Ian gets ready for diving
Donna and Maria help Ian get ready for diving.
Donna entering the hole
Maria descends into the hole
Click on the photos to see a larger view

When Ian Hawes, Donna Sutherland, and Maria Uhle said they were putting on suits, they weren't heading to the office or going to a business meeting. Instead, the three scientists were getting dressed in dive gear known as dry suits and preparing to lower themselves into an ice hole that was three feet in diameter, 15 feet long, and filled with water that had an average temperature of 32 degrees Fahrenheit (0 degrees Celsius).

The reason they did this was because it was the only way they could get themselves below the permanent ice that covered Lake Hoare in Taylor Valley, and they wanted to explore the waters and life beneath it.

Ian and Donna, who are with New Zealand's National Institute of Water and Atmospheric Research, and Maria, who is with the University of Tennessee in Knoxville were this year's Dry Valleys' (DV) dive team.

Their goal was to study photosynthesis in low light environments. DV lakes are about as low as you can go and still have photosynthesis take place.

Typically less than one percent of the light that falls on the lakes makes it through the 15 to 20 feet of ice that cover them, says Ian, who has been a part of the project since its inception in 1994. But the bottom of Lake Hoare, report the divers, is blanketed with several cm thick mats of organisms that make the most of that wee bit of light. These bottom, or benthic dwellers, include cyanobacteria and diatoms, which use light as an energy source to convert carbon dioxide and water to carbohydrates in a process known as photosynthesis. They also include protozoa and ordinary bacteria, which are involved in the cycling of carbon and nutrients.

Ice view from inside the dive hole
Maria gets a view of the lake ice from inside the dive
hole. Note the sediment trapped inside the ice.

Click on the photo to see a larger view

Studying photosynthesis in DV lakes is difficult because of the inherent problems of working under thick ice cover. It is also hard because the rates of photosynthesis are very low when compared to rates in temperate zones.

What happens to the benthic mats
during the winter?

Benthic mats
Benthic Mats

The benthic mats essentially enter a period of hibernation in which their metabolism is reduced to maintenance levels. So the mats gradually consume the carbon they fixed during the summer, explains Donna.

Despite this, the mats still manage to grow a few millimeters each year. Yet if the DV lakes had insects or mollusks that fed on the microbial mats, as is the case in many other lakes, winter would be a time when the mats would be depleted, notes Ian. But in DV lakes these consumers are absent, and the microbial mats are free to slowly accumulate to their spectacular formations. This is a slow process as a 10 cm high benthic mat may have taken many years to develop.Slow, undisturbed growth, coupled with the efficient use of energy and internal cycling of nutrients, is what allows the benthic mats to dominate the DV lakes.

To address this latter problem, the team made use of a new approach that measures photosynthesis with a fluorometer. "Photosynthesis," explains Ian, "is really just a flow of electrons." Fluorometers measure that flow. "The amount of photosynthesis taking place is pretty much directly proportional to the electron flow measured."

Fluorometer analyses have shown that DV lake cyanobacteria and diatoms are well-suited to the very low light regime that they get, says Donna. Photosynthetic organisms require a certain amount of light for their photosynthetic systems to kick into gear and start operating. The threshold for the DV lake cyanobacteria and diatoms is incredibly low. This specialization has a cost, however - the DV organisms can't use light at intensities much higher than 1% that of temperate zone noonday sunlight. But that's not a problem, notes Ian. They rarely receive more light than this.

Because DV lake cyanobacteria and diatoms get such low amounts of light, they photosynthesize at rates that are much slower than their temperate counterparts. Yet says Donna, they are able to use that light very efficiently. Scientists have been trying to figure out why.

They've found that it has a lot to do with how well the organisms absorb light energy. The amount and kind of light reaching the benthic mats is influenced by their location at the bottom of a lake, an ice-covered one at that. Water in general, and ice in particular, filter out wavelengths of light at different rates. Colors associated with longer wavelengths, such as red and yellow, are the first to be absorbed while colors associated with shorter wavelengths, such as blue, are absorbed last. So the little bit of light that does make it to the bottom of Lake Hoare is blue.

Both the DV cyanobacteria and the DV diatoms are very efficient at soaking up this blue light. They have pigments that allow them to absorb around 80% of the light that hits them. Of this light, an average 90% is converted into the electrochemical reactions that comprise photosynthesis. The pigments are an adaptation that helps the cyanobacteria and diatoms survive and thrive in the Dry Valleys.

Photos courtesy of Karen Cozzetto

- Karen