Asked by: Tristan Cook, Brighton
Well we don’t know for sure, but probably something with fast DNA replication and cells that can quickly expand in size – organisms that inflate their cells with lots of water are a good bet. For total growth, try giant kelp. That can grow about 60cm per day, although the average (in spring) is more like 27cm. Huge organisms like kilometre-scale underground honey fungus networks will probably beat this, although measuring it is difficult.
Bacteria such as certain Bacillus stearothermophilus strains can double in size, as a percentage of bodyweight, every 10 minutes under optimal conditions. So, given unlimited resources, in one day a single bacterium could produce more offspring than the combined weight of all the organisms in the world! Although of course, that is impossible because the resources run out long before this point, but the statistic does demonstrate their prodigious powers of reproduction.