The most important primary producers in marine ecosystems are . The most important primary producers in marine ecosystems are . euglenids dinoflagellates diatoms seaweeds

Diatoms.

Explanation:

Diatoms are single celled group of algae that are found in aquatic habitat both freshwater and marine habitat. The are the most important primary produce in the marine habitat. They are photosynthetic I .e they have chlorophyll which help them to trap light energy from sunlight and uses carbon dioxide and water to produce food. They produce food with the process of photosynthesis.