Oxygen Measured in Water by Making Movies
When asked to describe a new technique that measures sedimentary oxygen content arising from the activity of burrowing animals, researcher Lubos Polerecky of Max Planck Institute for Marine Microbiology in Bremen, Germany, turned to paraphrase. The traditional technique uses an electrochemical microelectrode for a point measurement. The new approach employs an oxygen-sensitive indicator dye that is embedded in a planar optical sensor, along with blue LEDs and a fast CCD camera to produce a movie in two dimensions.
Researchers have developed a modular luminescence lifetime imaging system that measures oxygen distribution in biological samples — mapping, for example, the distribution of O2 in lake sediments and correlating it with the activity of sedimentary animals. Courtesy of Lubos Polerecky.
“One two-dimensional oxygen movie is worth a thousand microelectrode profiles,” Polerecky said.
Animals burrow into sediment below rivers and oceans and then actively ventilate the tunnels. With this bioirrigation, they provide themselves with oxygen and, if they are filter feeders, food. The impact, however, is not confined to one animal. Bioirrigation increases the sediment surface area and affects the exchange of oxygen and nutrients between water and sediment, a major reason for scientists’ interest in the activity. Microelectrodes have been used to measure bioirrigation effects, but they can do so only at a single point with sufficient temporal resolution.
To overcome these limitations, the researchers — Polerecky and co-worker Peter Stief, who now is at Aarhus University in Denmark, and Nils Volkenborn of Alfred Wegener Institute for Polar and Marine Research in List, Germany — embedded an oxygen-sensitive fluorophore in a polymer and spread the mixture on a plastic foil.
The modular luminescence lifetime imaging system creates movies that show changes in O2 distribution during the burrowing activity of an insect larva in sediment. This single frame was acquired approximately 22 min after the larva was introduced to the sediment. The top of the image is near the sediment-water boundary.
When illuminated, the dye fluoresced with a known intensity and lifetime, but in the presence of oxygen, the fluorescence dimmed and the lifetime decreased. As reported in Environmental Science and Technology on Sept. 15, they placed the sensor in a tank with a plastic partition running its length, filled the tank with sediment and topped everything with water.
They illuminated the sensor with 450-nm LEDs made by Philips Lumileds Lighting Co. of San Jose, Calif., that operate in a pulsed mode and captured emissions above 605 nm using a fast, cooled camera from PCO AG of Kelheim, Germany. From these measurements, they determined the fluorescence lifetime — and thus the oxygen content — of the sediment near the sensor.
For their study, they introduced wormlike larvae collected from a freshwater lake into the tank and took two-dimensional oxygen-distribution images every 15 to 30 s.
The animals set to work immediately. “The burrowing larvae alternatively pumped and rested, with a great deal of variability in the duration of these periods and in the resulting volume of oxic [oxygen-bearing] sediment around the burrow,” Polerecky said.
On average, the larvae pumped for 5.4 min and rested for 9.2 min, with standard deviations of 1.7 and 5.8 min, respectively. There was no correlation between the length of pumping and the amount of rest.
As for the measurement technology, Polerecky reported that it still is being improved. One possibility might be the use of different oxygen indicators. Another might be to expand the number of measurable analytes, or to add the ability to measure such parameters as pH, carbon dioxide or temperature. It might even be feasible to measure multiple analytes using a single sensor.
Polerecky noted that biologists would like to address their ecological questions with more insight and that doing so will require device enhancements. “The goal is to develop a piece of useful instrumentation that is portable, reliable and simple to use,” he said.
Contact: Lubos Polerecky, Max Planck Institute for Marine Microbiology, Bremen, Germany; e-mail: firstname.lastname@example.org.
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