Seaweed qualifies as a resilient food source. If a huge volcano exploded or a nuclear war blocked the light, they would be a possible solution. Algae could save humanity. As?
«Investing in the construction of seaweed farms could prevent global famine. “It would happen in scenarios of abrupt reduction in sunlight.” This was said by David Denkenberger, professor at the University of Canterbury. The research was published in the journal Earth's Future.

Proteins
Seaweed is a versatile product, which can serve as a critical source of food and fuel when everything else is scarce. It can thrive in tropical oceans even after a nuclear war. The researchers built a model based on the algae Gracilaria tikvahiae. It is known as graceful red grass. They simulated the growth of algae after an adverse global event.
In a nuclear war, sunlight would be blocked for years. But algae could still photosynthesize. The light requirement of some species of algae to saturate photosynthesis is very low. From 50 to 100 micromoles of photons.
Seaweed is often eaten in salads or as dry food wraps. They are rich in proteins, minerals, vitamins, essential amino acids and fatty acids. Producing seaweed is a lifeline for many families living in poor coastal communities in low-income countries. The organism is largely protected underwater. It grows faster than any other known agricultural crop.
China currently produces more than 50% of the world's supply, followed by Indonesia. But a nuclear war could also, in a certain scenario, destroy all marine life. Global cooling would fill the seas with ice.

Dangerous scenarios
The recovery of the ocean, according to that study, would be very slow. Decades at the surface and hundreds of years at depth, with thousands of years in colder waters like the Arctic. Marine ecosystems would impact the global ecosystem.
Yes, algae could save humanity. But at a certain fundamental point, a nuclear war affecting the oceans would be a true point of no return.