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  • 1
    Language: English
    In: Limnologica, 2008, Vol.38(3), pp.360-366
    Description: Since amino acids represent an important component of dissolved organic carbon in lakes, we investigated the uptake and consumption of leucine by several phytoplankton species. Firstly, we measured the leucine uptake of 28 phytoplankton species (several cyanobacteria and chlorophytes, one diatom, and one euglenophyte) and the uptake kinetics by a chlorophyte ( ) compared to that of heterotrophic bacteria. Furthermore, we tested whether the algae can decrease the concentration of leucine in the light to lower levels than in darkness (hypothesis 1), and whether algae with high minimum substrate requirements exhibit higher consumption rates at plentiful concentrations compared to algae with high substrate reduction capability but low maximum consumption rate (hypothesis 2). Thirteen species of cyanobacteria and chlorophytes showed leucine uptake. Specific uptake rates by were lower in the light than in the dark and much lower than that of heterotrophic bacteria. In the consumption experiments, several algae consumed leucine with higher rates and to lower residual concentrations in the dark than in the light, but with lower rates and not to lower concentrations than heterotrophic bacteria. Residual concentrations and consumption rates were not related to algal cell volume and chlorophyll content. Consumption rates were negatively related to residual concentrations, i.e. algae with higher consumption rates also depleted leucine to lower concentrations. Although the hypotheses were not supported, several algae were capable of removing leucine to equally low concentrations as bacteria so that algal uptake of amino acids is potentially important in natural waters.
    Keywords: Algae ; Amino Acids ; Bacteria ; Competition ; Cyanobacteria ; Doc ; Leucine ; Mixotrophy ; Oceanography ; Ecology
    ISSN: 0075-9511
    E-ISSN: 1873-5851
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  • 2
    Language: English
    In: Limnologica, 2009, Vol.39(2), pp.107-114
    Description: The zooplankton of oligotrophic lakes in North Patagonia is often dominated by mixotrophic ciliates, particularly and Therefore, we tested whether spp. (i) is an important food for juvenile endemic ( , , , ) and introduced ( ) fish species, and (ii) represents a remarkable grazer of bacteria. Ingestion rates of fish estimated by disappearance of in feeding experiments ranged between 8 ( ) and 53 ( ) ciliates per fish and day, and assimilation rates measured by using radioactively labelled ranged between 3 ( ) and 52 ( ) ciliates per fish and day. However, although we detected the consumption of by fish, the daily consumption amounted to at most 0.2% of the fish biomass which can not cover the energy requirement of the fish. Furthermore, the daily consumption was equivalent to a maximum of 1.6% of the standing stock so that fish predation does not seem to be an important mortality factor for the ciliates. The clearance rate of sp. on natural bacteria was on average 3.8 μl cil h . The daily ingestion (mean 3.9 ng C cil d ) was about 3.5% of the individual biomass of sp. Therefore, bacteria ingestion might explain a ciliate growth rate of appr. 1% d , which was about 17% of the photosynthesis of endosymbiotic algae. The maximum density of sp. in the lake could ingest about 1 μg C L d bacteria which is only 3% of average bacterial production. Thus, grazing by sp. does not seem to be a main loss factor for the bacteria.
    Keywords: Mixotrophic Ciliates ; Stentor ; Fish Predation ; Bacteria ; North Patagonian Lakes ; Oceanography ; Ecology
    ISSN: 0075-9511
    E-ISSN: 1873-5851
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  • 3
    Language: English
    In: Limnologica, 2010, Vol.40(2), pp.161-166
    Description: Acidic mining lakes (pH 〈3) are specific habitats exhibiting particular chemical and biological characteristics. The species richness is low and mixotrophy and omnivory are common features of the plankton food web in such lakes. The plankton community structure of mining lakes of different morphometry and mixing type but similar chemical characteristics (Lake 130, Germany and Lake Langau, Austria) was investigated. The focus was laid on the species composition, the trophic relationship between the phago-mixotrophic flagellate sp. and bacteria and the formation of a deep chlorophyll maximum along a vertical pH-gradient. The shallow wind-exposed Lake 130 exhibited a higher species richness than Lake Langau. This increase in species richness was made up mainly by mero-planktic species, suggesting a strong benthic/littoral – pelagic coupling. Based on the field data from both lakes, a nonlinear, negative relation between bacteria and biomass was found, suggesting that at an biomass below 50 μg C L , the grazing pressure on bacteria is low and with increasing biomass bacteria decline. Furthermore, in Lake Langau, a prominent deep chlorophyll maximum was found with chlorophyll concentrations ca. 50 times higher than in the epilimnion which was build up by the euglenophyte sp. We conclude that lake morphometry, and specific abiotic characteristics such as mixing behaviour influence the community structure in these mining lakes.
    Keywords: Acidic Lake ; Mining Lake ; Plankton ; Mixotrophy ; Rotifers ; Flagellates ; Ochromonas ; Lepocinclis ; Chlamydomonas ; Deep Chlorophyll Maximum ; Oceanography ; Ecology
    ISSN: 0075-9511
    E-ISSN: 1873-5851
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