The limits of bacterial hitchhiking

This article was originally published on the Society for General Microbiology blog on 22-08-2012. Read the original here.

Bacteria are able to move across oceans, deserts and mountain ranges with relative ease, by hitching a ride in clouds of dust or water vapour. The rules that govern the dispersal, or ‘biogeography’, of larger organisms simply don’t apply to bacteria because they’re so small. So how do we know where they came from? And how do we know if a bacterial strain found in Alaska is the same as one found in Costa Rica?

A study published in the August issue of Microbiology has shed light on the dispersal of nitrogen-fixing rhizobia bacteria in North and Central America. By sequencing portions of six common genes from Bradyrhizobium strains, researchers at the State University of New York, Binghamton were able to compile phylogenetic trees which show how different strains are related to each other. Those that group together are likely to have evolved from a common ancestor.

The team found no evidence that any Bradyrhizobium strains had evolved from a common ancestor in any one region, but that the same strains were found simultaneously across North America. However, they also found that each location had its own distinctive population composition, suggesting some adaptation of each rhizobium to its environment and its legume host.


There was a trend towards higher diversity in genes from rhizobia in tropical regions: strains isolated from Panama had a significantly higher diversity than those from Washington State. This was not due to the number of legume hosts sampled in the study, or the considerably higher annual rainfall in lower latitudes. Of the genes analysed, nifD, which codes for an enzyme involved in nitrogen fixation, tended to have the greatest nucleotide diversity – double that of the other five genes.

One of the most surprising findings was the lack of community structure overlap in regions with similar characteristics, e.g. Washington State and the north-eastern US, yet overlap existed between dissimilar regions, e.g. north-eastern US and Chihuahua, Mexico. The two latter sites have distinct biotic communities (lowland temperate forest vs. mountainous evergreen oak-pine forest) with no common legume host species. However, previous studies have shown that these regions shared the same flora in the late Miocene era, which suggests the similarity may be a legacy of previous interactions.

The biogeography of legumes is affected by soil type, rainfall and temperature, meaning their distribution is not random. The dispersal of rhizobia, as symbionts of legumes, will be limited by these same factors, though perhaps not for the same reasons. The presence of rhizobia and legumes may also be limiting factors for each other.

This study provides some important insights into the biogeography of rhizobia, and emphasises a frustrating question in legume-rhizobia research: is legume distribution limited by rhizobia presence, or vice versa?

Peat free compost

Considering the attention the destruction of peat bogs has received in recent years (e.g. this from the BBC gardening pages and this in the Independent from 1992!), and how devastating the impact of their destruction can be, I was recently surprised to find how much of the compost available to me contains peat.

On a recent trip to our local garden centre they had actually put the peat content of various composts on their signs...

First up, standard garden centre compost:

So standard compost is 60% peat. This is a no-go for me.

What about John Innes? That's good stuff right?

Good stuff it may be, but it's still 44% peat.

How about organic compost?

Nope, that's still 50% peat!

So it seems the only way to be sure is to buy stuff that EXPLICITLY says it is PEAT FREE!

The problem I find is that while the organic peat free stuff may actually work out quite cheap, it is also pretty rubbish. It seems that it is mostly just bark chippings that have been composted. And I'm pretty sure that if I did a pH test it would come up pretty acidic (it has a rather pine-y scent to it)

We've had a few fungus-related issues too...

Not to mention a couple of sudden-death incidents...

So if I'm honest, I just don't think peat-free compost is up to scratch just yet. It needs to compete with the peat-full stuff if it's to stand any hope of replacing it (before we run out of peat, that is!).

If you want to make a few bob I reckon there's a market opening up here!

Tomato harvest

Today we had our first, long-awaited tomatoes. They were delicious.

We raised some from seed, but they just didn't put on the growth early in the season, and are miles behind the two plants we ended up buying from a local nursery (where these beauties came from). I suspect these latter plants have also had an advantage in being in pots near the house, where it's much warmer than on the allotment (and less swamp-like...). Next year I think we will buy in tomato plants rather than endure the emotional turmoil of massive seedling failure, as we just don't have the facilities to get them going well - and  early enough - just yet.

I'll let you guess which one we found hilarious...