6. Purple Bacteria

Since recovering from my illness (thanks, Mr Penicillin!) I’ve been working in the lab. Growing bacteria in a bottle is a lot more fun than in the tonsils. They are beautiful.

            These are the “purple” bacteria we’re growing for hydrogen production, after six

Aren't they beautiful?

days of growth. Purple bacteria are a group of organisms that photosynthesize using an entirely different set of molecular machinery than that found in plants or algae. The proteins they use to harness light for energy absorb at different wavelengths, which is what causes them to appear purple or red instead of green. This trait has proven so useful that the genes coding for these processes seem to have been transferred horizontally into a number of unrelated organisms, creating the extremely diverse group of bacteria lumped together by this shared trait.

            These bacteria possess an extremely diverse array of characteristics. Almost all (of those studied, anyway) are able to live aerobically or anaerobically; they can also grow via any of the four major metabolic routes (chemoheterotrophy [chemicals as energy and carbon source, like animals], photoautotrophy [light for energy and CO₂ for carbon, like plants], chemoautotrophy, or photoheterotrophy [only bacteria do these]). They are found in all kinds of environments: wastewater sludges, salt- or freshwater sediments, soil, etc. And, as far as I can tell, they do not cause any diseases in humans or other animals. So I caught my bacterial sickness from another source.

            My project involves testing the efficacy of these bacteria as a secondary treatment stage in the fermentation of wastewater. The wastes are first put into a normal (“dark”) fermentation digester, where anaerobic bacteria break them down into gasses (in this case, energy-rich hydrogen and waste CO₂), water, and simple organic molecules like acetic acid and butyric acid. These organic molecules still contain lots of energy, but the bacteria in the dark fermenter cannot break them down any further.

 

Reactors, just after inoculation. Nice, fresh sludge.

           The effluent of the dark fermenter, therefore, is treated further in a photo-fermentation reactor using purple bacteria. Their diverse metabolisms can break the remaining small molecules down, leaving nothing but hydrogen, CO₂, and biomass. They do not produce hydrogen from the initial substrates as efficiently as other bacteria, so are used only to finish the conversion; but, their ability to survive under so many conditions means they can be left in the dark fermenter broth. They can live there on the sidelines while the dark fermenters produce lots of hydrogen, and then take over to convert the residues once the lights are turned on.

            We’re working here with a whole diverse group of purple bacteria instead of single-strain monocultures – this is a little trickier than working with a single species, but is expected to yield better results on real wastewater mixes. Lots of different bacteria mean they can metabolize lots of different contaminants in the waste, leading to cleaner water and hopefully more hydrogen. This is something that hasn’t been done very much yet with purple bacteria, which is also a good thing – moving the technology a little closer to practical application!

Starting to grow!

Next time: Hiking trip (for real).

Previously: MAGIC