized by a high esterase/lipase activity. Centropages typicus as well as Paracalanus parvus and Temora longicornis can be expected to utilize various food items most efficiently. Centropages hamatus and Temora longicornis showed optimal abilities to digest carbohydrates while peptidases were most active in Centropages typicus and Paracalanus parvus.Keywords: North Sea copepods, protein, lipid, C:N, energy, zymograms, digestive enzymes, fluorogenic substrate

Synchronization of a genetic oscillator with the cell division cycle

Genetic circuits that control specific cellular functions are never fully insulated against influences of other parts of the cell. For example, they are subject to periodic modulation by the cell cycle through volume growth and gene doubling. To investigate possible effects of the cell cycle on oscillatory gene circuits dynamics, we modelled a simple synthetic genetic oscillator, the repressilator, and studied hallmarks of the resulting nonlinear dynamics. We found that the repressilator coupled to the cell cycle shows typical quasiperiodic motion with discrete Fourier spectra and windows in parameter space with synchronization of the two oscillators, with a devil's stair case indicating the Arnold tongues of synchronization. In the case of identical parameters for the three genes of the repressilator and simultaneous gene duplication, we identify two classes of synchronization windows, symmetric and asymmetric, depending on whether the trajectories satisfy a discrete three-fold rotation symmetry, corresponding to cyclic permutation of the three genes. Unexpectedly changing the gene doubling time revealed that the width of the Arnold tongues is connected to that three-fold symmetry of the synchronization trajectories: non-simultaneous gene duplication increases the width of asymmetric synchronization regions, for some of them by an order of magnitude. By contrast, there is only a small or even a negative effect on the window size for symmetric synchronization. This observation points to a control mechanism of synchronization via the location of the genes on the chromosome