Discovery of large conical stromatolites in Lake Untersee, Antarctica

Abstract

Lake Untersee is one of the largest (11.4 km(2)) and deepest (>160 m) freshwater lakes in East Antarctica. Located at 71°S the lake has a perennial ice cover, a water column that, with the exception of a small anoxic basin in the southwest of the lake, is well mixed, supersaturated with dissolved oxygen, alkaline (pH 10.4) and exceedingly clear. The floor of the lake is covered with photosynthetic microbial mats to depths of at least 100 m. These mats are primarily composed of filamentous cyanophytes and form two distinct macroscopic structures, one of which--cm-scale cuspate pinnacles dominated by Leptolyngbya spp.--is common in Antarctica, but the second--laminated, conical stromatolites that rise up to 0.5 m above the lake floor, dominated by Phormidium spp.--has not previously been reported in any modern environment. The laminae that form the conical stromatolites are 0.2-0.8 mm in thickness consisting of fine clays and organic material; carbon dating implies that laminations may occur on near decadal timescales. The uniformly steep sides (59.6 ± 2.5°) and the regular laminar structure of the cones suggest that they may provide a modern analog for growth of some of the oldest well-described Archean stromatolites. Mechanisms underlying the formation of these stromatolites are as yet unclear, but their growth is distinct from that of the cuspate pinnacles. The sympatric occurrence of pinnacles and cones related to microbial communities with distinct cyanobacterial compositions suggest that specific microbial behaviors underpin the morphological differences in the structures.

Biological modifiers of marine benthic seascapes: Their role as ecosystem engineers

“…They include chemical alteration to pore waters, biomineralisation and precipitation of specific inorganic or organic substances that form microscopically visible entities, larger scale organic and inorganic traces at the sediment surface, and animal burrows deep into the sediment (Schäfer, 1972;Ratcliffe and Fagerstrom, 1980;Bromley, 1996;Crowe et al, 2007;Hausrath et al, 2007;Jenkins et al, 2007;Sivan et al, 2007;Anderson et al, 2011). Microorganisms play a central role in biomineralisation and precipitation to produce new molecules and structures that will then be of use to other organisms.…”

Bacteriophages in Arctic and Antarctic low-temperature systems

“…Oxygen concentration in the oxic part of the lake (0-70 m) is high, up to 200% saturation; oxygen content in the transition zone (64-79 m) was from 160 to 0%, while below 80-98 m the water column was anoxic. High pH of the water (10.2-6.9) is evidence of erosion of the mostly anorthosite rocks of the lake basin (Andersen et al, 2011).…”

Ecological niches of Antarctic phototrophic communities during global glaciation

“…Cyanobacteria, belonging mostly to the genera Leptolyngbya and Phormidium, often form mats at the bottom, which sometimes occur at depths of 100 m. Mats with conical protrusions were observed in Lake Untersee (eastern Antarctica), which is per manently covered by ice ~3 m thick. These structures may be a modern model for formation of Conophyton type stromatolites [25]. The bottom sediments of Ant arctic lakes, including microbial mats, may reach con siderable thickness due to low rates of organic matter decomposition and burial by sedimentary materials.…”