Warming mediates the resistance of aquatic bacteria to invasion during community coalescence.

The immigration history of communities can profoundly affect community composition. For instance, early-arriving species can have a lasting effect on community structure by reducing the invasion success of late-arriving ones through priority effects. This can be particularly important when early-arriving communities coalesce with another community during dispersal (mixing) events. However, the outcome of such community coalescence is unknown as we lack knowledge on how different factors influence the persistence of early-arriving communities and the invasion success of late-arriving taxa. Therefore, we implemented a full-factorial experiment with aquatic bacteria where temperature and dispersal rate of a better adapted community were manipulated to test their joint effects on the resistance of early-arriving communities to invasion, both at community and population level. Our 16S rRNA gene sequencing-based results showed that invasion success of better adapted late-arriving bacteria equaled or even exceeded what we expected based on the dispersal ratios of the recipient and invading communities suggesting limited priority effects on the community level. Patterns detected at the population level, however, showed that resistance of aquatic bacteria to invasion might be strengthened by warming as higher temperatures (a) increased the sum of relative abundances of persistent bacteria in the recipient communities, and (b) restricted the total relative abundance of successfully established late-arriving bacteria. Warming-enhanced resistance, however, was not always found and its strengths differed between recipient communities and dispersal rates. Nevertheless, our findings highlight the potential role of warming in mitigating the effects of invasion at the population level.

Detecting aquatic pathogens with field-compatible dried qPCR assays.

Field-ready qPCR assays with extended shelf-life support monitoring programs for emerging aquatic pathogens and enable quick conservation and management decisions. Here, we developed, validated, and tested the shelf-life of qPCR assays targeting Gyrodactylus salaris and Aphanomyces astaci with lyophilization and air-drying.

Formation of mercury methylation hotspots as a consequence of forestry operations.

Earlier studies have shown that boreal forest logging can increase the concentration and export of methylmercury (MeHg) in stream runoff. Here we test whether forestry operations create soil environments of high MeHg net formation associated with distinct microbial communities. Furthermore, we test the hypothesis that Hg methylation hotspots are more prone to form after stump harvest than stem-only harvest, because of more severe soil compaction and soil disturbance. Concentrations of MeHg, percent MeHg of total Hg (THg), and bacterial community composition were determined at 200 soil sampling positions distributed across eight catchments. Each catchment was either stem-only harvested (n=3), stem- and stump-harvested (n=2) or left undisturbed (n=3). In support of our hypothesis, higher MeHg to THg ratios was observed in one of the stump-harvested catchments. While the effects of natural variation could not be ruled out, we noted that most of the highest % MeHg was observed in water-filled cavities created by stump removal or driving damage. This catchment also featured the highest bacterial diversity and highest relative abundance of bacterial families known to include Hg methylators. We propose that water-logged and disturbed soil environments associated with stump harvest can favor methylating microorganisms, which also enhance MeHg formation.

Diversity and abundance of aromatic catabolic genes in lake sediments in response to temperature change.

The abundance and composition of genes involved in the catabolism of aromatic compounds provide important information on the biodegradation potential of organic pollutants and naturally occurring compounds in the environment. We studied catechol 2, 3 dioxygenase (C23O) and benzylsuccinate synthase (bssA) genes coding for key enzymes of aerobic and anaerobic degradation of aromatic compounds in experimental incubations with sediments from two contrasting lakes; humic lake Svarttjärn and eutrophic Vallentunasjön, respectively. Sediment cores from both lakes were incubated continuously for 5 months at constant temperatures ranging from 1.0 to 21.0 °C. The difference in C23O gene composition of the sediment analyzed at the end of the experiment was larger between lakes, than among temperature treatments within each lake. The abundance of C23O gene copies and measured respiration was positively correlated with temperature in Vallentunasjön, whereas putative C23O genes were present in lower concentrations in Svarttjärn sediments. Putative bssA genes were only detected in Svarttjärn. For both lakes, the two catabolic genes were most abundant in the surface sediment. The results emphasize the important role of temperature and nutrient availability in controlling the functional potential of sediment microorganisms and reveal differences between systems with contrasting trophic status. A better understanding of catabolic pathways and enzymes will enable more accurate forecasting of the functional properties of ecosystems under various scenarios of environmental change.