Nouns ending in -isme are masculine.
Is organisme masculine or feminine?
l'organisme
The rule behind it
100% of -isme nouns are masculine
In context
Sans air, tous les organismes vivants mourraient.
But for air, all living things would die.
L'astronaute a trouvé un organisme intelligent.
The astronaut found an intelligent organism.
L'homme est un organisme complexe.
Man is a complex organism.