The oceans dominate the Earth, and about half of the world's primary production is the result of marine photosynthesis. This course introduces the mechanisms, functions and research tools needed to study this process. First, we will give an overview of the diversity of photosynthetic organisms in the ocean as well as the environment in which they develop. We will then look at the mechanisms of photosynthesis and carbon acquisition in marine plants. Finally, we will focus on the measurements and ecological aspect of marine photosynthesis. In this course, laboratory and field demonstrations/experiments will be combined as much as possible with theoretical knowledge (accompanied mini research projects). In particular, the use of an advanced underwater fluorometer will be illustrated to analyze photosynthetic organisms in their natural habitat.