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  • 1
    In: Journal of Phycology, August 2009, Vol.45(4), pp.807-811
    Description: Algae of various taxonomic groups are capable of assimilating dissolved organic carbon (DOC) from their environments (mixotrophy). Recently, we reported that, with increasing biomass of mixotrophs, heterotrophic bacteria did not increase. We hypothesized that algal uptake of external DOC may outweigh their release of DOC by exudation (H1). Here, we addressed an alternative hypothesis that algae did not assimilate external DOC but constrained the release of DOC (H2). In chemostat experiments, we cultured the mixotrophic Negoro together with heterotrophic bacteria. As external substrates, we used glucose, which was potentially available for both bacteria and algae, or fructose, which was available only for bacteria. We increased the biomass of algae by the stepwise addition of phosphorus. Bacterial biomass did not increase in experiments using glucose or when fructose was offered, suggesting that mechanisms other than algal mixotrophy (H1) kept concentrations of bacteria low. Measured exudation rates (percent extracellular release, PER) of mixotrophic algae ( W. Krüger) were very low and ranged between 1.0% and 3.5% at low and moderately high phosphorus concentrations. In contrast, an obligately phototrophic alga ( H. Ettl) showed higher exudation rates, particularly under phosphorus limitation (70%). The results support H2. If mixotrophy is considered as a mechanism to recycle organic exudates from near the cell surface, this would explain why algae retained mixotrophic capabilities although they cannot compete with bacteria for external organic carbon.
    Keywords: Algae ; Bacteria ; Chemostat ; Competition ; Doc ; Exudation ; Mixotroph
    ISSN: 0022-3646
    E-ISSN: 1529-8817
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  • 2
    Language: English
    In: Hydrobiologia, 2010, Vol.649(1), pp.379-383
    Description: To investigate the consequences of increased temperature and enhanced input of dissolved organic matter (DOM) into lakes for heterotrophicic bacteria and for mixotrophic algae which use DOM in addition to photosynthesis, the hypotheses were tested whether (1) both bacteria and mixotrophic algae benefit from increased input of DOM, or (2) increased DOM input enhances bacterial biomass and thereby decreases algal biomass. Growth experiments in batch cultures, exudation measurements, and competition experiments in chemostats were performed at two temperature levels. Increased temperature stimulated the autotrophic growth rate of Chlorella protothecoides . Bacteria and Chlorella increased their heterotrophic growth rates at higher DOM concentration at lower temperature whereas enhanced DOM concentration hardly stimulated their growth at higher temperature. In chemostats, enhanced input of soil extract increased both bacterial and algal biomass at lower temperature whereas bacterial biomass increased only slightly and algal biomass decreased at higher temperature. Thus, the temperature determines the response of microorganisms to enhanced DOM concentration.
    Keywords: Bacteria ; Mixotrophic algae ; Temperature ; DOM ; Climate change ; Chemostat
    ISSN: 0018-8158
    E-ISSN: 1573-5117
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  • 3
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 28 October 2003, Vol.100(22), pp.12776-81
    Description: The majority of organisms can be grouped into those relying solely on photosynthesis (phototrophy) or those relying solely on the assimilation of organic substances (heterotrophy) to meet their requirements for energy and carbon. However, a special life history trait exists in which organisms combine both phototrophy and heterotrophy. Such "mixotrophy" is a widespread phenomenon in aquatic habitats and is observed in many protozoan and metazoan organisms. The strategy requires investment in both photosynthetic and heterotrophic cellular apparatus, and the benefits must outweigh these costs. In accordance with mechanistic resource competition theory, laboratory experiments revealed that pigmented mixotrophs combined light, mineral nutrients, and prey as substitutable resources. Thereby, they reduced prey abundance below the critical food concentration of competing specialist grazers [Rothhaupt, K. O. (1996) Ecology 77, 716-724]. Here, we demonstrate the important consequences of this strategy for an aquatic community. In the illuminated surface strata of a lake, mixotrophs reduced prey abundance steeply. The data suggest that, as a consequence, grazers from higher trophic levels, consuming both the mixotrophs and their prey, could not persist. Thus, the mixotrophs escaped from competition with and losses to higher grazers. Furthermore, the mixotrophs structured prey abundance along the vertical light gradient, creating low densities near the surface and a pronounced maximum of their algal prey at depth. Such deep algal accumulations are typical features of nutrient-poor aquatic habitats, previously explained by resource availability. We hypothesize instead that the mixotrophic grazing strategy is responsible for deep algal accumulations in many aquatic environments.
    Keywords: Food Chain ; Chlamydomonas -- Physiology
    ISSN: 0027-8424
    E-ISSN: 10916490
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  • 4
    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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  • 5
    Language: English
    In: Journal of phycology, 2008, Vol.44(3), pp.616-623
    Description: Dissolved organic carbon (DOC) constitutes the bulk of organic carbon in aquatic environments. The importance of DOC utilization by mixotrophic algae is unclear since heterotrophic bacteria are regarded as more efficient users. We tested the hypothesis that algae decrease the DOC concentration in the light to lower levels than in darkness resulting in competitive exclusion of heterotrophic bacteria according to the mechanistic competition theory. We investigated (a) the uptake kinetics of glucose as a model substrate by two cultured algae and mixed bacteria populations, (b) the competition for glucose between algae and bacteria in chemostats, (c) the effect of discontinuous glucose supply in chemostats, and (d) the minimum glucose concentrations achieved in cultures of algae and bacteria. Bacteria showed higher specific-glucose-uptake rates than algae. In chemostats, algae became extinct in the dark and coexisted in the light where they decreased bacteria to lower densities. Discontinuous glucose supply promoted the algae compared to continuous substrate addition. Several algae consumed glucose to lower concentrations in the dark than in the light and showed lower or equal residual glucose concentrations than bacteria. Residual concentrations were not related to allometric traits (cell volume) and photosynthetic potential (chl content). Overall, the hypothesis was not supported, and mechanisms of competition for DOC obviously differed from those for particulate prey. However, since some algae showed lower or equal residual glucose concentrations than bacteria, algal dark uptake of DOC may be important in deep layers of many waters. ; Includes references ; p. 616-623.
    Keywords: Bacteria ; Algae ; Mixotrophy ; Doc ; Competition
    ISSN: 0022-3646
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  • 6
    In: Freshwater Biology, 09 February 2002, Vol.47(2), pp.243-250
    Description: 1. The ability of roach to use cyanobacterial food is generally believed to be one reason for the dominance of roach over perch in eutrophic European lakes. The aim of this study was to test whether cyanobacteria really are a suitable food for juvenile roach. Special attention was paid to differences between the two cyanobacteria species and which are common in eutrophic lakes and are ingested by roach there. 2. We performed growth and behaviour experiments with juvenile roach fed with zooplankton and the different cyanobacteria. Growth rate with was lower than with but significantly higher than without food, whereas growth rate with was as low as without food. 3. In cultivation experiments of roach faeces, was found not to have been digested and grew exponentially after passing through the gut whereas stayed at low biomass. Differences in growth were not related to the toxin content of cyanobacteria. Investigations of roach motility showed no differences whether fed with or . 4. In contrast to , can be regarded as a suitable food source for juvenile roach probably because of its better digestability. We conclude that the ability to feed on cyanobacteria is not a general competitive advantage for roach, but the outcome depends on the species composition of the cyanobacteria.
    Keywords: Aphanizomenon ; Cyanobacteria ; Microcystis ; Roach ; Zooplankton
    ISSN: 0046-5070
    E-ISSN: 1365-2427
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  • 7
    In: Journal of Phycology, June 2008, Vol.44(3), pp.616-623
    Description: Dissolved organic carbon (DOC) constitutes the bulk of organic carbon in aquatic environments. The importance of DOC utilization by mixotrophic algae is unclear since heterotrophic bacteria are regarded as more efficient users. We tested the hypothesis that algae decrease the DOC concentration in the light to lower levels than in darkness resulting in competitive exclusion of heterotrophic bacteria according to the mechanistic competition theory. We investigated (a) the uptake kinetics of glucose as a model substrate by two cultured algae and mixed bacteria populations, (b) the competition for glucose between algae and bacteria in chemostats, (c) the effect of discontinuous glucose supply in chemostats, and (d) the minimum glucose concentrations achieved in cultures of algae and bacteria. Bacteria showed higher specific‐glucose‐uptake rates than algae. In chemostats, algae became extinct in the dark and coexisted in the light where they decreased bacteria to lower densities. Discontinuous glucose supply promoted the algae compared to continuous substrate addition. Several algae consumed glucose to lower concentrations in the dark than in the light and showed lower or equal residual glucose concentrations than bacteria. Residual concentrations were not related to allometric traits (cell volume) and photosynthetic potential (chl content). Overall, the hypothesis was not supported, and mechanisms of competition for DOC obviously differed from those for particulate prey. However, since some algae showed lower or equal residual glucose concentrations than bacteria, algal dark uptake of DOC may be important in deep layers of many waters.
    Keywords: Algae ; Bacteria ; Competition ; Doc ; Mixotrophy
    ISSN: 0022-3646
    E-ISSN: 1529-8817
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  • 8
    Language: English
    In: SIL Proceedings, 1922-2010, 01/1995, Vol.26(2), pp.712-714
    Description: .
    Keywords: Secchi Discs ; Dominant Species ; Grazing ; Zooplankton ; Phosphorus ; Phytoplankton ; Competition ; Melosira ; Cyclotella ; Daphnia ; Cryptomonas ; Germany, Sachsen, Bautzen Reservoir ; Freshwater ; Plankton ; Biomass ; Cryptomonas ; Cyclotella ; Melosira ; Secchi Discs ; Biomanipulation ; Biomass ; Competition ; Dominant Species ; Grazing ; Phosphorus ; Phytoplankton ; Zooplankton;
    ISBN: 3510540468
    ISSN: 0368-0770
    Source: Informa - Taylor & Francis (via CrossRef)
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