Effects of increasing levels of C02 and 02 derived compounds on biogeochemical cycles and shaping of the biosphere

Plants reduce C02 to molecular species of increasing energy content. These compounds are then moved by heterotrophic organisms through decreasing energy levels as the organic matter is returned to inorganic form by successive oxidations. Thus, C02 and 02 are the major substrates of biospheric reactions. Oxygen is toxic for anaerobic and, in excess, for aerobic organisms. Its toxicity is overcome by aerobic organisms by using it as an electron terminal acceptor in respiration and by developing biochemical defenses. It still determines phenomena such as anaerobiosis, the depth limit of higher aquatic plants, or bioluminiscence, and perhaps others like the diurnal vertical rnigration of zooplankton. Low photosynthetic efficiency in light use, shape and color of vegetation may result from the peculiar availability of light and C02. Therefore, slight changes in these factors would produce life shapes deeply alien to the world we now perceive. Atmospheric C02 levels have increased by about 70 Ilmol mor 1 over the past 240 years, as has been shown by ice core studies and Mauna Loa data. Parallel to this increase, sorne plant changes have already been found: an overall decrease from 121 % to the present-day 1 00% level in stomatal density, and from 144 % to present-day 1 00% leaf nitrogen content of 14 herbarium specimens of trees, shrubs and herbs collected over the last 240 years in Catalonia, a Mediterreanean area. Similar declines have also been reported in experimental studies on leaves of plants grown in enriched CU2 environments. Consequently the C/N ratio has increased in leaves which together with the decrease in the specific leaf area (SLA) may have consequences for herbivores, decomposers and ecosystems. Increases in oxygen-derived compounds, mainly ozone, have potentially large biospheric effects. Nowadays, they seem to play an important role in forest decline and sorne decreased crop productions, as has been shown in laboratory experiments. Increased photosynthetic rates are expected with increasing C02 levels in the decades to come, but regional ozone increase is expected to decrease plant production. Responses of ecological systems are however complex and difficult to predict. Finally, a possible change in photosynthetic efficiency, plant shape and color is speculated to be expected because of increasing C02 levels, and the role of anthropospheric activities in the balance between reduction and oxidation in the biosphere is emphasized.