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- Institut für Biochemie und Biologie (19) (remove)
The fate of allochthonous dissolved organic carbon (DOC) in aquatic systems is primarily controlled by the turnover of heterotrophic bacteria. However, the roles that abiotic and biotic factors such as light and DOC release by aquatic primary producers play in the microbial decomposition of allochthonous DOC is not well understood. We therefore tested if light and autochthonous DOC additions would increase allochthonous DOC decomposition rates and change bacterial growth efficiencies and community composition (BCC). We established continuous growth cultures with different inocula of natural bacterial communities and alder leaf leachates (DOCleaf) with and without light exposure before amendment. Furthermore, we incubated DOCleaf together with autochthonous DOC from lysed phytoplankton cultures (DOCphyto). Our results revealed that pretreatments of DOCleaf with light resulted in a doubling of bacterial growth efficiency (BGE), whereas additions of DOCphyto or combined additions of DOCphyto and light had no effect on BGE. The change in BGE was not accompanied by shifts in the phylogenetic structure of the BCC, but BCC was influenced by the DOC source. Our results highlight that a doubling of BGE is not necessarily accompanied by a shift in BCC and that BCC is more strongly affected by resource properties.
It is currently controversially discussed if the same freshwater microorganisms occur worldwide wherever their required habitats are realized, i.e., without any adaptation to local conditions below the species level. We performed laboratory experiments with flagellates and ciliates from three acidic mining lakes (AML, pH similar to 2.7) to investigate if similar habitats may affect similar organisms differently. Such man-made lakes provide suitable ecosystem models to test for the significance of strong habitat selection. To this end, we analyzed the growth response of three protist taxa (three strains of the phytoflagellate Chlamydomonas acidophila, two isolates of the phytoflagellate Ochromonas and two species of the ciliate genus Oxytricha) by exposing them to lake water of their origin and from the two other AML in a cross-factorial design. Population growth rates were measured as a proxy for their fitness. Results revealed significant effects of strain, lake (= habitat), and strain X habitat interaction. In the environmentally most adverse AML, all three protist taxa were locally adapted. In conclusion, our study demonstrates that (1) the same habitat may affect strains of the same species differently and that (2) similar habitats may harbor ecophysiologically different strains or species. These results contradict the 'everything is everywhere' paradigm.
Terrestrial-derived dissolved organic carbon (DOC) contributes significantly to the energetic basis of many aquatic food webs. Although heterotrophic bacteria are generally considered to be the sole consumers of DOC, algae and cyanobacteria of various taxonomic groups are also capable of exploiting this resource. We tested the hypothesis that algae can utilise DOC in the presence of bacteria if organic resources are supplied in intervals by photolysis of recalcitrant DOC. In short-term uptake experiments, we changed irradiation in the range of minutes. As model substrates, polymers of radiolabelled coumaric acid (PCA) were used, which during photolysis are known to release aromatic compounds comparable to terrestrial-derived and refractory DOC. Three cultured freshwater algae readily assimilated PCA photoproducts equivalent to a biomass-specific uptake of 5-60% of the bacterial competitors present. Algal substrate acquisition did not depend on whether PCA was photolysed continuously or in intervals. However, the data show that photoproducts of terrestrial DOC can be a significant resource for osmotrophic algae. In long-term growth experiments, interval light was applied one hour per day. We allowed cultured Chlamydomonas to compete for ambient DOC of low concentration. We found higher abundances of Chlamydomonas when cultures were irradiated intermittently rather than continuously. These data suggest that photolysis of DOC supports algal heterotrophy, and potentially facilitates growth, when light fluctuations are large, as during the diurnal light cycle. We concluded that osmotrophic algae can efficiently convert terrestrial carbon into the biomass of larger organisms of aquatic food webs.
The zooplankton of oligotrophic lakes in North Patagonia is often dominated by mixotrophic ciliates, particularly Stentor amethystinus and Stentor araucanus. Therefore, we tested whether Stentor spp. (i) is an important food for juvenile endemic (Cheirodon australe, Galaxias maculatus, Odontesthes mauleanum, Percichthys trucha) and introduced (Oncorhynchus mykiss) fish species, and (ii) represents a remarkable grazer of bacteria. Ingestion rates of fish estimated by disappearance of Stentor in feeding experiments ranged between 8 (G. maculatus) and 53 (C australe) ciliates per fish and day, and assimilation rates measured by using radioactively labelled Stentor ranged between 3 (P. trucha) and 52 (C australe) ciliates per fish and day. However, although we detected the consumption of Stentor 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 Stentor standing stock so that fish predation does not seem to be an important mortality factor for the ciliates. The clearance rate of Stentor sp. on natural bacteria was on average 3.8 mu l cil(-1) h(-1). The daily ingestion (mean 3.9 ngC cil(-1) d(-1)) was about 3.5% of the individual biomass of Stentor sp. Therefore, bacteria ingestion might explain a ciliate growth rate of appr. 1% d(-1), which was about 17% of the photosynthesis of endosymbiotic algae. The maximum density of Stentor sp. in the take could ingest about 1 mu g C L-1 d(-1) bacteria which is only 3% of average bacterial production. Thus, grazing by Stentor sp. does not seem to be a main loss factor for the bacteria.
We investigated the response of the microbial components of the pelagic food web to re-oligotrophication of large, deep Lake Constance where total phosphorus concentrations during mixing decreased from a maximum of 2.81 mu mol L- 1 in 1979 via 1.87 mu mol L-1 in 1987 to 0.26 mu mol L-1 in 2007. Measurements of heterotrophic bacteria, autotrophic picoplankton (APP) and heterotrophic nanoflagellates (HNF) in 2006 and 2007 were compared to values from 1987 to 1997. We hypothesized that the biomass and seasonal variability of all groups will decrease under more oligotrophic conditions due to reduced resource availability, particularly for APP and HNF but less for the competitively stronger bacteria. Average bacterial biomass between spring and autumn was unrelated to phosphorus, whereas the ratio of bacterial biomass to chlorophyll a concentration increased with decreasing trophy due to declining chlorophyll concentrations. In contrast, a unimodal relationship was found between APP and phosphorus with low biomass at low and high phosphorus concentrations and maximum biomass in between. Average HNF biomass decreased strongly by a factor of 10-30 with decreasing trophy, and chlorophyll-specific HNF biomass was unimodally related to phosphorus. The relative seasonal biomass variability did not change for any group during re-oligotrophication. To conclude, HNF responded much more strongly and bacteria less so than chlorophyll concentrations to oligotrophication, whereas APP exhibited a more complex pattern.
Lake morphometry and wind exposure may shape the plankton community structure in acidic mining lakes
(2010)
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 Ochromonas 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 Ochromonas biomass was found, suggesting that at an Ochromonas biomass below 50 mu g CL-1. the grazing pressure on bacteria is low and with increasing Ochromonas 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 Lepocinclis sp. We conclude that lake morphometry, and specific abiotic characteristics such as mixing behaviour influence the community structure in these mining lakes.
Bacterivory by mixotrophic flagellates may contribute to their nutrient supply, providing a competitive advantage in oligotrophic waters. We hypothesized an increase in Dinobryon biomass during the re-oligotrophication process in the large and deep Lake Constance. To estimate whether bacterivory contributed substantially to the flagellates' phosphorus supply, we determined ingestion rates. Dinobryon biomass increased with decreasing total phosphorus concentrations in the lake over a period of 17 years (P = 0.0005). The promotion of Dinobryon biomass during re-oligotrophication may be explained by the increasing light availability due to the decreasing biomass of other phytoplankton yielding a release from competition. The date of the Dinobryon abundance maximum shifted to earlier time points in the year, probably because a smaller phosphorus pool was depleted more quickly. Ingestion rates of Dinobryon ranged between 0.5 and 13 bacteria cell(-1) h(-1) (0.2-5.4 fg C pg C-1 h(-1)), and clearance rates varied between 0.2 and 3.2 nL cell(-1) h(-1) (4-78 pL pg C-1 h(-1)), leading to bacterial losses of up to 30% day(-1) of bacterial standing stock. The ingestion of bacteria covered 77% of the phosphorus need of the flagellate during the period of maximum growth in 1996 (net growth rate 0.34 day(-1)), and it fully covered the need at all other times.
In experiments with axenic cultures of Microcystis aeruginosa, we tested whether this cyanobacterium incorporates leucine, a compound that is often used for the measurement of heterotrophic bacterioplankton production. Microcystis showed significant leucine incorporation, and the uptake of exponentially growing cells was higher than the uptake of cells in stationary growth phase. Therefore, the leucine method may not be suitable for measuring bacterial production in highly eutrophic waters with a dominance of cyanobacteria.
The filtration rate of Daphnia galeata was determined in in situ experiments in Bautzen Reservoir and in laboratory experiments, where daphnids were exposed to filtrates that previously contained either natural phytoplankton or cultured eukaryotic algae (Scenedesmus obliquus or Asterionella formosa), respectively. Individual filtration rate (FR) was measured using fluorescent beads, taking into account ingested beads in the gut only. Compared to heated control treatments (100°C), dissolved compounds released by the nutritious cultured algae during the preconditioning phase or by the natural phytoplankton assemblages from Bautzen Reservoir strongly reduced the filtration rate of D. galeata (down to 60 %). Heating deactivated these dissolved compounds. A significant correlation was found between primary production measured in situ and the reduction of FR in the filtrate of reservoir water, indicating that extra- cellular products released during photosynthesis triggered the reduction of the filtration rate. The ratio of ingested to collected beads was used to quantify the proportion of food, which was not only collected but passed the mouth of D. galeata. The ratio of ingestion to collection was compared between filtered and unfiltered reservoir water both media identical with respect to the concentration of dissolved compounds, whereas other factors (e. g. food concentration, temperature, filtration rate) were different. The changes in this ratio between filtered and unfiltered reservoir water suggest that D. galeata is capable of a chemosensory control of the ingestion behaviour by detecting external metabolites.
1. After observing that juvenile roach fed intensively on cyanobacteria and that cyanobacteria were densely colonized by heterotrophic bacteria, we tested whether the bacteria are used by underyearling roach and the extent to which they contribute to the energy requirements of the fish. 2. We radiolabeled attached bacteria in a natural cyanobacterial suspension, fed the fish with these particles, and estimated their assimilation by roach. Biomass of attached bacteria on cyanobacteria increased with the proportion of the cyanobacterium Microcystis in total cyanobacteria. Biomass-specific thymidine incorporation of attached bacteria was higher than that of free bacteria. 3. In feeding experiments, we detected assimilation of bacterial biomass into muscle tissue of underyearling roach. Fish consumed Microcystis to a lesser extent compared to Aphanizomenon but assimilation of attached bacteria was higher when roach fed on Microcystis due to the higher biomass of epibacteria on this cyanobacterium. However, biomass of attached bacteria was too low to be an important food source for underyearling roach. 4. We conclude that assimilation of epibacteria from cyanobacteria cannot explain the success of roach in eutrophic lakes.