Feeding in the dinoflagellate Oxyrrhis marina

Protists grazing activities account for 60-70% of daily phytoplankton consumed. Protists are major consumer of phytoplankton and bacterioplankton, thus playing a key role in carbon cycling and nutrient regeneration in the marine environment. Feeding mechanisms of planktonic prostists have been intensively studied in particular Oxyrrhis marine, which has been used extensively as a model predator in in-vitro studies. This is mainly because it is easy to culture and manipulate and also due to its versatile but selective feeding behaviour. This paper looked into the mechanism underlying selective feeding behaviour of O. marina.

Diet of O. marina

O. marina has been shown to be able to feed on a spectrum of different size organisms. It ranges from small algae and bacteria, as small as <1µm, to feeding on protists species as large as itself, for example Cricosphaera elongata (20– 30 mm) (Droop, 1966; Dodge and Crawford, 1971).  This diverse diet of O. marina allows researchers to use this species as a versatile model.

Selective feeding behaviour

As much as O. marina exhibits plasticity in dietary, it also has distinct feeding preferences. O. marina has been shown to discriminate between prey based on size, biochemical composition and charge (Hammer et al., 1999, 2001; Wootton et al., 2007). Hammer et al., 1999, shown that ingestion rate is four times higher on 4µm beads compared with 1µm beads. Additionally, other studies have shown that O. marina prefers beads coated with mannose-BSA over N-acetylgalactosamine-BSA thus, this concludes that even though O. marina ingest artificial particles, it prefers live prey (Wootton et al., 2007).

The paper describes a study carried out by Evans and Wilson, 2008, which looks into prey selectivity within species. Virus infected Emiliania huxleyi were flavoured over healthy ones when equal density was present to O. marina. It is assumed that prey parameters such as cell surface properties and dimethylsulphoniopropionate (DMSP) lyase activity differ between healthy and virus infected E. huxleyi. Other studies have also investigated the ability of O. marina to discriminate between E.huxleyi strains with different DMSP lyase activities (Wolfe et al., 1997; Strom et al., 2003a).    

Another factor that affect grazing, which is discussed in this paper, is the prey’s carbon:nitrogen ratio. When O. marina is grown in dense culture of Isochrysis galbana, grazing ceases as prey become nitrogen-limited. It has been suggested that this rejection of grazing might be due to the build-up of an inhibitor within predator cells or a change in prey recognition by the predator (Flynn et al., 1996; Martel, 2009). Many studied have thus applied the use of carbon:nitrogen ratio as a measure of prey quality.

Mechanism of feeding

This paper categorised the feeding mechanism of O. marina into five stages: searching, contact, capture, processing and ingestion. The ability of motile protists to search for prey is influence by its swimming speed and changes in the direction and frequency of turning (Montagnes et al., 2008). O. marina swim in a helical path and it increases as prey concentration decreases. This change in helical movement is controlled by longitudinal and transverse flagella movement. At low prey concentration, it is observed that O. marina invest more energy in using transverse flagellum for movement, which results in an increase in helical trajectories. It is suggested that this optimised random searching in three-dimensional environments with low prey densities (Bartumeus et al., 2003). It is well documented that O. marina exhibits positive motile response towards chemical cues such as DMSP, which are released by their prey (Breckels et al., 2011). The response is initiated when a chemoattractant molecule bind to a specific cell receptor on the protists. Preliminary studies have shown that G-protein-coupled receptors and protein kinases in involved in the signalling pathway which initiate motility (Hartz et al., 2008.    

Transverse flagellum movement not only cause helical movement, it also creates a feeding current that carried bacterial prey towards its cingular depression where phagocytosis takes place. Engulfment occurs when receptors on protists bind to specific ligands on cell surface of prey. Experimental evidence indicates the involvement of a mannose-binding lectin in prey adhesion and recognition by O. marina (Wootton et al., 2007).  However, further experiments could be done to determine other interacting receptors that are involved in phagocytosis.

From a molecular perspective, feeding mechanisms used by O. marina are poorly understood. A complete genome sequence has yet to be done and it would be useful in further understanding the mechanism underlying feeding behaviour of O. marina. Furthermore, O. marina could be used as a model species in a wider context.  

Roberts, E. C., Wootton, E. C., Davidson, K., Jeong, H. J., Lowe, C. D., and Montagnes, D. J. (2010). Feeding in the dinoflagellate Oxyrrhis marina: linking behaviour with mechanisms. Journal of plankton research, fbq118

http://plankt.oxfordjournals.org/content/33/4/603.full.pdf