Nod you later

Lupin, a nodulating legume -- Creative Commons photo by Flickr user aussiegall

Nitrogen seems like a strange element to have problems acquiring. After all, it’s by far the most abundant element in the atmosphere -- about two thousand times as abundant as CO₂ -- and, on average, plants only need one nitrogen atom per 30 carbon atoms. Yet, as any gardener will tell you, nitrogen is also one of the most important limiting resources for plants, and is the sole reason we have a lucrative international trade in bird crap. Despite being bathed in nitrogen for their entire lives, plants lack the metabolic machinery to break the strong triple bond between the two nitrogen atoms. They need someone else do the hard work for them. 

Defining the Relationship

C. pennsylvanicus photo courtesy the excellent Alexander Wild

Having dealt last week with the termites of the sea, it would seem reasonable to leave the littoral and move along to honest-to-goodness termites of the land. But we’re not (termites will come later, I promise!). Like termites and shipworms, this week’s host organism can frequently be found boring holes in wood, and has caused consternation to many a homeowner. Unlike those aforementioned mutualists, though, this animal doesn’t consume the wood or digest any cellulose -- it just chews holes through it to make a nest. 

Carpenter ants should be familiar to pretty much everyone who’s ever seen an ant; the species commonly found here in the Eastern United States (Camponotus pennsylvanicus) is, at up to 2 cm in length, fairly hard to ignore. The ‘tribe’ to which carpenter ants belong, the Camponotini, is one of the largest and most successful groups of ants, composed of over a thousand species distributed across the globe. With such a common host species, it’s little surprise that their symbiotic bacteria were one of the first such associations to be discovered: endosymbiotic microbes were first described in the gut epithelium and ovaries of Camponotus way back in 1882 by F. Blochmann. What is surprising is that it’s taken scientists over a century to figure out what the heck the bacteria are doing there.

Camponotus, coming soon to a blog near you

Unexpectedly explosive day today; you'll get your carpenter ants soon enough...

Fish, and plankton, and sea greens, and termites from the sea!

shipworm damage; photo CC flickr user alaskaent

Wood makes for a pretty tough meal. To start with, cellulose -- wood’s primary constituent -- is indigestible by most organisms. If that weren’t bad enough, the cellulose itself is packed in a dense matrix of other intransigent biopolymers like hemicellulose, pectin, and lignin, making it structurally difficult even to get enzymes in and working. And to top it all off, wood contains very little nitrogen -- about an order of magnitude less than most organisms need to grow, relative to carbon.

With such a difficult hurdle to overcome, it’s pretty strange that one of the world’s champion wood-eating animals lives in an environment where no wood grows (the ocean) and belongs to a class that has never even evolved teeth (the bivalves). Thus the somewhat unfortunately named shipworms (family Teredinidae) are something of an enigma.