In situ interactions between photosynthetic picoeukaryotes and bacterioplankton in the Atlantic Ocean: evidence for mixotrophy

Mixotrophy is classically the trophic strategy of specialized aplastidic protists in acquatic environments. In this article is presented evidence of mixotrophy carried out by phototrophic picoeukaryotes belonging Prymnesiophyceae, Chrysophyceae and Pelagophyceae groups of microalgae (<5 μm size).

Authors using flow cytometric cell sorting and dual tyramide signal amplification fluorescence in situ hybridization, present the interaction of individual picoeukaryotes cell and bacterioplankton. They show graphically the presence of picocyanobacteria cell inside phototrophic picoeukaryotes. Prochlorococcus and SAR11 clade are two major groups of bacterioplankton studied in this work, and is known that they both can’t establish symbiotic relationship with eukariotes.

Interesting results are that different groups of picoeukaryotes (predators) have likely favorite prey (e.g. Prymnesiophyceae incorporate mainly Prochlorococcus), opening a key role in the microbial food web of these important and abundand groups. Authors also suppose that global distribution of picosized Primnesiophyceae might be explained by their mixotrophic behaviour and the nutritional flexibility potentially gives a significant competitive advantage under different light and nutrient regimes in open ocean waters.

Another interesting result is the ingestion/assimilation ratio of prey biomass by predator that is equal about 50% meaning phototrophic picoeukaryotes might be as hightly efficient in prey assimilation as specialized protistan predators, such as microflagellates and planktonic ciliates. Is also supposed a selective predation on Prochlorococcus cell respect SAR11 clade cells because the last one are significantly more abundant in the environments studied here but are also smaller and hence less nutritious.

During the seminar we examined other two recent scientific articles about interactions between microalgae and cyanobacteria in the upper holigotrophic ocean waters. We talked  about other cases of symbiotic relationship between procariotic simbiont and eucariotic host, highlighting that phytoplankton organisms (one "simple" green cell… (not so simple!)) are not only able to express great metabolic plasticity in response to changing environmental conditions, but also can adapt their trophic strategy based on the most convenient available resources (e.g. light and nutrients or, for exemple, preys like delicious take-away Prochlorococcus cells) and partners for exemple exchanging fixed N and gaining back organic C with different group of eukariotic microalgae.

Hartmann, M., Zubkov, M. V., Scanlan, D. J., & Lepère, C. (2013). In situ interactions between photosynthetic picoeukaryotes and bacterioplankton in the Atlantic Ocean: evidence for mixotrophy. Environmental Microbiology Reports. Online Early.