Thursday, 14 November 2013

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.


  1. Hi, Roberto,
    Assuming that the picoeukaryotes identify potential prey through cell surface receptors, is there a proposed mechanism for selection, i.e. do they reject less preferential prey that they contact or is there a method for locating the preferred food species? Perhaps cell surface receptors aren't solely responsible for identification?

    1. Hi George, thanks for comment and sorry if I reply only now.

      In this work the authors based their selectivity thesis on statistical data, so they just count significant difference in microbial cell numbers inside predator cell. For these specific groups studied in this paper there isn’t a specific model but however about prey-predator interactions in planktonic organisms there are a lot of information and models proposed for each steps of the whole predatory process (chemical-mediated prey identification, adhesion, phagocytosis or other intake mechanism, digestion vesicle making, and final digestion). More detailed studies to molecular level have been done on non-planktonic species like soil protists that graze on bacteria. About cell-cell interactions and relative cascade effects, just to give you an example, in some species of phagocytic protists is known that integrins (heterodimeric adhesion receptors) and lectins (high-specific carbohydrate-binding proteins that agglutinate cells) are key molecules of adhesion and recognition process.
      Obviously selection can happen earlier than adhesion stage especially for free living planktonic organisms. In this case, chemoattractants and chemorepellents are the most important activator of specific signal transduction pathways.
      In response, prey possess a multitude of chemical-based anti-predator strategies that can also explain rejection events. Example of this are numerous toxins produced and excreted as grazing response. Another important bacterial features in anti-predator strategies seems to be the presence of S-layer as defensive and camouflage tool.

      If you want know more about, I recommend you these interesting reviews that I find on the web:
      • HORIZONS:
      Emily C. Roberts, Catherine Legrand, Michael Steinke, and Emma C. Wootton - Mechanisms underlying chemical interactions between predatory planktonic protists and their prey - J. Plankton Res. (2011) 33 (6): 833-841 first published online February 19, 2011 doi:10.1093/plankt/fbr005