By Charlie Whittaker
Life in water isn’t all plain sailing, particularly if you’re photosynthetic. As well as the problem that you’re wet all the time, it actually poses pretty big problems for a cell’s CO2 uptake (something essential for photosynthesis). Most CO2 in water is dissolved, and in the form HCO3-. As well as this, stuff in general diffuses far more slowly in water than air, all in all causing a massive problem for the organisms that need it.
In response to these challenges, aquatic organisms have evolved a wide array of means of stuffing themselves with as much CO2 as possible. These are known as Carbon Concentrating Mechanisms (CCMs for short) and are found in a huge number of different algal species, as well as the critters that gave rise to the chloroplasts, the cyanobacteria. In algae, the CCM relies upon the usage of a subcellular structure called the pyrenoid. All of the Rubisco (the enzyme that uses the CO2) aggregates in a specific part of the chloroplast, and CO2 delivered, in doing so generating awesomely high local concentrations. Cynaobacteria on the other hand use Carboxysomes, which are protein covered boxes chock full of Rubisco.
Either way, they’re pretty neat, and enable these guys to generate over 50% of the world’s primary productivity, despite the unfavourable conditions!