If all algae sudenly disappeared from Earth's waters what would happen to living things on Earth?

Answer 1

Algae provide 70% to 80% of the oxygen in the atmosphere. Without algae, land organisms would slowly suffocate. Ocean animals would also starve and suffocate. All the dead, rotting organisms would release massive amounts of greenhouse gases, heating up the climate and wiping out lots of plant species. Few, if any, complex, aerobic organisms other than plants would survive. Plants might not even survive, in which case Earth would become a world with only anaerobic, single called organisms. A slightly luckier scenario would be that surviving plants would colonize the ocean before everything was destroyed, and fill the niches that algae left open. It is unlikely they could do that fast enough, though.

Answer 2

If all algae suddenly disappeared from Earth's waters, it would disrupt the entire marine food chain. Algae are primary producers that form the base of the marine ecosystem, providing food and oxygen for a wide range of organisms. This would have cascading effects on fish, marine mammals, and even some land animals that rely on marine resources for survival.