Organism of the week #3 – Switch off all electrical items on the approach

There are many good reasons to visit San Francisco, some of them thoroughly unsafe for work, but one I didn’t consider was to see this amazing sight on the approach to the airport:

Halobacterium salinarum [CC-By-SA-3.0 Steve Cook]

Salt ponds at the southern end of San Francisco Bay

The pink colour in the water is caused by a bloom of single-celled organisms I must refrain from calling bacteria, since they are not. Nor are they eukaryotes, the group to which you and I, and plants and fungi and protists belong. The things living in the water are halo’bacteria’, named before it was realised we needed a whole new classificatory scheme to accommodate their weirdness. Halobacterium is a member of the Archaea, which includes many extremophiles. Some are capable of living in near-boiling water, hot acid, or – as here – in very salty water indeed, but some are much more commonplace and allow some humans to set light to their farts.

As well as being not-bacteria and not-eukaryotes, halobacteria also confound naïve human assumptions about how to make a living. If you look only at multicellular organisms, you find they fall mostly into two camps: photosynthetic organisms like plants, which need light, water, carbon dioxide, and not much else to grow; and heterotrophic organisms like humans and fungi, which steal almost everything they need to grow either directly or indirectly from photosynthetic organisms.

Halobacteria are pink because they contain a pigment called bacteriorhodopsin. They use this pigment to capture light from the Sun. This energy is used to run their biochemistry and to help pump salt into the cell to prevent it from popping from osmotic stress. But unlike green plants, they can’t fix carbon dioxide into sugars and other complex organic compounds. Like us, they have to get their carbon in a fixed organic form, mostly in the form of amino acids. Many bacteria and archaea have metabolisms that don’t fit the dreary ‘plant-like’ vs. ‘animal-like’ division, and this is one of the many reasons I love them so.

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