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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.
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.
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.
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.
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.
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.
We compared growth rates and efficiencies of pelagic bacteria from an extremely acidic mining lake (pH 2.6, mean depth 4.6m) supplied with different sources of carbon: (1) excreted by phytoplankton, (2) derived from benthic algae, (3) entering the lake via ground water, and (4) leached from leaf litter. Bacteria exhibited high growth rate and efficiency on exudates of pelagic and benthic algae. In contrast, they showed a lower growth rate and efficiency with organic carbon from ground water, and grew at a very high rate but a very low efficiency on leaf leachate. Results from stable isotope analyses indicate a greater importance of benthic exudates and leaf leachate for bacteria in the epilimnion, and a higher impact of ground water sources in the hypolimnion. Given the magnitude of differential source inputs into the lake, we suggest that benthic primary production was the most important carbon source for pelagic bacteria. The benthic-pelagic coupling seems to be more relevant in this shallow acidic lake with low pelagic carbon dioxide concentrations than in neutral lakes
The acidic mining lakes of Eastern Germany are characterized by their extremely low pH and high iron concentrations. Low concentrations of CO2 in the epilimnion due to the low pH and reduced light transmission due to dissolved ferric iron potentially limit phytoplankton primary production (PP), whereas dissolved organic carbon (DOC) may promote heterotrophic production of bacteria (HP). We, therefore, tested whether HP exceeds PP in three lakes differing in pH and iron concentration (mean pH 2.3-3.0, 23-500 mg Fe L-1). Bacterial biomass and HP achieved highest values in the most acidic, most iron-rich lake, whereas PP was highest in the least acidic lake. HP was often higher than PP (ratio HP/PP up to 11), indicating that planktonic PP was not the main carbon source for the bacteria. HP was not related to PP and DOC, but HP as well as bacterial biomass increased with decreasing pH. Light stimulated the formation of ferrous iron, changed the DOC composition, and increased the HP in laboratory experiments, suggesting that iron photoreduction caused DOC degradation. This may explain why we found the highest HP in the most acidic and most rich lake. Overall, the importance of bacteria in the cycling of matter and as a basis for the whole food web seemed to increase in more acidic lakes with higher iron concentrations
Inorganic carbon limitation and mixotrophic growth in Chlamydomonas from an acidic mining lake
(2005)
Plankton communities in acidic mining lakes (pH 2.5-3.3) are species-poor because they face extreme environmental conditions, e.g. 150 mg l(-1) Fe2++Fe3+. We investigated the growth characteristics of the dominant pigmented species, the flagellate Chlamydomonas acidophila, in semi-continuous culture experiments under in situ conditions. The following hypotheses were tested: (1) Low inorganic carbon (IC) concentrations in the epilimnion (e.g. 0.3 mg l(-1)) arising from the low pH limit phototrophic growth (H-1); (2) the additional use of dissolved organic carbon (mixotrophy) leads to higher growth rates under IC-limitation (H-2), and (3) phagotrophy is not relevant (H-3). H- 1 was supported as the culture experiments, in situ PAR and IC concentrations indicated that IC potentially limited phototrophic growth in the mixed surface layers. H-2 was also supported: mixotrophic growth always exceeded pure phototrophic growth even when photosynthesis was saturated. Dark growth in filtered lake water illuminated prior to inoculation provided evidence that Chlamydomonas was able to use the natural DOC. The alga did not grow on bacteria, thus confirming H-3. Chlamydomonas exhibited a remarkable resistance to starvation in the dark. The compensation light intensity (ca. 20 mu mol photons m(-2) s(-1)) and the maximum phototrophic growth (1.50 d(-1)) fell within the range of algae from non-acidic waters. Overall, Chlamydomonas, a typical r-strategist in circum-neutral systems, showed characteristics of a K-strategist in the stable, acidic lake environment in achieving moderate growth rates and minimizing metabolic losses. (c) 2005 Elsevier GmbH. All rights reserved