CONTENTS

Introduction
Human Skin Cancer
Sunscreen and Fabric
The Mouse Model of Cancer
Studies Using Skin Tissue
Drugs and Sunlight
Plant and Algae Growth
Conclusion
Glossary
Bibliography

Methods

The method of measuring photosynthesis is to measure the oxygen produced, which is a by-product of photosynthesis. Oxygen electrodes are put into dishes and then sea water is stirred in. A bit of the benthic algae or a sample of the phytoplankton is then placed in the dish, the light turned on and a measurement taken of the rate at which oxygen is produced. The algae can be irradiated at various levels of UV light starting with no UVB then gradually increasing amounts of UV light.

The sequence of spectral colours going from the visible region is from blue to violet and into ultra-violet. Ultra-violet is divided into different ranges of wavelength for convenience. UVA is the one closest to visible light in the range of 320-380 nanometers. UVB is the middle range of the ultraviolet radiation from 280-320 nm. Below the UVB region is the UVC region which is from 200-280nm. “The most damaging radiation is the UVC, then UVB and then UVA, which is not very damaging at all,” says Larkum. “Radiation up around the level of 320nm doesn’t do a great deal of damage but as it gets down to 300nm damage increases dramatically. Down at 280nm the damage is potentially 10 times greater than at 320nm. So that if there is a very small change in the curve due to ozone depletion it can create conditions where there is a really large effect.”

The reason that most of the research is concentrated on UVB and not UVC is that there is no UVC hitting the earth’s surface; it is all absorbed by oxygen in the stratosphere and that is what creates ozone in the first place. The cut off point, between those wavelengths that get through to the earth and those that don’t, is right in the UVB region. Below 290 nm there is very little or no radiation which reaches the earth’s surface. So even though UVB is defined from 280nm this wavelength of UVB radiation doesn’t in fact reach us.

One of the practical problems Larkum has to master first is how to measure UVB levels accurately enough to really know how much damage is being done by UV light in each range of wavelengths. “We have got a very large light source which has been set up for work on skin cancer out at Westmead hospital,” he says. “You select the wavelength you want. We are gradually going through the wavelengths to find out what the most damaging wavelengths are.”

The technique is to study a succession of 2nm wave bands. To start with the algae are exposed to 280-282nm for a given length of time with a given amount of energy and the effect is observed. Then the band between 282-284 nm is observed in the same way under the same conditions and so on through the whole UVB range. In this way he will get an “action spectrum” of damage to photosystem 2. The object is to identify a peak where the damage is the greatest.

This has already been done for other processes. For example skin cancer shows a peak at about 300nm. It has also been done for the Hill reaction which is an overall reaction for photosynthesis. Here the peak has been found to be around 290nm so the peak for damage to photosystem 2 is expected to be somewhere in that region.

A second study Larkum is doing is to look at screening compounds. Screening compounds are water soluble and are easily extracted with methanol from parts of plants. Once they are extracted it is then possible to measure how effective they are in absorbing UV. That gives the first estimate. Then they are put through a high performance liquid chromatograph where the individual compounds can be separated out. A good laboratory is needed because the chromotographic equipment is very expensive and quite sophisticated.

“We do field experiments where we screen plants to filter out UV light,” says Larkum. “We allow plants to exist for lengths of time, perhaps a week under these conditions. Then we look to see whether they are still producing their own screening compounds. This is how we know that the level of screening compounds goes down when you remove the UVB light.”