Marine plankton—spanning tiny phytoplankton to zooplankton—are crucial for regulating Earth’s climate and supporting marine food webs, yet face growing threats. Warming seas and acidification reduce plankton productivity, causing phenological mismatches that disrupt entire ecosystems. Expanding oxygen minimum zones confine plankton to shrinking oxygen-rich areas, while overfishing of apex predators further alters delicate population balances. Despite these challenges, plankton hold promise as a sustainable food source for a rapidly growing global population. Phytoplankton and zooplankton farming could offer high-quality nutrition with a smaller environmental footprint than conventional agriculture. However, large-scale cultivation must be carefully managed to prevent ecological imbalances. Protecting plankton populations is vital for both ocean health and future food security.