Phytoplankton are microscopic organisms that live in watery environments, both salty and fresh. Some phytoplankton are bacteria, while some are protists. Most are single-celled.
Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to turn it into chemical energy. They consume carbon dioxide (CO2), and release oxygen into the atmosphere. All phytoplankton photosynthesise, but some get additional energy by consuming other organisms.
Phytoplankton growth depends on the availability of carbon dioxide, sunlight, and nutrients.