Alison Smith

From seaweed to a partnership between bacteria and algae.

For years, algae have puzzled researchers. Some species require vitamin B₁₂ to live, others are perfectly happy without it — but the variation between, and even within, species seemed completely random. More to the point, it wasn't at all clear where the vitamin-dependent algae were getting their supplies from — were they making it themselves?

Alison Smith, see page 90, a plant scientist at the University of Cambridge, UK, had long followed the debate, and discussed it with her colleague and frequent collaborator Martin Warren at the University of Kent. They knew that seaweed was often used by vegetarians as a vitamin B₁₂ supplement.

“For a long time, Martin and I wondered whether seaweed makes vitamin B₁₂,” Smith says. They both leaned towards the affirmative as algae are generally considered to be autotrophic; in other words, they can produce everything they need to sustain themselves.

To explore the idea, they set out to grow various algae in the lab. But they soon found that natural growth conditions were difficult to mimic, and their algae were quickly covered in bacteria. The pair wrote off the experiment as a failure.

A subsequent literature search showed up quite clearly the differences between various algae and their relationship with vitamin B₁₂. After a little digging, Smith and Warren found papers on 326 different algae — half of which required vitamin B₁₂ and half of which didn't. “We realized that the whole algae kingdom had been sampled more or less,” says Smith.

They returned to the lab, and looked at the genomic sequences of three types of algae — one that required vitamin B₁₂ to live and two that didn't. The vitamin-B₁₂-dependent alga contained only one gene that matched any of the 19 known to be necessary for bacteria to make the vitamin. So they decided the vitamin B₁₂ must be coming from somewhere else.

The hunt was on. First they examined the sea and pond water that algae live in, but to no avail. Then they remembered how difficult they'd found culturing algae in the lab. They realized that most researchers who had grown algae in the past either added bacteria to their culture or encouraged their growth along with the algae. They isolated and assayed the bacteria in question and found that, sure enough, they produced vitamin B₁₂.

Smith says that their findings point to the important of symbiosis — not just in algae, but in many organisms. “We've provided a very succinct piece of evidence that organisms don't live on their own,” she says. Even humans require bacteria in the gut to thrive. Highlighting the relationship between algae and bacteria illustrates that simple organisms depend on each other, too.

Smith and Warren next want to examine the mechanism by which the algae get the vitamin from the bacteria and also what the bacteria get from the transaction. “Clearly they're getting something — we don't know what.” That question gives Smith and Warren another algal puzzle to tackle.