Blue-green algae, or cyanobacteria, can contain complex toxins called microcystins.
They even have their own website: http://www-cyanosite.bio.purdue.edu/cyanotox/toxins/microcystins.html
Here is what one molecule looks like:
Microcystins consist of a seven-membered peptide ring which is made up of five non-protein amino acids and two protein amino acids. It is these two protein amino acids that distinguish microcystins from one another, while the other amino acids are more or less constant between variant microcystins.
Discover more about the microcystins here.