After the K-Pg impact darkened oceans, why did the tiniest plankton survive best?

Answer: Lower energy demand

Tougher mineral shells — Tough shells can help against predators or abrasion, but they do not solve a dark-ocean food shortage. Many shelled plankton groups were actually among the hardest hit. The surprising filter was closer to an energy budget: when photosynthesis falters, a large body needs more carbon just to keep a viable population above extinction.

Lower energy demand ✓ — Lower energy demand is the key. The Nature model made extinction depend on biomass thresholds that scale with body size, so tiny plankton could remain viable after light and food crashed. The payoff is that survival was not just about being hardy; it was about needing little enough energy to outlast a century-scale bottleneck.

Faster sinking bodies — Sinking faster would usually make things worse, because it carries cells away from the lit surface where photosynthesis is possible. Some sinking organic matter can feed deep communities, but that is different from keeping a plankton population alive. The K-Pg pattern points to small size and low energy demand, not a shortcut to the seafloor.