17 Jun 2002
Marine algae adapt their photosynthesis rates rapidly when the light 'goes out', even in turbulent seawater. During the eclipse in August 1999, Jonathan Sharples and colleagues at the University of Southampton, UK, studied the photosynthesis of marine phytoplankton in situ, with a fast repetition rate fluorometer.
These organisms convert carbon dioxide from the sea into carbon, to build up their cell structure, and oxygen, which they release back into the sea. This process may play an important role in regulating the amount of carbon dioxide in the atmosphere and controlling our climate.
Phytoplankton have a limited number of photon receptors and each takes a finite amount of time to process a photon of light, during which time it cannot receive any other light, and is 'closed'. The cell's protective pigments absorb any light that hits a 'closed' receptor and fluoresce it back as red light.
The team's fast repetition rate fluorometer, manufactured by Chelsea Instruments, UK, is based on a technique developed at Brookhaven National Laboratory in the US. It delivers a rapid series of high-frequency (200 kHz) light flashes and measures the fluorescence as the algae become saturated with light. The amount of fluorescence observed corresponds to the number of photoreceptors that are already processing light for photosynthesis.
Sharples explained that this study is very complicated because the organisms adjust to ambient light conditions. For example, if a cell is under low light conditions, the receptor size increases to make better use of the light. The proportions of different pigments can also change, with more photo-protective pigments under high light intensity.
He added that instruments for these studies are only just becoming commercially available and that researchers work closely with the manufacturers. There is also good communication between the five or so main groups using this technique because there is still uncertainty about how to process the results.
These groups are all studying photosynthesis, but chemists want to apply the technique to detect hydrocarbons in water.