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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Microbes and Climate Change

Since the very first little blobs of entropy-defying life first appeared around four billion years ago, micro-organisms have played a major role in shaping the temperature of the planet by adjusting the balance of gases in the air. It could even be argued that global warming was one of the first effects of life, when the first methanogens (methane producing bacteria) started pumping greenhouse gasses into the new atmosphere. The evolution of photosynthesis lead to the great oxidation event, and over time the balance of gasses in the atmosphere stabilised into its current composition: lots of nitrogen (controlled by nitrogen fixing bacteria), medium amounts of oxygen (controlled by photosynthesis) and much smaller amounts of carbon dioxide (also controlled by photosynthesis, of both plants and bacteria).

Until humans, the general gaseous air composition was controlled almost exclusively by bacteria, with plants (mostly algae) having a lesser effect on carbon and oxygen levels. Animals didn't really get much of a look in until humans started releasing all the locked up carbon in fossil fuels.

Bacteria that are currently contributing to global warming are the methanogens, most notably those in the gut of ruminant mammals (i.e cows, sheep and other edible things). Cows and sheep can't break down cellulose in the plant material that they eat, so they have bacteria that do it for them. Unfortunately this process releases huge amounts of methane, and methane is around 20 more planet-warming than carbon dioxide.

When I went to Copenhagen last year (I didn't go for the conference, in fact I didn't realise it was on until I started wondering why it was so hard to find a hostel room!) someone handed me a leaflet saying that climate change could be prevented if everyone in the world became a vegetarian. It was an ... interesting point of view, but you could see where the idea came from. Cows are little methane factories.

Just in case anyone forgot what a cow was.

However bacteria are also heavily involved in keeping climate change under control with photosynthesis, which uses up carbon and releases oxygen into the environment. Despite being very leafy and green, forests (even rainforests) tend not to be huge carbon sinks, they take up carbon during the day certainly, but at night they respire and use most of it up again, and anything they've stored tends to be released once they die and decompose. Marine cyanobacteria, however, take in carbon like its going out of fashion, and when they die they sink down to the bottom of the ocean and lock it all away in calcified rocks. One of the most prolific carbon-eating bacteria is Prochlorococcus. Around 100 million Prochlorococcus can be found in every litre of seawater and, along with fellow bacteria Synechococcus it removes about 10 billion tons of carbon from the air every year.

In terms of helping to moderate climate change, there are plenty of ideas floating around as too how bacteria could be useful, but one of the more helpful ones is trying to make a bacterial-based carbon neutral biofuel. The idea is that if you find bacteria that take up as much carbon for their growth as they release while being used as fuel they are technically 'carbon-neutral'. You can grow pretty much anything in bacteria, up to and including oils that can drive cars, it's just currently not very efficient.

Whether or not anything can be done to stop climate change (or, more importantly, whether or not people can agree to do anything) may be an unresolved issue, but its becoming clear that the issue of how the worlds climates are changing is a subject for microbiologists and plants-scientists as much as for meterologists. Whatever happens to the climate in the future, bacteria will still have a large part to play.

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Exams and course-work are all over, so from now on I am hoping to keep this blog purely for the prokaryotes: