Unicellular microscopic algae inhabiting the world oceans generate half the oxygen available on the planet and fuel marine food webs. Despite this, we do not understand many aspects of their basic ecology. A clear example is mixotrophy, the ability for micro-algae to combine both photosynthesis and feeding on prey to meet their nutritional needs. Discovered many decades ago for eukaryotic micro-algae, it was considered an anecdotal phenomenon until a series of studies starting around the 1990s revealed that this strategy could have significant ecological relevance. Here we focus on small phytoflagellates (unicellular eukaryotic micro-algae < 20 µm in size that possess filaments or flagella used for motion and/or to capture prey). Mixotrophic small phytoflagellates (abbreviated to MSP) are present in all marine systems where there is light, and are normally very abundant (in the order of thousands in any given drop of seawater). They generally prey on bacteria, and can be a major mortality factor for this group. For example, in the North Atlantic Ocean they have been seen to account for up to 90 % of all bacterial losses, and they generally account for 30 - 60 % in most other tested marine systems. However, most of these studies are based on single measurements and more information is needed on how MSP feeding changes along long and short temporal scales; how much different MSP groups contribute to total bacterial consumption (e.g. green vs red micro-algae groups); and how MSP feeding is influenced by different environmental parameters, such as light or nutrient concentration.
In the present study we focused on coastal zones and conducted a field and culture studies to determine: (i) How important is MSP feeding in temperate and polar coastal zones? (ii) Can changes in feeding be observed along short and long space and time scales? (iii) How do different environmental parameters influence MSP feeding? And (iv) How different is the feeding magnitude and regulation for different MSP groups?