Phytoplankton, a group of single-celled organisms capable of photosynthesis, are vital to marine ecosystems due to their primary production and role in biogeochemical cycles. They adapt to diverse environments and utilize complex nutritional strategies. Phytoplankton contribute to the global carbon cycle through carbon fixation and sequestration, particularly by diatoms and coccolithophores. However, climate change and anthropogenic impacts are altering their dynamics, with rising temperatures, acidification, and pollution affecting growth, productivity, and community structure. Harmful algal blooms (HABs), driven by eutrophication and increased nutrient loads, pose significant threats to marine ecosystems, causing toxic effects and disrupting food webs. Effective management of these issues requires extensive research, international cooperation, and informed policy measures. Understanding phytoplankton’s responses to environmental changes is crucial for predicting marine ecosystem futures and developing strategies to mitigate the adverse effects of climate change and pollution.