This is saxitoxin, a potent neurotoxin produced by some species of dinoflagellates, a type of microbial algae. Saxitoxin causes paralytic shellfish poisoning (PSP), by interacting with the sodium ion channel in the nervous system.
The smell of earth after rain; is called Petrichor, and it is caused by Geosmin. Geosmin is an organic compound with the formula C12H22O. It’s produced by several classes of microbes, including cyanobacteria (blue-green algae) and actinobacteria (especially Streptomyces), and released when these microbes die. The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.
This molecule is tetrodotoxin, an zwitterionic compound originally isolated from puffer fish. Wild puffer fish carry bacteria that produce this toxin, while those raised in capacity usually lack the microbes and toxin. Tetrodotoxin prevents the sodium ion channel from working in the nervous system. A few other organisms produce it, such as the blue-ringed octopus and a type of newt.