Primary producers are a key part of the food chain, being the basis of marine ecosystems. In a simple way, these organisms use light energy and inorganic nutrients (e.g. nitrogen) to produce organic matter through a process called photosynthesis.
Marine plants and macroalgae are two well-known primary producer groups. However, microalgae and bacteria are also important autothrophs (producers).
In fact, microalgae are responsible for generating most of the primary production in the oceans. Although we do not see these organisms with naked eye, the effects they cause are evident when occuring phenomena such as algal blooms.
These occur when there are such favorable conditions of light, temperature and nutrients that rapid growth of phytoplankton occurs.