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The main goal of this research was to evaluate whether the mixture of fresh labile dissolved organic matter (DOM) and accumulated refractory DOM influences bacterial production, respiration, and growth efficiency (BGE) in aquatic ecosystems. Bacterial batch cultures were set up using DOM leached from aquatic macrophytes as the fresh DOM pool and DOM accumulated from a tropical humic lagoon. Two sets of experiments were performed and bacterial growth was followed in cultures composed of each carbon substrate (first experiment) and by carbon substrates combined (second experiment), with and without the addition of nitrogen and phosphorus. In both experiments, bacterial production, respiration, and BGE were always higher in cultures with N and P additions, indicating a consistent inorganic nutrient limitation. Bacterial production, respiration, and BGE were higher in cultures set up with leachate DOM than in cultures set up with humic DOM, indicating that the quality of the organic matter pool influenced the bacterial growth. Bacterial production and respiration were higher in the mixture of substrates (second experiment) than expected by bacterial production and respiration in single substrate cultures (first experiment). We suggest that the differences in the concentration of some compounds between DOM sources, the co-metabolism on carbon compound decomposition, and the higher diversity of molecules possibly support a greater bacterial diversity which might explain the higher bacterial growth observed. Finally, our results indicate that the mixture of fresh labile and accumulated refractory DOM that naturally occurs in aquatic ecosystems could accelerate the bacterial growth and bacterial DOM removal.

Four experiments covering different seasons were performed to test the impact of increased benthic and planktonic resource availability on the structure of biofilm-dwelling ciliate communities which were cultivated in river bypass systems. The growth of benthic bacteria was stimulated by the addition of dissolved organic carbon. The enrichment of the planktonic resource was achieved by supplementation with suspended bacteria. It was shown that both resource enrichments can differentially influence abundance and taxonomic structure of ciliate communities. Furthermore, both resources can influence different stages during biofilm colonization. Increased benthic bacterial growth mainly resulted in both an accumulation of primarily grazing-resistant bacterial filaments and in an increase in the number of vagile heterotrophic flagellates. This can stimulate nanophagous ciliates (feeding on flagellates) in addition to the direct stimulation of bacterio vorous ciliates. The effects of the planktonic bacteria enrichments were twofold: They could have been utilized either directly by suspension-feeding ciliates or indirectly through an enhanced growth of suspension-feeding attached heterotrophic flagellates, which were then in turn grazed upon by ciliates. The magnitude of responses of the total ciliate abundance to the two resource enrichments further depended on the background conditions, thereby showing temporarily variable limitations of these resources. Furthermore, the particular taxonomic groups stimulated by one resource type sometimes differed between the experiments, an observation which demonstrates that the response depends on different environmental factors and is not easily predictable based simply on resource type. Taken together, our results emphasize the need of a differentiated view on the effects of resources on complex biofilm-dwelling consumer communities with respect to both the origin of carbon source as well as the particular environmental conditions.

The occurrence of bloom-forming cyanobacteria is one of the most obvious sign of eutrophication in freshwaters. Although in eutrophic lakes water transparency in the ultraviolet (UV) region is strongly reduced, bloom-forming cyanobacteria are exposed to high solar UV radiation at the surface. Here, we show that, in a natural phytoplankton community from a very eutrophic lake, Microcystis synthesizes UV sunscreen compounds identified as mycosporine-like amino acids (MAAs). The biomass-specific MAA concentration was significantly correlated with the occurrence of Microcystis but not with other algal groups, even though they were dominant in terms of biomass. Based on a photo-optical model, we estimated that the maximum MAA concentration per cell observed (2.5% dry weight) will confer only ~40% of internal screening to a single layer of Microcystis cells. Thus, the formation of a colony with several layers of cells is important to afford an efficient UV screening by internal self-shading. Overall, we propose that Microcystis uses a combination of photoprotective strategies (MAAs, carotenoids) to cope with high solar UV radiation at the water surface. These strategies include also the screening of UV radiation by d-galacturonic acid, one of the main chemical components of the slime layer in Microcystis.

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (-N), and phosphorus limited (-P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in -P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in -N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in -P PAR compared to -P PAB, and the opposite radiation effect was observed in -N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.

Iron- and sulfate-reducing microorganisms play an important role for alkalinity-generating processes in mining lakes with low pH. In the acidic mining lake 111 in Lusatia, Germany, a passive in situ remediation method was tested in a large scale experiment, in which microbial iron and sulfate reduction are stimulated by addition of Carbokalk (a mixture of the nonsugar compounds of sugar beets and lime) and straw. The treated surface sediment consisted of three layers of different pH and geochemical composition. The top layer was acidic and rich in Fe(III), the second and third layer both showed moderately acidic to circum-neutral pH values, but only the second was rich in organics, strongly reduced and sulfidic. Aim of the study was to elucidate the relative importance of neutrophilic heterotrophic, acidophilic heterotrophic, and acidophilic autotrophic iron-reducing microorganisms in each of the three layers. In order to distinguish between them, the effect of their respective characteristic electron donors acetate, glucose, and elemental sulfur on potential iron reduction rates was investigated. Limitation of iron reduction by the availability of Fe(III) was revealed by the addition of Fe(OH)₃. The three groups of iron-reducing microorganisms were quantified by most probable number (MPN) technique and their community composition was analyzed by cloning and sequencing of 16S rRNA genes. In the acidic surface layer, none of the three electron donors stimulated iron reduction; acetate even had an inhibiting effect. In agreement with this, no decrease of the added electron donors was observed. Iron reduction rates were low in comparison to the other layers. Iron reduction in layers 2 and 3 was enhanced by glucose and acetate, accompanied by a decrease of these electron donors. Addition of elemental sulfur did not enhance iron reduction in either layer. Layer 2 exhibited the highest iron reduction rate (4.08 mmol dm⁻³d⁻¹) and the highest cell numbers in MPN media. In MPN enrichments from all layers, Acidithiobacillus-like sequences were frequent. In addition to these, sequences related to Fulvimonas and Clostridium dominated in layer 1. MPN enrichments of layer 2 were diverse, containing Rhodocyclaceae-related sequences and surprisingly low numbers of Geobacteraceae. In layer 3, Sulfobacillus and Trichococcus spp. were also important. It was concluded that in the surface layer mainly acidophilic, probably autotrophic and heterotrophic, iron reducers were active, whereas in layers 2 and 3 mainly neutrophilic heterotrophs were important for iron reduction. These differ from well-studied Fe(III) reducers in other environments, so they deserve further study. The potential for acid-producing sulfur-driven Fe(III) reduction seemed not to be critical for in situ remediation.