by Sharon Beder
The Basic Mechanism of Photosynthesis
Photosynthesis has two components, light reactions and carbon dioxide fixation. There are two major types of light reactions in photosynthesis called photosystem 1 and photosystem 2. They are linked in series, a bit like a bicycle with two wheels, both of which are needed to get along. These two photosystems work together to take up light and produce the necessary energy to be able to use carbon dioxide from the air. It appears at this stage that only photosystem 2 is affected by the kind of increase in UV that can be expected.
In photosystem 2 water is split to form oxygen. It is the first part of the process in which electrons are released from water and are passed on to carbon dioxide together with protons. Over the last couple of years a lot has been discovered in this field. It is now fairly precisely known where the UVB is acting within photosystem 2. This is largely fallout from the work of Michel and Diesenhofer in Germany who got a Nobel prize for working out the basic mechanism for photosystem 2 in 1988.
After their work was published it was then possible to ask where UVB might take affect because all the components were known. It turned out to be one component which is on the oxidising side of photosystem 2 very close to where water is split and it can probably now be identified as a single amino acid residue which absorbs UVB. This seems to modify the amino acid and stop the transfer of electrons through photosystem 2, which knocks out photosystem 2 and so knocks out photosynthesis.
Once one of the proteins is damaged in this way the system is impaired. There are repair processes going on all the time in all organisms to replace proteins and that happens with this particular protein too. It is known that this protein is the most highly turned over protein of any in plants. That is probably because there is always UV coming in when there is light around. So there is natural damage to it and a natural turnover. It takes about 18 hours for the repair process to take effect. So if a plant is heavily damaged out in the midday sun or if it is subjected to UVB by artificial means, it will take about 18 hours to repair before it can photosynthesise properly again.
So if there is a whole string of sunny days a plant may end up with a very impaired system at the end. The situation is much worse in the polar regions because this repair process is temperature dependent and won’t take place under cold conditions. This process of damage and repair is continually going on under present conditions in algae in temperate waters around Sydney. But in Antarctica all the conditions are there for much greater damage.
Apart from repair, plants have a second mechanism for coping with UVB damage. This involves the production of compounds which absorb UVB. There is a family of these compounds called screening compounds which are produced by nearly all plants and algae. The class of compounds produced by algae in response to UVB are called mycrosporine compounds. When these compounds are present they can absorb UVB, up to a point, and block the radiation from harming the photosystem, but they are not entirely effective. Also, if the UVB is removed their production slows down.
All these compounds appear to be water soluble. They are a bit like a sunscreen in that they only screen out a proportion of radiation. Some of it still gets through. The production of these compounds is not a guarantee of absolute protection. They were first discovered in coral and a patent has been taken out to use this type for human sunscreen. It is currently being tested to see if there are any side affects.
All algae have these compounds and it can be shown that they produce more under high radiation levels. But they apparently have problems dealing with variable weather conditions. A common condition in Antarctica is one where there may be a long period, perhaps weeks, of very cloudy weather. During this period there may be very little UV or visible light and so the plants stop producing these protective compounds. Then suddenly the weather might clear up and there will be a number of brilliant days. Larkum has demonstrated that in these circumstances the algae are not always well prepared. When the sun comes out the compounds are immediately produced but they don’t get up to a protective level for a few days.
The effect of the ozone hole on life in Antarctica is not easy to determine. It is a very difficult situation to assess because the ozone hole occurs just at the end of winter. At this time there is almost no light and then the hole disappears again by the beginning of summer when normal levels of ozone resume. The task is to understand what the effect of this 50% reduction of ozone is over those 3 months.
It is complicated by the fact that the ice sheet extends over a very large area, stretching for hundreds of miles off the coast of Antarctica. The ice will absorb a fair amount of visible and UV radiation and extends beyond the known latitude of the ozone hole. From a protective point of view, the annual appearance of the ozone hole might be at a time of year when it does less damage than it otherwise would because the ice sheet is at its largest when the weather is coldest.
However, it does come at the time of the spring blooms of phytoplankton. The spring bloom happens right on the edge of the ice sheet, well away from Antarctica, and there have been very few measurements of UV levels at that periphery. There are some instruments now working at the bases in Antarctica but that is in the wrong place. At the moment very little is known about the ozone hole, certainly not nearly enough to predict whether it has an affect on the phytoplankton in this critical period.
What is known is that in the Antarctic summer the normal levels of UV are high enough to inhibit photosynthesis, both by the phytoplankton and by the benthic algae. What remains unknown is what effect the ozone hole has. It seems clear however that there is a lot more work required before this is can be determined.