Life thrives on Antarctic lake floor
Scientists from NIWA have made the first measurements of photosynthesis in the icy gloom of an Antarctic lake floor.
3 July 2006
The discovery, published in the July issue of Limnology and Oceanography, may help to shed light on past Antarctic climate, and even the origins of life on Earth and other planets.
The measurements were made by aquatic ecologists Dr Kay Vopel and Dr Ian Hawes in November 2004 at Lake Hoare, an 18 m deep perennially ice-covered meltwater lake in the Dry Valleys of Antarctica. Scientists have long been interested in the bacteria and microscopic algae that inhabit such lakes, because they offer insights into the extreme conditions under which life can exist.
The floor of Lake Hoare is covered with bright pink mats of cyanobacteria and microscopic algae. 'The idea that these microbial mats may be capable of photosynthesising has been quite controversial, because the light levels are so low. We were pretty sure that this was a photosynthesis-based community, having demonstrated photosynthesis in the lab, but the unusual conditions meant we needed actual measurements on the lake bottom to be absolutely sure', says Dr Hawes.
The scientists got their proof by diving under 5 m thick ice and measuring fine-scale gradients in concentrations of dissolved oxygen around the mats, nearly 17 m below the surface. They measured photosynthesis at lake depths where noon irradiance was less than 0.05% of surface light intensity, and results suggest that it may be possible at intensities 10 times lower still.
These are among the lowest natural light levels at which photosynthesis has ever been recorded. 'There are some tantalising observations of marine plants growing on rocks in very deep water, and of algae and cyanobacteria growing in caves where there may be even less light, but these are certainly amongst the lowest where field observations have been made', says Dr Hawes.
The microbial mats form layers corresponding to summer growth and winter stasis. "Because growth is related to the transparency and thickness of the lake’s ice cover, these layers may give us a fine-scale record of climate conditions in Antarctica that supplement the coarser scale records from ice and snow cores," explains Dr Vopel.
The scientists plan to return to Antarctica this November to study microbial mats at greater depths in Lake Hoare and in another meltwater lake, Lake Fryxell.
- A copy of the paper will soon be freely available at: aslo.org/lo/toc/vol_51/issue_4/1801.pdf
Vopel, Kay, and Ian Hawes (2006). Photosynthetic performance of benthic microbial mats in Lake Hoare, Antarctica. Limnology and Oceanography, 51 (4), 1801-1812.
- Photosynthesis is the process by which plants convert sunlight to energy, absorbing carbon dioxide and releasing oxygen in the process.
- Lake Hoare is a meltwater lake in the Dry Valley region of Southern Victoria Land, the largest ice-free region of Antarctica, and one of the coldest and driest deserts on Earth.
- The micro-organisms living in the lakes may offer insights into the origins of life on earth or other planets: "We're studying a community that appears almost exactly the same as the earliest communities in the fossil record, 2 billion years ago," says Dr Hawes. "They give clues as to how the earliest communities were organised on Earth and the conditions they could survive in, either here or on other planets," he adds.
- The scientists had to first drill through 5m of ice, then dive down to depths of around 16 m in special suits, attached to an air and communication line. Using very fragile underwater microelectrodes, they measured changes in oxygen concentrations over distances of less than 0.5 mm around the mats.
- "It’s dark down there, but once you turn on a light, it’s very colourful," says Dr Hawes. The microbial mats are bright pink (the pink pigment helps them absorb the low levels of blue light that penetrate the lake ice) and have "weird and wonderful structures, with small pinnacles and folds protruding above the lake floor."
- In the past, scientists thought that all biological production happened in the body of these lakes. "We now know that at least half the production is occurring on the bottom. There needs to be a substantial rethink about how these lakes operate," says Dr Hawes.
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