Some eerily beautiful images of animals turned to stone have been making their rounds around science news outlets and social media over the last couple of weeks. These black and white images are the work of Nick Brandt in his book “Across the Ravaged Land.”* At first glance they may lead you to believe that a lake in eastern Africa is so caustic that simple contact can turn animals to stone. This isn’t entirely true. It’s certainly true that the conditions in the lake will ossify animals that die there, but these stone corpses have been positioned in seemingly lifelike poses for artistic effect only. In fact, these black and white images do not do justice to the truly colorful landscape of Lake Natron.
While shallow Lake Natron is an extreme environment with a pH greater than 10 and water temperatures of more than 100 degrees Fahrenheit, it supports a thriving ecosystem. At the producer trophic level, it supports several different species of cyanobacteria that give the lake its red color. The predominant strains include species of Spirulina, Cyanospira, Synechococcus and Chroococcus. These salt-loving superphotosynthesizers also transfer their color to the millions of lesser flamingoes that feed on them and use the lake as their nesting ground.
As it turns out, the conditions in Lake Natron make it one of the most productive environments, photosynthetically speaking. The characteristic warm temperatures and abundance of direct sunlight in this shallow lake may make sense at first glance, but what about that awful pH? Haven’t we been taught that life prefers a pH closer to 7? Well, not necessarily if you are a cyanobacterium that feeds on carbon dioxide.
Still confused? Here’s a quick chemistry refresher. The solubility of carbon dioxide (i.e. the amount of it that can stay dissolved in the water) is greater at higher pH. Well, for those of you sticklers out there, this isn’t entirely true either. The solubility of the hydrated form of carbon dioxide (bicarbonate) is what increases in higher pH solutions.
These caustic aka ‘soda lakes’ ensure that the waters there are saturated with carbon dioxide and bicarbonate. All of these carbonates are coming from the alkaline lava found in the general area of the Rift Valley. There’s so much carbon dioxide and carbonate around that it exceeds the amount of other ions in the water (calcium and magnesium) which react and precipitate out as insoluble carbonate salts. Cyanobacteria have strategies for taking up both carbon dioxide and bicarbonate from their environment to ensure a steady supply to the Rubsico enzyme that can fix them into sugar. These conditions work together to create the algal blooms that give the lake its palette of red, orange and pink.
So don’t be fooled into thinking that Lake Natron is really a Death Valley.** Also, I’m not sure if I would even call the cyanobacteria that live there superphotosynthesizers either. By many accounts they have won the environmental lottery and live a pretty decadent lifestyle as far as basic photosynthetic requirements go (light, carbon dioxide, water***). But because they thrive in this seemingly hostile environment, I’m filing them under Superphotosynthesizers here on this blog.
* Nick Brandt’s book explores beyond Lake Natron and is a photographic safari across Eastern Africa featuring the region’s disappearing species. The stark images are meant to emphasize the dark effects of human activity on these ecosystems.
** LSU fans know that Death Valley is in Baton Rouge, LA. It’s an extreme environment where little life exists beyond the native tiger species.
*** Water may be the only limiting factor during some times of the year in this particular location.