Members of the archaeal Miscellaneous Crenarchaeotic Group (MCG) are among the most successful microorganisms on the planet. During its evolutionary diversification, this very diverse group has managed to cross the saline-freshwater boundary, one of the most important evolutionary barriers structuring microbial communities. However, the current understanding on the ecological significance of MCG in freshwater habitats is scarce and the evolutionary relationships between freshwater and saline MCG remains poorly known. Here, we carried out molecular phylogenies using publicly available 16S rRNA gene sequences from various geographic locations to investigate the distribution of MCG in freshwater and saline sediments and to evaluate the implications of salinefreshwater transitions during the diversification events. Our approach provided a robust ecological framework in which MCG archaea appeared as a core generalist group in the sediment realm. However, the analysis of the complex intragroup phylogeny of the 21 subgroups currently forming the MCG lineage revealed that distinct evolutionary MCG subgroups have arisen in marine and freshwater sediments suggesting the occurrence of adaptive evolution specific to each habitat. The ancestral state reconstruction analysis indicated that this segregation was mainly due to the occurrence of a few saline-freshwater transition events during the MCG diversification. In addition, a network analysis showed that both saline and freshwater MCG recurrently co-occur with archaea of the class Thermoplasmata in sediment ecosystems, suggesting a potentially relevant trophic connection between the two clades.

The Miscellaneous Crenarchaeotic Group (MCG) is an archaeal lineage whose members are widespread and abundant in marine sediments. MCG archaea have also been consistently found in stratified euxinic lakes. In this work, we have studied archaeal communities in three karstic lakes to reveal potential habitat segregation of MCG subgroups between planktonic and sediment compartments. In the studied lakes, archaeal assemblages were strikingly similar to those of the marine subsurface with predominance of uncultured Halobacteria in the plankton and Thermoplasmata and MCG in anoxic, organic-rich sediments. Multivariate analyses identified sulphide and dissolved organic carbon as predictor variables of archaeal community composition. Quantification of MCG using a newly designed qPCR primer pair that improves coverage for MCG subgroups prevalent in the studied lakes revealed conspicuous populations in both the plankton and the sediment. Subgroups MCG-5a and -5b appear as planktonic specialists thriving in euxinic bottom waters, while subgroup MCG-6 emerges as a generalist group able to cope with varying reducing conditions. Besides, comparison of DNA- and cDNA-based pyrotag libraries revealed that rare subgroups in DNA libraries, i.e. MCG-15, were prevalent in cDNA-based datasets, suggesting that euxinic, organic-rich sediments of karstic lakes provide optimal niches for the activity of some specialized MCG subgroups.

Archaea inhabiting marine and freshwater sediments have a relevant role in organic carbon mineralization, affecting carbon fluxes at a global scale. Despite current evidences suggesting that freshwater sediments largely contribute to this process, few large-scale surveys have been addressed to uncover archaeal diversity and abundance in freshwater sedimentary habitats. In this work, we quantified and high-throughput sequenced the archaeal 16S rRNA gene from surficial sediments collected in 21 inland waterbodies across the Iberian Peninsula differing in typology and trophic status. Whereas methanogenic groups were dominant in most of the studied systems, especially in organic-rich sediments, archaea affiliated to widespread marine lineages (the Bathyarchaeota and the Thermoplasmata) were also ubiquitous and particularly abundant in euxinic sediments. In these systems, Bathyarchaeota communities were dominated by subgroups Bathyarchaeota-6 (87.95 ± 12.71%) and Bathyarchaeota-15 (8.17 ± 9.2%) whereas communities of Thermoplasmata were mainly composed of members of the order Thermoplasmatales. Our results also indicate that Archaea accounted for a minor fraction of sedimentary prokaryotes despite remarkable exceptions in reservoirs and some stratified lakes. Copy numbers of archaeal and bathyarchaeotal 16S rRNA genes were significantly different when compared according to system type (i.e., lakes, ponds, and reservoirs), but no differences were obtained when compared according to their trophic status (from oligotrophy to eutrophy). Interestingly, we obtained significant correlations between the abundance of reads (Spearman r = 0.5, p = 0.021) and OTU richness (Spearman r = 0.677, p < 0.001) of Bathyarchaeota and Thermoplasmata across systems, reinforcing the hypothesis of a potential syntrophic interaction between members of both lineages.

Coordinated distributed experiments (CDEs) enable the study of large-scale ecological patterns in geographically dispersed areas, while simultaneously providing broad academic and personal benefits for the participants. However, the effective involvement of early-career researchers (ECRs) presents major challenges. Here, we analyze the benefits and challenges of the first CDE exclusively led and conducted by ECRs (i.e. ECR-CDE), which sets a baseline for similar CDEs, and we provide recommendations for successful CDE execution. ECR-CDEs achieve most of the outcomes identified in conventional CDEs as well as extensive benefits for the young cohort of researchers, including: (i) receiving scientific credit, (ii) peer-training in new concepts and methods, (iii) developing leadership and communication skills, (iv) promoting a peer network among ECRs, and (v) building on individual engagement and independence. We also discuss the challenges of ECR-CDEs, which are mainly derived from the lack of independence and instability of the participants, and we suggest mechanisms to address them, such as resource reallocation and communication strategies. We conclude that ECR-CDEs can be a relevant tool to empower ECRs across disciplines by fostering their training, networking and personal well-being.