Metabolic versatility of freshwater sedimentary archaea feeding on different organic carbon sources.

Members of the phylum Bathyarchaeota and the class Thermoplasmata are widespread in marine and freshwater sediments where they have been recognized as key players in the carbon cycle. Here, we tested the responsiveness of archaeal communities on settled plant debris and sediment from a karstic lake to different organic carbon amendments (amino acids, plant-derived carbohydrates, and aromatics) using a lab-scale microcosm. Changes in the composition and abundance of sediment and biofilm archaeal communities in both DNA and RNA fractions were assessed by 16S rRNA gene amplicon sequencing and qPCR, respectively, after 7 and 30 days of incubation. Archaeal communities showed compositional changes in terms of alpha and beta diversity in relation to the type of carbon source (amino acids vs. plant-derived compounds), the nucleic acid fraction (DNA vs. RNA), and the incubation time (7 vs. 30 days). Distinct groups within the Bathyarchaeota (Bathy-15 and Bathy-6) and the Thermoplasmata (MBG-D) differently reacted to carbon supplements as deduced from the analysis of RNA libraries. Whereas Bathyarchaeota in biofilms showed a long-term positive response to humic acids, their counterparts in the sediment were mainly stimulated by the addition of tryptophan, suggesting the presence of different subpopulations in both habitats. Overall, our work presents an in vitro assessment of the versatility of archaea inhabiting freshwater sediments towards organic carbon and introduces settled leaf litter as a new habitat for the Bathyarchaeota and the Thermoplasmata.

Phosphorus deficiency and kinetics of alkaline phosphatase in isolates and natural populations of phototrophic sulphur bacteria.

Phosphorus deficiency was analysed in the oxic-anoxic gradient of the karstic sulphurous lakes Vilar and Sisó during the stratification period. The distribution of planktonic photosynthetic populations along a vertical gradient coincided with an increase in alkaline phosphatase activity (APA). A multiple stepwise correlation analysis of data yielded a positive correlation of APA with planktonic phototrophic populations. MUF-P hydrolysis saturation curves were used to estimate the enzyme kinetics. High-affinity phosphatases (i.e. low K(M) saturation constant) coincided with the oxic-anoxic gradient and progressively declined through both the epi- and the hypolimnion. Changes in the K(M) values are likely due to phosphate inhibition and the contribution of different planktonic populations in the induction of alkaline phosphatases. Extremely low organic phosphorus turnover times (as short as 0.37 h) were also estimated in the gradient zone, indicating a high dependence of the bacterial populations on organic phosphate esters. Phosphatase saturation kinetics revealed K(M) values from 0.53 to 8.45 microM MUF-P, perfectly matching those found in the isolates Thiocapsa sp. UdG3513, Chlorobium limicola UdG6050 and UdG6055 and Chlorobium phaeobacteroides CL1401. The results obtained indicate that a relevant adaptation of sulphur phototrophic bacteria may occasionally face periods of phosphate limitation despite thriving in nutrient-rich anoxic waters.