What limits bacterial production in the suboxic region of permanently ice-covered Lake Bonney, Antarctica?

Abstract

Bacterial production assays (thymidine incorporation rates) were used to evaluate the activity of heterotrophic bacteria at the chemocline region in both the East (ELB) and West (WLB) Lobes of permanently ice-covered Lake Bonney, in the Taylor Valley of Antarctica. The magnitude of activity varied dramatically within the depth interval of 1 to 2 m from moderate to very low levels below the chemocline, especially in the East Lobe, where chemical distributions indicate the absence of a normally functioning nitrogen cycle. Several parameters (e.g. addition of nutrients or chelators, dilution) were manipulated in incubation experiments in order to identify factors that would enhance activity in the suboxic deep waters of the East Lobe. Activity, in terms of thymidine incorporation, was consistently detected in the deep-water communities, implying that, although the water may be 'toxic', the cells remain viable. None of the treatments resulted in consistent enhancement of thymidine incorporation rates in samples from below the chemocline. Bacterial populations above the chemocline appear to be phosphorus-limited. The nature of the limitation, toxicity or inhibition that limits bacterial activity in the suboxic waters has not been identified.

Denitrification in the hypolimnion of permanently ice-covered Lake Bonney, Antarctica

“…Samples were collected in November and December of 1999 and 2000. Water was collected from a central station in each lobe of Lake Bonney using trace metal clean sampling techniques (Bruland et al 1979) as described previously (Ward et al 2003). Briefly, a peristaltic pump equipped with acid-leached C-Flex tubing (Cole Parmer) was used to pump water from depth through an acid-cleaned teflon tube which was lowered through a hole drilled through the 4 m thick permanent ice cover.…”

“…Polyethylene-lined trilaminate bags (Pollution Measurement Corporation) were used for sample collection and incubation (Ward et al 2003). Bags were acidwashed and flushed with N 2 gas prior to use.…”

“…In the 4°C room in a glove box flushed with N 2 , the water was diluted in a large beaker with appropriate amounts of either Milli-Q water or artificial lake water (Ward et al 2003) that had also been flushed with N 2 , and then pumped into the 500 ml incubation bags using a small peristaltic pump. Dilution with artificial lake water was designed to remove the metal compo-nent of the solutes without changing the salinity, which was approximately 80 at the chemocline depth of the east lobe.…”

“…Although thymidine incorporation rates were very low below 19 m in the east lobe, the lake is not in fact completely devoid of biological activity. In an extensive set of incubation experiments, we made multiple measurements of thymidine incorporation rates in subchemocline waters of the east lobe (Ward et al 2003). Very low rates of bacterial production could be detected, but no combination of manipulations such as organic carbon or phosphorus additions, dilution of the salt and metal content and chelation of bioactive metals in the deep water significantly enhanced production rates in the hypolimnion samples.…”

Supersaturated N2O in a perennially ice‐covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict

“…Other studies have shown that little microbial activity of any kind occurs in the deep brine of ELB despite the presence of viable cells (Priscu 1997;Ward and Priscu 1997). This lack of microbial metabolism may be the result of metal toxicity associated with the deep brine (Ward et al , 2005 or thermodynamic constraints that have evolved over time (Lee et al 2004). In the absence of modern activity consistent with the observed N 2 O depth distribution, it has been suggested that the extreme N 2 O supersaturation in ELB may be a biogeochemical relict from past microbial activity that was present during the climate-driven evolution of the lake Priscu 1997).…”