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The James Ross archipelago houses numerous lakes and ponds. In this region, a vast diatom and cyanobacterial variety has been reported; however, the prokaryotic diversity in microbial mats from these lakes remains poorly explored.
Here, a high-throughput sequencing of 16S rRNA gene in microbial mats from Lake Bart-Roja in James Ross Island and lakes Pan Negro and North Pan Negro located in Vega Island was performed. Combined with mineralogical and environmental characteristics, we analyzed the diversity and structure of the microbial communities. Sequences assigned to Archaea were extremely low, while Bacteria domain prevailed with the abundance of Proteobacteria (mostly Betaproteobacteriales) followed by Bacteroidetes, Verrucomicrobia, Firmicutes, and Cyanobacteria.
Local environmental conditions, such as conductivity and Eh, provided differential microbial assemblages that might have implications in the oligotrophic status of the lakes. Consequently, a clear segregation at the family level was observed.
In this sense, the assigned diversity was related to taxa recognized as denitrifiers and sulfur oxidizers. Particularly, in Lake Pan Negro sulfur-reducing and methanogenic representatives were also found and positively correlate with alkalinity and water depth.
Moreover, Deinococcus-Thermus was observed in Lake Bart-Roja, while Melainabacteria (Cyanobacteria)-poorly reported in Antarctic mats-was detected in Lake Pan Negro. Epsilonbacteraeota was exclusively found in this lake, suggesting new potential phylotypes. This study contributes to the understanding of the diversity, composition, and structure of Antarctic benthic microbial ecosystems and provides highly valuable information, which can be used as a proxy to evaluate environmental changes affecting Antarctic microbiota.
Climate change and human-driven eutrophication promote the spread of harmful cyanobacteria blooms in lakes worldwide, which affects water quality and impairs the aquatic food chain. In recent times, sedimentary ancient DNA-based (sedaDNA) studies were used to probe how centuries of climate and environmental changes have affected cyanobacterial assemblages in temperate lakes. However, there is a lack of information on the consistency between sediment-deposited cyanobacteria communities versus those of the water column, and on the individual role of natural climatic changes versus human pressure on cyanobacteria community dynamics over multi-millennia time scales.
Therefore, this thesis uses sedimentary ancient DNA of Lake Tiefer See in northeastern Germany to trace the deposition of cyanobacteria along the water column into the sediment, and to reconstruct cyanobacteria communities spanning the last 11,000 years using a set of molecular techniques including quantitative PCR, biomarkers, metabarcoding, and metagenome sequence analyses.
The results of this thesis proved that cyanobacterial composition and species richness did not significantly differ among different water depths, sediment traps, and surface sediments. This means that the cyanobacterial community composition from the sediments reflects the water column communities. However, there is a skewed sediment deposition of different cyanobacteria groups because of DNA alteration and/or deterioration during transport along the water column to the sediment. Specifically, single filament taxa, such as Planktothrix, are poorly represented in sediments despite being abundant in the water column as shown by an additional study of the thesis on cyanobacteria seasonality. In contrast, aggregate-forming taxa, like Aphanizomenon, are relatively overrepresented in sediment although they are not abundant in the water column. These different deposition patterns of cyanobacteria taxa should be considered in future DNA-based paleolimnological investigations. The thesis also reveals a substantial increase in total cyanobacteria abundance during the Bronze Age which is not apparent in prior phases of the early to middle Holocene and is suggested to be caused by human farming, deforestation, and excessive nutrient addition to the lake. Not only cyanobacterial abundance was influenced by human activity but also cyanobacteria community composition differed significantly between phases of no, moderate, and intense human impact.
The data presented in this thesis are the first on sedimentary cyanobacteria DNA since the early Holocene in a temperate lake. The results bring together archaeological, historical climatic, and limnological data with deep DNA-sequencing and paleoecology to reveal a legacy impact of human pressure on lake cyanobacteria populations dating back to approximately 4000 years.