Sources of solutes and carbon cycling in perennially ice-covered Lake Untersee, Antarctica

Abstract

perennially ice-covered lakes that host benthic microbial ecosystems are present in many regions of Antarctica. Lake Untersee is an ultra-oligotrophic lake that is substantially different from any other lakes on the continent as it does not develop a seasonal moat and therefore shares similarities to sub-glacial lakes where they are sealed to the atmosphere. Here, we determine the source of major solutes and carbon to Lake Untersee, evaluate the carbon cycling and assess the metabolic functioning of microbial mats using an isotope geochemistry approach. The findings suggest that the glacial meltwater recharging the closed-basin and well-sealed Lake Untersee largely determines the major solute chemistry of the oxic water column with plagioclase and alumino-silicate weathering contributing < 5% of the Ca 2+-na + solutes to the lake. the tic concentration in the lake is very low and is sourced from melting of glacial ice and direct release of occluded co 2 gases into the water column. The comparison of δ 13 c tic of the oxic lake waters with the δ 13 c in the top microbial mat layer show no fractionation due to non-discriminating photosynthetic fixation of HCO 3 in the high pH and carbon-starved water. the 14 C results indicate that phototrophs are also fixing respired CO 2 from heterotrophic metabolism of the underlying microbial mats layers. The findings provide insights into the development of collaboration in carbon partitioning within the microbial mats to support their growth in a carbon-starved ecosystem. Numerous perennially ice-covered lakes have been inventoried in Antarctica, including in the McMurdo Dry Valleys (MDV), Bunger Hills, Vestfold Hills, Schirmacher Oasis, and Soya Coast 1,2. These lakes have varied chemistries as a result of source water and Holocene history of the lakes, but many are oligotrophic and support benthic cyanobacterial mats and heterotrophic bacterial communities 3-5. Primary productivity is often limited by light attenuation through the ice-cover and nutrient availability (e.g., C and P), however, summer moating and streams provide seasonal recharge of nutrients to the lakes 4,6. Analysis of carbon isotopes (e.g., δ 13 C, 14 C) can provide insights about the source of carbon and transformations of organic matter as photosynthesis and remineralization control the isotopic composition of most organic matter. For example, carbon isotope geochemistry of dissolved inorganic carbon (δ 13 C DIC) and organic carbon (δ 13 C DOC) have been used to trace carbon sources and cycling in the MDV lake ecosystem 3,7-9. Lake Untersee is a 169-m deep ultra-oligotrophic lake that is substantially different from other lakes in Antarctica 10,11. It is recharged by subaqueous melting of glacial ice and subglacial meltwater, and the lake remains ice-covered with no open water along the margin (summer moating) that would provide access to nutrients, CO 2 and enhanced sunlight 12-14. In the absence of large metazoans, photosynthetic microbial mats cover the floor of the lake from just below the ice cover ...

Microbial Diversity of Pinnacle and Conical Microbial Mats in the Perennially Ice-Covered Lake Untersee, East Antarctica

“…For example, a diverse community of diatoms including, Navicula , Nitszchia , and Diadesmis have been reported in lakes Hoare ( Spaulding et al, 1997 ), Fryxell ( Jungblut et al, 2016 ) and Vanda ( Sumner et al, 2016 ). In Lake Untersee, the paucity of diatoms has been linked to the extreme high pH ( Andersen et al, 2011 ), which has been linked to weathering of clay minerals in an ice-sealed water body ( Wand et al, 1997 ; Andersen et al, 2011 ; Marsh et al, 2020 ). High pH extends many millimeters into the mat surface in Lake Untersee ( Hawes et al, 2019 ) and this has been shown to constrain growth in diatoms ( Chen and Durbin, 1994 ), although there are examples of diatoms tolerant of high pH ( Sutherland, 2009 ).…”

“…The feldspar was shown to be Ca plagioclase, and the clays minerals typically had a Al:Si:O mole ratio of 1:2:5–6 ( Andersen et al, 2011 ). The organic C abundances of the microbial mats is 10.14% in the upper pigmented sections (0 – 0.5 cm) (Andersen, unpublished data) and range from 1.0 to 5.8 % in the lower laminations, decreasing with depth within the mats ( Marsh et al, 2020 ). Microbial mats have been observed to extend from 8 to 130 m deep in the lake; pinnacles were observed at depths up to around 20 m whilst cones were observed regularly throughout the lake to depths of 130 m ( Figure 1B ).…”

Effects of Seasonal Freezing-thawing on the Protistan Communities in a Mountain Lake

“…This suggests that terrestrial carbon subsidies make an important contribution to seasonal trends in microbial communities in the mountain lakes. TOC can be utilized by aquatic heterotrophic microorganisms [25,26]. Because organic carbon is an important energy material for heterogeneous microorganisms, changes in organic carbon lead to changes in protistan community structure.…”

Glacial lake outburst floods enhance benthic microbial productivity in perennially ice-covered Lake Untersee (East Antarctica)

“…The presence of the boulders is evidence for the presence of a thick, continuous ice cover for at least the past hundred years or perhaps throughout the Holocene 20 . The lake is recharged entirely by subaqueous melting of glacial ice and subglacial meltwater 21 , and it has a Na(Ca)-SO 4 geochemical facies with high pH (10.6) that reflects a heritage of recharge from glacial meltwater, with minor contribution of Ca 2+ -Na + solutes from in situ weathering of plagioclase and aluminosilicate minerals 22 . The total inorganic carbon (TIC) and total organic carbon (TOC) concentrations in the lake are very low (0.3-0.4 mg C L −1 ) 22 , 1-3 orders of magnitude lower than in the MDV lakes and Antarctic subglacial lakes [23][24][25][26] .…”