Bacterial Hitchhikers

There was an interesting post over at Culturing Science about the widespread dispersal of bacteria which, as well as sporting an amazing hand-drawn MS Paint picture also put forward the argument that bacterial evolution occurs in very selective environmental pockets and niches, while a sort of general 'less-evolving' population floats around the world. This helps to explain why you can find almost identical species of bacteria all over the world, yet still find very specialised colonies in distinct environmental niches. This can all be summed up with a rather nice little quote by the Dutch biologist Lourens Baas-Becking:

"Everything is everywhere; the environment selects"

It's a quote that can be hotly debated, but it certainly is true that bacteria have a remarkable capacity for spreading around the world. Even before humans and air-travel allowed pathogenic bacteria to go on holiday environmental bacteria have been spreading through the sea, air and soil not by their own (rather poor) motile ability but by hitching rides on other organism. In the case of soil bacteria they can also travel in spore form, which significantly reduces the harm they sustain by (e.g) being trampled all over the savannah's by an elephant.

In the sea bacteria have even more help with movement. Currents, tides, waves and general water movement can help to move bacteria large distances horizontally and a recent paper (that Lucas sent me, reference is below) provides evidence that bacteria might move vertically in the oceans by taking a ride on zooplankton, small eukaryotic ocean-dwelling creatures, a selection of which are shown below:

This isn't just a careless picking up of bacteria by idly floating zooplankton though, this is the bacteria actively attaching and dissociating from the zooplankton as they move through the water. The paper proposes a "conveyor belt hypothesis" which states that bacteria attach at one level, travel either upwards or downwards on the migrating zooplankton, and then dissociate when they reach where they want to be.

The reason bacteria would want to travel around between the different depths is due to nutrient availability (this may also act as a biochemical signal for the bacteria to fall off their zooplankton transporters). Deeper waters have a higher concentration of inorganic nutrients, while waters closer to the surface contain a higher concentration of oxygen, and algal derived organic matter.

In order to estimate how much travelling the bacteria were doing, the researchers used. Three different bacteria (that were thought to travel between layers, rather than bacteria that have adapted to the layer they are in) were isolated and labelled with GFP - a protein which fluoresces green. They were then added too migration columns, filled with zooplankton called daphnids which are very phototaxic (i.e they move towards light). Running a light up and down the migration columns lead to the daphnids moving up and down, and the movement of the bacteria could be tracked by following the spots of green florescence.

Sure enough, they found that the green bacterial dots would gather either at the bottom or the top of the migration column, but only when the daphnids were added (a column containing just bacteria and water with a light running up and down the side produced no results). They also found that the more Daphnids they added, the move bacterial movement was found, strongly supporting their hypothesis that the bacteria were taking a ride on the far larger zooplankton.

Graph showing the number of bacteria in the upper layer of the migration column per migration cycle of the dahpnia (i.e one movement up and down - 2 hours). The circle shows columns with no daphnia added, while triangle and square show 20 and 80 daphnia respectively. Numbers to the right are the regression slope.

Among other things, this study shows just how dependant ocean species can be on the other organisms they live amongst. The bacteria which use zooplankton for conveyor-belt style hitchhiking now have their own survival intrinsically linked with the continued well-being of the zooplankton species that they rely on. This knowledge can then be added to models of how large-scale changes to the oceanic environment will affect the creatures within it and ultimately, given the importance of zooplankton on the foodchain and the importance of oceanic bacteria on the environment, the fate of many other organisms.

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Grossart HP, Dziallas C, Leunert F, & Tang KW (2010). Bacteria dispersal by hitchhiking on zooplankton. Proceedings of the National Academy of Sciences of the United States of America PMID: 20547852

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