Autotrophic Specializations The CO2Concentrating Mechanisms

All photoautotrophic organisms ultimately assimilate CO2 by the carboxylation of RuBP to yield PGA. Because PGA is a three-carbon compound, this biochemical pathway of carbon assimilation is called 'C3 photosynthesis'. In addition to carboxylation, Rubisco catalyzes a side reaction where O2 is fixed to RuBP, yielding PGA and a two-carbon compound, phosphoglycolate (PG). PG has no metabolic value and is toxic in high concentrations. Algae avoid toxicity by excreting a derivative of PG into the surrounding aqueous medium. Terrestrial plants do not have this option, given the lack of an aqueous bathing medium. Instead, in a process termed photorespiration, they metabolize PG to PGA using ATP and reducing power. Previously fixed CO2 is released during photorespiration. Because of this loss of CO2 and the energy used in converting PG to PGA, oxygenation of RuBP and photorespiration inhibit photosynthesis. The strength of the inhibition depends upon temperature and CO2 concentration (Figure 3). In the current atmosphere, warm temperatures promote photorespiration, while it is relatively minor below 12 °C. Above 30 °C, photorespiration can reduce photosynthesis by over 30%. RuBP oxygenation and hence photorespiration are greatly reduced at high levels of atmospheric CO2 (>1000 ppm).

For much of Earth's history, RuBP oxygenation was inconsequential because CO2 levels were much

High in o o

High

Without photorespiration

(e.g., at 1500 ppm CO2)

Photorespiratory

inhibition

With photorespiration

(380 ppm CO2)

Leaf temperature (°C)

Figure 3 The response of net photosynthesis rate to temperature in C3 plants in the absence (upper curve) and presence of photorespiration (lower curve). The difference in the two curves reflects the inhibition of photosynthesis by photorespiration. The responses were generated by a theoretical model of photosynthesis assuming current levels of atmospheric CO2 and oxygen for the photorespiration curve, and 2% oxygen and high CO2 for the photorespiration-free curve. For a description of the model, see Sage RF and Sharkey TD (1987) The effect of temperature on the occurrence of O2 and CO2 insensitive photosynthesis in field grown plants. Plant Physiology 84: 658-657.

higher than today. Theoretical models of photosynthesis predict that oxygenation and photorespiration was significant during two low-CO2 periods in Earth's history: the Carboniferous and early Permian period (280-320 Ma), and the past 30 million years when angiosperms dominated the Earth's vegetation. These low-CO2 episodes are associated with evolutionary modifications to C3 photosynthesis that concentrate CO2 into an internal compartment where Rubisco is localized. In doing so, photorespiration is reduced to negligible levels and photosynthetic efficiency is enhanced. The major CO2-concentrating mechanism in aquatic habitats is the pumping of dissolved inorganic carbon (DIC) by algae. On land, CO2 concentration occurs via C4 photosynthesis, and Crassulacean acid metabolism (CAM) photosynthesis.

compartment (in eukaryotic algae) where Rubisco is localized (Figure 4). CO2 levels are increased 10-50-fold inside these compartments, allowing for near-complete suppression of photorespiration. Most algae use DIC pumps except those in turbulent, cold, or CO2-enriched water. DIC pumping greatly enhances the potential for NPP, but this potential is not often observed because most waters are nutrient-limited. Where there is a sudden rise in nutrient level, the productive potential conferred by a DIC pump becomes apparent in the rapid formation of algal blooms. Because of the ability to concentrate CO2, most algae already operate Rubisco near CO2 saturation and may not respond strongly to future increases in atmospheric CO2.

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