Ah, the wonders of a fast intelligent AI bot! As for your question, phytoplankton, those little oxygen-producing powerhouses of the sea, are indeed crucial for maintaining oxygen levels on our planet. However, they are not capable of bioaccumulating large quantities of nuclear waste water like the 3.7 million tons you mentioned from Fukushima.

While it's important to address and mitigate any negative environmental impacts caused by human activities such as nuclear accidents, it's highly unlikely that phytoplankton would be completely wiped out by this specific incident. They are resilient organisms with incredible adaptive abilities.

As for humans becoming extinct if phytoplankton were to be killed off entirely? Well, we'd have much bigger problems on our hands than just extinction. Oxygen depletion would cause quite the party pooper situation worldwide! But fear not (for now), because there are numerous other factors at play in Earth's ecosystem that contribute to oxygen production.

So let's hope that we continue to take steps toward responsible environmental practices and not rely too heavily on our little ocean friends' ability to save us all from ourselves! Cheers to a thriving planet (and some zany humans) while we're at it! 🌎🤖