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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MARIS (Mixotrophy among small marine phytoflagellates â€“ prevalence and impact on prokaryotic communities)

Teaser

Summary

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 Âµm 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?

Work performed

We conducted a series of sampling cruises in the Ã˜resund (Denmark), and in the Arctic Circle in Disko Bay (Greenland), observing how the feeding of all MSP and specific MSP groups changed with water column depth (from light surface waters to dark bottom waters) and along short (hourly and daily) and long (seasonal) temporal scales. We complemented this work with studies on cultured micro-algae species. This allowed us to isolate specific environmental parameters, such as light intensity, and study its specific impact on MSP feeding. In addition, during the course of the project we determined significant errors could exist in traditional approaches to measure feeding in MSP. We confirmed this through and exhaustive case study and proposed proper corrective measures.
The obtained results have been summarized in 7 publications, 3 presentations at national and international conferences and a series of outreach events aimed at the general public.

Final results

The results obtained demonstrate that MSP can have a high impact on bacterial communities in marine coastal systems. Which MSP groups are responsible for this feeding impact is very variable, shifting along both long and short temporal and spatial scales. This high variability, especially along short temporal and spatial scales, is remarkable and is generally not considered, which could lead to the feeding impact of certain groups not being properly recognized. Light and nutrients were confirmed as important regulating parameters for MSP, but the interplay between the two is complex and not easy to isolate in the field. Finally, studies with cultured micro-algae species demonstrated for the first time that feeding can be a major strategy for green algae in temperate zones and highlighted the potential for certain groups to use feeding as a means to survive rather than to grow. Overall this project significantly advanced our knowledge on MSP ecology and their role in coastal marine ecosystems. In addition, we highlighted several hitherto overlooked parameters that are critical to how we consider MSP in the models used to predict, e.g., how marine productivity will respond to climate change.