“Ocean Bacteria Glow to Turn Themselves Into Bait”

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Not Exactly Rocket Science has an interesting article on glowing ocean bacteria and why they do what they do, with a bit of an introduction here:

On 25 January 1995, the British merchant vessel SS Lima was sailing through the Indian Ocean when its crew noticed something odd. In the ship’s log, the captain wrote, “A whitish glow was observed on the horizon and, after 15 minutes of steaming, the ship was completely surrounded by a sea of milky-white color.” The eerie glow appeared to “cover the entire sea area, from horizon to horizon . . . and it appeared as though the ship was sailing over a field of snow or gliding over the clouds”. The ship took six hours to sail through it.

These glowing seas have featured in sailor stories for centuries. The crew of the Nautilius encountered the phenomenon in Jules Verne’s Twenty Thousand Leagues Under the Sea. And in 2006,  Steven Miller actually managed to recover satellite images of the very same patch seen by the crew of the SS Lima – it stretched over 15,000 square kilometres, the size of Connecticut or Yorkshire.

The glowing waters are the work of bioluminescent bacteria – microbes that can produce their own light. They are found throughout the oceans, although usually in smaller numbers than the giant bloom responsible for the SS Lima’s sighting. In many cases, they form partnerships with animals like fish and squid, taking up residence inside their hosts and paying their rent by providing light for navigation or defence.

But many glowing bacteria live freely in the open ocean, and they glow nonetheless. Creating light takes energy, and it’s not something that’s done needlessly. So why do the bacteria shine? One of the most common answers – and one that Miller proposed to explain his satellite images – is that the bacteria are screaming “Eat me!” at passing fish. A fish’s guts are full of nutrients, and it can carry bacteria across large distances. The bacteria, by turning themselves into glowing bait, get a lift and a meal.

The article goes on to explain a study that showed this in action: basically, zooplankton – a classification for small organisms that drift around in bodies of water – were given a choice of eating glowing or non-glowing bacteria, and they tended to choose the glowing bacteria. Presumably this is because the bacteria only glow when they’re grouped together, which happens when they find a tasty piece of detritus to hang onto. Glowing bacteria means there’s food around, so they’re a nice target.

As a side effect, organisms that eat glowing bacteria will sometimes end up glowing themselves, making them big targets for fish. A deadly circle of life, all so that some bacteria can hitch a ride. 

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