Gravity-driven membrane (GDM) filtration is a promising tool for low-cost decentralized drinking water production. The biofilms in GDM systems are able of removing harmful chemical components, particularly toxic cyanobacterial metabolites such as microcystins (MCs). This is relevant for the application of GDM filtration because anthropogenic nutrient input and climate change have led to an increase of toxic cyanobacterial blooms. However, removal of MCs in newly developing GDM biofilms is only established after a prolonged period of time. Since cyanobacterial blooms are transient phenomena, it is important to understand MC removal in mature biofilms with or without prior toxin exposure. In this study, the microbial community composition of GDM biofilms was investigated in systems fed with water from a lake with periodic blooms of MC-producing cyanobacteria. Two out of three experimental treatments were supplemented with dead biomass of a MC-containing cyanobacterial strain, or of a non-toxic mutant, respectively. Analysis of bacterial rRNA genes revealed that both biomass-amended treatments were significantly more similar to each other than to a non-supplemented control. Therefore, it was hypothesized that biofilms could potentially be 'primed' for rapid MC removal by prior addition of non-toxic biomass. A subsequent experiment showed that MC removal developed significantly faster in mature biofilms that were pre-fed with biomass from the mutant strain than in unamended controls, indicating that MC degradation was a facultative trait of bacterial populations in GDM biofilms. The significant enrichment of bacteria related to both aerobic and anaerobic MC degraders suggested that this process might have occurred in parallel in different microniches.
During the last decades, the planktonic cyanobacterium Planktothrix rubescens became a dominant primary producer in many deep pre-alpine lakes. While altered physiochemical conditions due to lake warming seem to favour this cyanobacterial species, its dominance is partly attributed to factors conferring grazing resistance. The rigid structure of the cyanobacterial filaments and toxic secondary metabolites (e.g. microcystins) protect against diverse grazers. Nonetheless, species of the protistan genus Nuclearia (Nucleariidae, Opisthokonta) are able to overcome this grazing protection. Time lapse video documentation served as tool to record slow feeding processes of N. thermophila and N. delicatula. Three different feeding strategies could be distinguished: (i) Phagocytosis of small fragments, (ii) serial break-ups of cyanobacterial cells and (iii) bending and breaking of filaments. While observations revealed mechanical manipulation to be important for the efficient breakdown of P. rubescens filaments, the toxin microcystin had no pronounced negative effects on nucleariid cells. Growth experiments with N. thermophila/N. delicatula and different accompanying bacterial assemblages pointed to a pivotal role of distinct prokaryotic species for toxin degradation and for the growth success of the protists. Thus, the synergistic effect of nucleariids and specific bacteria favours an efficient degradation of P. rubescens along with its toxin.
We studied the cm to m scale spatial distribution of dissolved free amino acids (DFAA) in the upper epilimnion of oligomesotrophic Lake Zurich in 14 sampling campaigns over > 3 years and at various periods of the growing season. During each campaign, 10 sets of 10 simultaneously drawn samples (10 mL, 2 cm distance) were collected from 5 m depth. DFAA concentrations varied by one to > 3 orders of magnitude within sets, providing field evidence for DFAA release from macroscopic point sources and for substantial variability of the insitu growth conditions of bacterioplankton metacommunities. There was a tight relationship between the median DFAA concentration per sampling campaign and the compositional heterogeneity of the 15 most common AA: their composition was similar in samples from campaigns with high median DFAA concentrations, indicating that spatial distribution patterns were mainly a result of physical mixing. By contrast, AA composition was spatially variable in campaigns with low median DFAA concentrations, and serine, aspartate, and glycine were disproportionally high in the 10% samples with highest DFAA concentrations. We hypothesized that pelagic bacteria would preferably target pulses of such locally overrepresented AA. Short-term incubations with radiolabeled tracers revealed substantially higher microbial uptake of serine and, to a lesser extent, aspartate, than of two amino acids with consistently low in situ concentrations (leucine, isoleucine). This illustrates a "preparedness" of the bacterioplankton to preferably incorporate those AAs that are more available in DFAA hotspots.Dissolved organic carbon represents one of the largest exchangeable carbon reservoirs in aquatic systems (Carlson 2002). It overlaps with the pool of dissolved organic nitrogen in the subset of labile low-molecular-weight compounds that are readily available to microorganisms, such as dissolved free amino acids (DFAAs) (
Microbial biofilms in gravity-driven membrane (GDM) filtration systems can efficiently degrade the cyanotoxin microcystin (MC), but it is unclear if this function depends on the presence of MC-producing cyanobacteria in the source water habitat. We assessed the removal of MC from added Microcystis aeruginosa biomass in GDMs fed with water from a lake with regular blooms of toxic cyanobacteria (ExpL) or from a stream without such background (ExpS). While initial MC removal was exclusively due to abiotic processes, significantly higher biological MC removal was observed in ExpL. By contrast, there was no difference in MC degradation capacity between lake and stream bacteria in separately conducted liquid enrichments on pure MC. Co-occurrence network analysis revealed a pronounced modularity of the biofilm communities, with a clear hierarchic distinction according to feed water origin and treatment type. Genotypes in the network modules associated with ExpS had significantly more links to each other, indicating that these biofilms had assembled from a more coherent source community. In turn, signals for stochastic community assembly were stronger in ExpL biofilms. We propose that the less “tightly knit” ExpL biofilm assemblages allowed for the better establishment of facultatively MC degrading bacteria, and thus for higher overall functional efficiency.
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