Cyanobacteria are very abundant in Arctic aquatic ecosystems ranging from lakes, pond and stream to ice-based meltwater ponds and supraglacial cryoconite holes. In particular, benthic cyanobacterial communities can generate high biomass accumulations in the form of cyanoabcteria-based microbial mats where grazers are absent. Here, they are important contributors to overall biological productivity in the Arctic.
Cyanobacterial mats
Cyanobacterial mats are often multilayered three-dimensional structures with filamentous cyanobacteria forming the main structuring agent. They grow on … and are mm – and in some cases cm – thick. They produce exopolymeric substances that hold the filamentous structures in a gel-like matrix (Zakhia et al, 2010, references within).
Most of these benthic aquatic microbial mats have a carotenoid-rich layer on the surface. High performance liquid chromatography analyses of Arctic benthic cyanobacterial mats in the Canadian Arctic showed a stratification of different types of pigments within the mats:
- From high concentrations of UV-screening scytonemin in the upper mat layer to increasing zeaxanthin and myxoxanthin in the bottom layer, and
- An overall shift from photoprotective to photosynthetic carotenoids further down the mat-profile, where cyanobacteria grow in increasingly shaded conditions with lower irradiance levels. At only 2 mm from the mat surface the maximum irradiance reaching the cells may be only 5% of that at surface. High surface concentrations of photoprotective pigments allows the cells deeper inside the mat community to grow under milder conditions, free of UV radiation (e.g. Quesada et al. 1999, Tanabe et al. 2010).
Benthic cyanobacterial mats have a deep chorophyll maximum, which is enriched in phycocyanin and chlorophyll a. The mat communities are also often characterised by vertical physical and biochemical stratification, including gradients in variables such as irradiance, redox potential, pH, and concentrations of dissolved oxygen, carbon dioxide, methane and nutrients. These gradients are generated in part by external physical factors such as light, sediment composition and water characteristics, but also through the zonation of metabolic activities of the functionally diverse microorganisms within the mats, such as sulphate-reduction and oxidation, photosynthesis, respiration, nitrification, denitrification, nitrogen fixation, fermentation and methanogenesis (Stal 2012).
Arctic freshwater microbial mats are diverse assemblages. Macroscopically, they are dominated by phototrophic and pigment-rich cyanobacteria, comprising at least 88% of total cell counts on ice shelf ecosystems (Vincent et al., 2004). However, cyanobacterial mats can comprise diverse communities including heterotrophic bacteria, Archaea and Viruses, e.g. known from some High Arctic Ice Shelfs. Here, Proteobacteria actually contributed the largest proportion of total DNA content within the cyanobacterial mats, while Archaea were rare (Varin et al. 2010). This implies that a broad range of bacterial decomposition, nutrient recycling and scaveninges processes happen within the cyanobacterial mats in addition to phototroph activities. Virus, like Alpha-, Beta-, Gammaproteobacteria phages and cyanophages – identified through their DNA-, likely contribute to cellular lysis and recycling within the mats (Varin et al. 2012).
Arctic cyanobacterial mat communities are also rich in microfauna and microbial eukaryotes. 18S rRNA metagenomic surveys not only detected the already well described Chlorophyceae and Bacillariophyceae as well as Rotifera, Tardigrada, Nematoda, and Platyhelminthes. The surveys also found 18S rRNA gene ribotypes grouping within various clade of Ulvophyceae, Trebouxiophyceae, Chrysophyceae, Ciliophora, Chrysophyceae , Amoebazoa, Ciliophora, Euglenoids , Cercozoa and fungi (Varin et al. 2012; Jungblut et al. 2012, Aguilera et al 2011 ).
Cryoconite communities
Cyanobacteria can also be found on glaciers as par of supraglacial cryoconites communities. They have been studied in particular on White Glacier in the Canadian high Arctic, and Midtre Lovenbreen in Svalbard as well as the Greenland Ice Sheet (Cameron et al. 2011, Edwards et al. 2009, Mueller et al 2001). Extensive growth of Cyanobacteria and microalgae on glacial ice may affect the glaciers albedo due to the extensive accumulation of pigments (Yallop 2012).
In these systems, cyanobacteria are likely to play an important role in aggregating formations that allow the development of more complex microbial communities (Langford et al. 2010). Cyanobacteria in cryoconite holes include Leptolyngbya, Crinalium, Phormidium, and Chaemosphiphon (Christner et al. 2003, Cameron et al. 2011, Edwards et al. 2009). Heterotrophic bacteria, eukaryotic micro-algae, protists and even metazoans such as rotifers, nematodes and tardigrades have been found in cryoconites communities (ref). These supraglacial, microbially dominated environments are thought to contribute to regional and global carbon cycling, driven by cyanobacteria together with green algae as primary producers (Stibal et al. 2012).
Planktonic cyanobacteria
Planktonic cyanobacteria (< 2 µm) in the overlying often nutrient-poor water in lakes and ponds occur in dilute concentrations. In Lake A, Canadian High Arctic, chlorophyll a in the water ranges from 0.02-0.68 µg L-1 indicating a sparse phytoplankton communities (Vincent et al. 2004). Cyanobacteria represented 25% of the communities total 16SrRNA gene sequences in the upper mixed layer of the lake(Comeau 2012). In comparison, chlorophyll a concentrations in the benthic communities ranges from 5.4 to 448 mg m-2 with an average of 147 mg m-2 in High Arctic lakes and ice shelve meltwater ponds (Vincent et al. 2004, Mueller et al. 2005).
Planktonic picocyanobacterial communities of Arctic lakes comprise mainly Synechococcus and can be separated into fresh and saline ecotypes (Villeneuve et al. 2001, Van Hove et al. 2008, Comeau et al. 2012). Picocyanobacteria are also a common constituent of high latitude rivers fed by lakes, or where the flowing water transit time is long enough to allow the development of phytoplankton communities. However, we still know little about the taxonomic diversity. In the Great Whale River, subarctic Canada, picocyanobacteria achieved concentrations up to 104 µm L-1 and contributed about half of the chlorophyll a biomass (Rae and Vincent 1998). Picocyanobacteria concentrations ranged up to 5 x 104 cells mL −1 along a 300-km transect down the Mackenzie River, in Canada’s Northwest Territories,, but dropped across the estuary to around 30 cells mL -1 in the offshore ocean (Vallières et al. 2008).
