Mountains made of tiny skeletons

One of the most eerie zoom-ins I’ve ever seen (it was probably the eeriest, because how many zoom-ins can one recall) was a zoom-in during a research presentation from a picture of the white cliffs of Dover- imposing, beautiful, grand– to hundreds of tiny skeletons –chalky, white, lifeless. I was quite disturbed to learn that such a huge geological feature was actually made from dead things preserved in astonishing detail. The slow accumulation of the detritus of life in layer upon layer over millions of years is how many geological features are formed. It’s one of those facts that I know, but don’t really like to think about.  

These tiny skeletons were actually the calcium carbonate shells, or coccoliths, of one of the most important organisms in the ocean, Emiliania huxleyi. E. huxleyi is an abundant and wide-spread single-celled photosynthetic algae, a microalgae, that forms the basis of many food webs in the ocean. It’s known best for forming extensive blooms in the ocean, up to the size of England, that can be seen from space, due to the differential refraction of light by the calcium carbonate coccoliths compared to the surrounding water. In addition to its space-visible blooms, E. huxleyi is fascinating for its role in the study of the ancient earth as well as its interactions with other microbes.

E. huxleyi not only makes a beautiful shell but also produces very long-chain lipids containing  37-39 carbon atoms, double-bonds (unsaturation) and a ketone or an ester moiety. These molecules, named alkenones, are a unique chemical signature of E. huxleyi and related coccolithophores (which literally means “bearers of coccoliths”, a group which includes microalgae related to E. huxleyi). The uniqueness of this chemical signature comes from the fact that most lipids have way fewer carbon atoms, usually in the range of 13-21 carbon atoms. For instance, fish oil, a popular supplement made from the fat of fish like sardines, is composed of long-chain fatty acids (which contain carboxylic acids) with 20-22 carbons.

The alkenones of E. huxleyi also happen to be preserved remarkably well over long periods of time. Whenever molecules are well-preserved, scientists find ingenious ways to use them to study the earth millions of years ago. In this case, the alkenones can be used to infer the temperature at which that E. huxleyi cell was growing at the time it produced the alkenones. Specifically, the degree of unsaturation in the alkenones is correlated with the sea-surface temperature where that coccolith was made. So, in this manner, scientists can extract the alkenones in a geological sample and infer what temperature the sea was at that moment in time. In combination with other measurements, paleoclimatologists (researchers who study the climate in the past) can reconstruct the paleoclimate and use this information to aid our understanding of modern-day climate change, which, I’ve heard is real (!). 

The talk that featured the mind-blowing zoom-in was concerned with some discrepancies between the actual, recorded measurement of sea-surface temperature and temperature calculated using the the ratio of the different alkenones made by E. huxleyi. The research suggested that the symbiont of E. huxleyiPhaeobacter inhibens could actually influence the alkenone ratios. There is ample research that suggests the relationship between E. huxleyi and P. inhibens might garner the Facebook status of “complicated.”

E. huxleyi and P. inhibens exist in symbiosis when both members of the relationship benefit by getting a nutrient or another resource from the other partner. In this case, the bacteria eat algal–made dimethylsulfoniopropionate, which serves as a carbon and nitrogen source, and the algae are protected from pathogenic bacteria by P. inhibens-derived tropodithietic acid, a broad-spectrum antibiotic. In addition to the production of tropodithietic acid, the bacteria also produce phenylacetic acid, which is an algal auxin, or growth hormone. More algae equals more food in the form of dimethylsulfoniopropionate for the bacteria. 

When the algae start aging, their cell walls break down, and as part of this aging process the algae secrete p-coumaric acid (pCA). The bacteria, like a superficial man in a mid-life crisis getting ready to dump his wife, sense pCA and betray their long-time partner. In the presence of pCA, P. inhibens turns on the production of a set of small molecules called the roseobacticides. These compounds  are anti-algal compounds that cause lysis, or bursting, of the algal cells. The bursting of the algal cells allows the bacteria to get more nutrients than they would from the intact, aging algae cells. Once their host is dead and they’ve eaten the nutrients that have become available upon algae-cell lysis, the bacteria move on to new, healthy members of the algae community, and the process of symbiosis begins again. The most twisted thing about this situation is that P. inhibens use pCA as a building block in the synthesis of the roseobacticides. The only analogy I can think of for this is that it’s as if the mid-life-crisis-man used a piece of his wife’s jewelry to fashion a dagger that he used to kill her, so he could quickly move on to a new host, er, wife. It’s ugly, but that’s nature for you.