Learn Turkish: Doğanın korunması hepimizin işi. - The protection of nature is everyone's job.

You could, but it changes the nuance. Korunum is a pure abstract noun derived with the -m suffix, but it is mostly used in scientific or highly formal contexts (like enerjinin korunumu — conservation of energy). Korunması is a verbal noun (korun-ma-sı) focusing on "the act of being protected." Using the verbal noun makes this sentence feel more active and natural for everyday speech.

Why is the passive form korunması used here instead of the active koruması?

In Turkish possessive phrases with verbal nouns, the first word acts like the "doer" of that action. If we said doğanın koruması, it would mean nature is doing the protecting (e.g., nature protecting its animals). Because nature is the thing being protected, we must use the passive stem korun-. Literally, it translates to "Nature's being protected."

Why does hepimiz (all of us) take the genitive suffix -in to become hepimizin?

Because "everyone's job" is a possessive relationship, acting like "the job of all of us." Just as you would use the genitive for "my" in benim işim (my job), you must put hepimiz into the genitive case (hepimizin) to properly link it to the possessed noun işi (job).

There is no verb at the end of the sentence. Is it grammatically complete as just işi?