What is the name of the microscopic plants that start the food chain in the seas of the world?

Phytoplankton are the microscopic plants that play a crucial role at the beginning of the marine food chain. These tiny organisms, usually found in the upper layers of oceans and other bodies of water, are capable of photosynthesis, which allows them to convert sunlight into energy. This process not only supports the phytoplankton themselves but also produces oxygen and forms the foundational base for many aquatic food webs.

Phytoplankton serve as the primary producers for various marine species, providing the essential nutrients and energy needed for higher trophic levels, such as zooplankton, small fish, and ultimately larger predators. The other options, while related to marine ecosystems, do not serve the same primary role as phytoplankton. For example, zooplankton are small animals that consume phytoplankton, while nutrients are chemical substances needed for growth and maintenance but are not organisms themselves. Photosynthetic bacteria, though they also participate in the ecosystem and contribute to the process of energy conversion, are distinct from phytoplankton, which are classified as plants. Thus, phytoplankton are fundamentally important for supporting life in the oceans as they initiate the food chain.