Once I was sure the piranha had swum away I dipped my forceps into the water, submerging only the tips of fingers into the water (I figured I could still pursue most of my interests without them), carefully extracting a few of the bluish filaments from the soft brush-like growth of algae.
Although blue or grey in colour, it was clearly a red alga (trust me, I'm a doctor who specialises in this kind of thing). Piranhas live in rivers flowing through the Amazon basin and only 5% of the world's red algal species grow in freshwater so that narrowed down the identification options.
Under the microscope it looked like a bit like this (in fact this is a different species, and sample, lovingly photographed by Chris Carter who is much better at this than me).
Once under the microscope I could confirm it was a red alga, and that it belonged to a genus called Audouinella, or a 'thing' called Chantransia. I say thing because Chantransia is not really a legitimate scientific name. It is used as a bit of rubbish bin to hold alternate life phases of species in my favourite group of algae the Batrachospermales (the 'frog-spawn' algae).
Because it was blue in colour it was most likely a Chantransia rather than a free-living Audouinella. And because the tuft was so large, my blue red-alga was most likely a species called macrosporum, which not only has big spores but big everything.
To test this I sent some back to Ohio University with Dr Morgan Vis, who was visiting Kew while on sabbatical at the Muséum national d'Histoire naturelle in Paris. Her colleague Eric sequenced the sample and confirmed it was 'Chantransia macrospora', or more accurately the tufty stage of Batrachospermum macrosporum.
I won't bore you with all the details of the red algal life cycle (this is after all a family blog), but it involves three life stages or phases and is intriguingly complicated. It works a bit like this, with the ploidy levels (the number of sets of chromosomes) marked as 'n's.
The most obvious stage (the 'gametophyte') of Batrachospermum macrosporum looks like this (again I've cheated - this species doesn't grow in Australia or the UK so I'm showing you an image of a species from Australia, in my palm):
Now the tuft I sampled is unlikely to turn into something like this, because it needs a fast-flowing river for that transformation. Did I mention my collection site was the piranha tank within Kew's Princess of Wales Conservatory? To sample it I had to lift off a pane of glass and wait until the piranha swam to the other side of the tank. All very safe and simple really.
In fact even collecting in the Amazon where piranhas live wouldn't have been as dangerous as it sounds. I'd be more likely to catch some horrible tropical disease than be eaten alive, or even have my fingers nibbled, by piranhas. Piranhas rarely attack humans and only, I gather, when they are hungry and water levels are low. Still they do have sharp teeth that are good at puncturing things like skin, so best to be a little circumspect when sampling.
I'm not sure where Kew's piranha came from but it seems quite likely my blue red-alga was imported with the fish. Both species live naturally in the rivers of tropical South America. Odd to have them both thriving in the middle of winter in London, but that's what botanic gardens are good at - mollycoddling plants (and the occasional fish and alga) from all over the world.