Whew! Time for a break from all this philosophical pondering!
Today I'm going to instead celebrate the nitty-gritty — organisms so nitty-gritty that when they die they are used in flea powder, toothpastes, cat litter, insulation, and as a component of dynamite. They live mostly in oceans, and they carry out about 20 percent of all the photosynthesis on earth, fixing more carbon per year than do all the rainforests combined.
And they're arguably the most beautiful single-celled creatures on the planet.
Yesterday my lab collaborators and I were the first people on the planet to witness their molecular architecture in pristine form: we froze them instantaneously at liquid-helium temperatures and used special techniques to visualize them with the electron microscope. So it's hard to contain my excitement.
They're collectively called diatoms, with an estimated 200,000 species alive today. We looked at Thallasiosira pseudonanna, a species with the distinction of being the first diatom to have its genome sequenced a few years ago. Each diatom species surrounds itself with a distinctive enclosure, called a frustrule, that's made out of hydrated silicon dioxide, the primary component of glass. The frustrule of T. pseudonanna is constructed like a Petri dish, a top fitting over a slightly smaller bottom, and its ornamentation would strike envy in the heart of any glassblower.
An electron micrograph we produced yesterday, shown at the top of the post, zooms in on this ornamentation to reveal its marvelous detail — for a sense of scale, the black opening in the center is 75 nm in diameter. The regular patterning of the tiny pits and crossbars, and the organic material in which they are embedded, is the collective effort of an estimated 150 genes in the T. pseudodonna toolkit. If you click here you'll be treated to lower-mag views of many other such diatomaceous creations.
Not only are diatoms gorgeous, and key players in marine ecosystems. They also have a fascinating evolutionary story.
Some 1.5 billion years ago, a eukaryotic organism with a nucleus and a mitochondrion engulfed a photosynthetic bacterium such that it came to reside in its cytoplasm, an event called endosymbiosis. Subsequent host generations "tamed" the progeny of this bacterium until they became permanent cellular constituents called chloroplasts. Some of the genes of the original bacterium were imported into the host nucleus and became permanent parts of the host genome. Most of the others were discarded, but not all — modern chloroplasts retain tiny genomes of their own that are copied in pace with the nuclear genomes of their "tamers."
The modern organisms that most closely resemble these first endosymbionts are the red algae whose primary chlorophylls are called a and c. But at some deep time, some of the early organisms switched to making chlorophylls a and b, and founded a second lineage that has given rise to all the modern green algae, including my beloved Chlamydomonas. And then, to finish this part of the story, some of the green algae took to forming primitive leaf and root systems, founding the land-plant lineage some 450 million years ago.
Meanwhile, the oceans were the dominion of the red and green algae and free-living photosynthetic bacteria until perhaps 250 million years ago, when an event occurred called secondary endosymbiosis. This time, a host organism with a nucleus and mitochondrion engulfed an entire red alga – nucleus, chloroplast and all – and initiated a second "taming" process, its descendants evolving into the modern diatoms and brown algae (e.g. the familiar kelps). In the diatom lineage, the taming process entailed importing some 170 genes from the red algal nucleus to the host nucleus and discarding the rest. The new creatures, it turns out, were also prone to picking up additional genes from bacteria via a process called horizontal gene transfer. Hence modern diatom genomes are a melee of genetic ideas – some host, some red algal, and some bacterial.
If I've gotten you a bit hooked on diatoms, I encourage you to peruse a beautifully written review article by Ginger Armbrust, from which I've gleaned much of my information, that details their many roles in ocean ecology and voices concerns about their ability to flourish in our rapidly changing climate. We instinctively (ah, that word!) think of our macroscopic bretheren when we think of endangered species, but in fact, of course, the whole shebang is intensely interconnected, with diatoms key participants in complex marine food chains. As Ginger concludes, the hope is that ongoing research will document the changes occurring in critical groups like the diatoms "before they become the new canaries in the coal mine."
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