Plants and Geothermal CO₂ Exhalations — Survival in and Adaptation to a High CO₂ Environment

Abstract

Modern plants live in a rather “low CO₂”- world when compared to CO₂ concentrations in the atmosphere during prehistoric evolution (Petit et al. 1999). Similar to Mars and Venus, CO₂ on planet Earth might have been as high as 90–98% during the early days of photosynthetic evolution (Emiliani 1992; Raven 1995; Grace and van Gardingen 1997). But concentrations decreased gradually during epochs to reach only a few hundred ppm, although [CO₂]¹ is again steadily increasing since the last 200 years (Bowes 1993). Values nowadays are dramatically lower by a factor of 3,000 than in those ancient times. Nevertheless, a further increase from presently 360 ppm (0.036% w/v) to ca. 700 ppm (0.07% w/v) is thought to take place within the current century (WMO 1990; Bowes 1993; IPCC 1996). Predictions for the second half of the present century range from 415 to 575 ppm depending on a CO₂ emission rate of ±2% (Houghton et al. 1990; Cook et al. 1997). During evolution, plants had to cope with and adapt to a slowly but permanently changing CO₂ environment including periods with increasing and others with decreasing CO₂. Although ambient CO₂ does nowadays not saturate C₃ photosynthesis, plants have evolved mechanisms to rather effectively capture and photo-reduce the oxidised carbon to the level of carbohydrates.