Bacteria are everywhere! On Earth, at least. That includes land, air, and in the sea too! All seven of them. With animals, the ones living in the sea are pretty different to the ones on land; there aren't many tentacled land animals, for example! Most land animals wouldn't be too happy if you dropped them in the middle of the atlantic, and nearly all the fish would be similarly unhappy if you put them in the middle of a car park. Is it the same with bacteria? How have marine microbes evolved to survive and thrive in the deep?
|This is the sea, in case you weren't sure! Taken from outside my house last week.|
The environment in the sea is a bit different to pretty much every other habitat on the planet. At first glance, you'd think that any barrier to living things being happy there would be that it's too wet; it is, after all, where most of the water is. But at the scales cells experience, the opposite is true; it's too dry! Human bodies, like yours, are mostly water. Cells are like little water balloons full of proteins, and without water the proteins wouldn't be able to do their jobs, and the cell would die. If you dunk a cell, animal or bacterial, into salty water then osmosis pulls water out of the cell, making them shrivel up like a balloon that's been deflating for a few days, and for similar reasons; when there's more air inside the balloon than outside, it leaks out (because of air pressure rather than osmotic pressure, but it's close enough!). This isn't ideal for many bacteria, so they have a hard time living in the sea. So what about the ones who like salt water? What makes them so special?
Back to the balloon-cell analogy; to keep the air in the balloon, there are a few things we can tweak. If the wall of the balloon-cell (celloon?) get less permeable to water/air, less will flow through, keeping the celloon nice and happy. Or, we could put a little pump in the wall of the celloon to bring back water/air, re-inflating the celloon! It doesn't quite work for balloons but bacteria can also make themselves more salty on the inside in general, lowering the difference between inside the cell and outside of it, which slows down the outflow of water.
Great! So seafaring bacteria can regulate or reduce how much water they lose to their saline surroundings. That all costs more than not having those features, in terms of energy and nutrients, so outside of the sea they're less happy, but the good thing about the ocean is that there's food everywhere! Even at the deepest depths where the sun never reaches, minerals and metals from the Earth feed communities of microbes, which then feed bigger organisms too!
In all, it sounds like a pretty sweet deal for the microbes if they can just evolve to be sea-worthy! Although of course, a planet-sized volume of nutritious water full of bacteria will also be a great thing for those that feed on them, and so there are more bacteria-killing viruses in the sea than anywhere else in the world... Life's never free from danger for bacteria, even in the sea! Although if it was, the ocean might be a bit more soupy with all the bacteria, and swimming would be trickier for we humans...