Limitations on Photosynthesis
While light is often assumed to be the limiting factor on photosyntheis, that is usually not the case. Photosynthesis is very complex, involving multiple reactions taking place in different compartments of the chloroplast. This complexity means that there are many different factors which can affect the rate of photosynthesis, but the availability of CO2 is often crucial, especially for algae.
Multiple factors contribute to the scarcity of bioavailable CO2 in marine/freshwater environments:
1. CO2 diffuses 10,000 times slower in water than in air.
2. CO2 is slow to equilibrate between water and air, often resulting in CO2 depleted waters which are slow to gain CO2 from the air.
3. In water, CO2 equilibrates with HCO3–, on a pH dependent basis – the pH of seawater (7.2) results in relatively low CO2 availability, with most of it existing as HCO3– (bicarbonate).
In addition to the above limitations on photosynthesis set by the aquatic environment, a complication faced by all photosynthetic organisms is the non-specificity of RuBisCO for CO2 over oxygen. The enzyme RuBisCO – whilst undoubtedly the most abundant and most important enzyme on the planet – is very old and very slow, with a turnover rate of about 3 molecules of CO2 per second. RuBisCO evolved at a time when atmospheric O2 concentrations were close to zero (compared to now, when about 21% of our atmosphere is oxygen), and as such did not evolve to distinguish between its “intended” substrate, CO2, and similarly-sized O2. This results in the competing carboxylase activity (which produces 3PGA directly for entry into the CBB cycle) and oxygenase activity (which produces 2-phosphoglycolate, that must be converted to 3PGA (which can enter the CCB cycle) in the energy-demanding process of photorespiration, to retrieve the lost carbon):
This results in a further reduction in the efficiency of carbon fixation.
To learn how plants and algae overcome these limitations, see Carbon Concentrating Mechanisms.