187 resultados para onus of proof
Resumo:
The variability in microbial communities (abundance and biomass), bacterial production and ectoaminopeptidase activity, particulate and dissolved organic carbon (POC, DOC), and particulate and dissolved lipids was examined in spring 1995 in the northwestern Mediterranean, where a transition from the end of a bloom to pre-oligotrophic conditions was observed. Four time series of 36 h each and 4 h sampling intervals were performed at 5 m and at the chlorophyll maximum (30 m) between 11 and 31 May. Simultaneous measurements of pigments, abundance of hetero- and autotrophic flagellates, bacteria and POC enabled the estimation of living POC (defined as autotrophic-C plus heterotrophic-C biomass), and thus the detrital organic carbon. During the first 2 time series (11 to 15 May), the bacterial-C biomass was higher than the autotrophic-C biomass at 5 m (ratio 1.4 and 1.7), whereas the opposite trend was observed in the chlorophyll peak (ratio 0.7 for the first cycle). However, at the end of May, autotrophic-C biomass was equivalent to bacterial-C biomass at both depths studied. The detrital pool remained a more or less constant fraction of the POC (52, 53 and 47% on 11-12 May, 14-15 May and 30-31 May) at the chlorophyll peak, whereas it decreased significantly with time (62 to 53%) at 5 m. Relationships between bacterial activities and evolution of available resources were not systematically evidenced from our 36 h diel cycle data. Nevertheless, at the monthly scale, comparison of bacterial carbon demand (BCD) to potential carbon resources (detrital POC and DOC) showed that bacteria fed differently on the various pools. From ectoaminopeptidase turnover rates and detrital POC, the potential hydrolysis rate of detritus was calculated. Depending on the choice of conversion factors for bacterial production and estimates of hydrolysis turnover rates, it was shown that bacterial hydrolysis of detritus could be one of the DOC accumulation sources. We observed that the percentage of BCD supplied by detrital POC hydrolysis increased in the surface and decreased in the chlorophyll peak. An index of lipid degradation in POC, the lipolysis index, increased during the month at 5 m, also indicating a higher hydrolysis of POC. The opposite trend was observed in the chlorophyll maximum layer. The selective decrease in dissolved lipids in DOC in the chlorophyll maximum layer, particularly free fatty acids, also suggests that bacteria utilized increased fractions of carbon sources from the DOC. We concluded that partitioning between DOC and detritus as resources for bacteria can change during the rapid transition period from mesotrophy to oligotrophy in the northwestern Mediterranean.
Resumo:
Abundance distribution and cellular characteristics of picophytoplankton were studied in two distinct regions of the equatorial Pacific: the western warm pool (0°, 167°E), where oligotrophic conditions prevail, and the equatorial upwelling at 150°W characterized by high-nutrient low-chlorophyll (HNLC) conditions. The study was done in September-October 1994 during abnormally warm conditions. Populations of Prochlorococcus, orange fluorescing Synechococcus and picoeukaryotes were enumerated by flow cytometry. Pigment concentrations were studied by spectrofluorometry. In the warm pool, Prochlorococcus were clearly the dominant organisms in terms of cell abundance, estimated carbon biomass and measured pigment concentration. Integrated concentrations of Prochlorococcus, Synechococcus and picoeukaryotes were 1.5x10**13, 1.3x10**11 and 1.5x10**11 cells/m**2, respectively. Integrated estimated carbon biomass of picophytoplankton was 1 g/m**2, and the respective contributions of each group to the biomass were 69, 3 and 28%. In the HNLC waters, Prochlorococcus cells were slightly less numerous than in the warm pool, whereas the other groups were several times more abundant (from 3 to 5 times). Abundance of Prochlorococcus, Synechococcus and picoeukaryotes were 1.2x10**13, 6.2x10**11 and 5.1x10**11 cells/m**2, respectively. The integrated biomass was 1.9 g C/m**2. Prochlorococcus was again the dominant group in terms of abundance and biomass (chlorophyll, carbon); the respective contributions of each group to the carbon biomass were 58, 7 and 35%. In the warm pool the total chlorophyll biomass was 28 mg/m**2, 57% of which was divinyl chlorophyll a. In the HNLC waters, the total chlorophyll biomass was 38 mg/m**2, 44% of which was divinyl chlorophyll a. Estimates of Prochlorococcus, Synechococcus and picoeukaryotes cell size were made in both hydrological conditions.
