The different defence responses of haploid and diploid Emiliania huxleyi

Marine phytoplankton are significant contributors to global primary production, as a result they are an important food source for many small ocean predators. One of the most important causes for mortality is by predation from microzooplankton (20-200mm), which is thought to be responsible for consumption of a global average of 67% of phytoplankton daily production. Although many potential defences have been suggested, there has been relatively little documentation for defences against predation in phytoplankton. Two potential avenues of research in this area include whether defences are either induced or constitutive and whether different life cycles (haploid and diploid) in phytoplankton have different defence responses. Kolb, A (2012) set out to test these two hypotheses with Emiliania huxleyi.

                                                                                         

In order to carry out this experiment, two strains (haploid and diploid) of E. huxleyi were either exposed to the ciliate predator Strombidinopsis acuminatum for 24hrs or were left naïve (no exposure). The ciliate ingestion rates of predator exposed versus naïve E. huxleyi were then compared at 3 points over 30 minutes.  The prey was considered to have a defence response when ingestion rates on naïve cells were higher than ingestion rates on predator-exposed cells.

The results showed that there was indeed an induced defence response in E. huxleyi, however, the method of this defence was not identified. The strongest candidate recognised by the author was one of a chemical nature, this included DMSP which we have previously discussed with Colin. Due to the continued feeding on other organisms with low DMSP levels in this experiment by S. acuminatum, it can be hypothesised that DMSP could act as a deterrent only when the predator is in close contact. As well as high DMS levels potentially deterring predators through being a warning of toxic acrylate, the author theorises there is the possibility for E. huxleyi to increase exudation of DMS to “play stressed”. The release of DMS can signal low nitrogen levels, so capable E. huxleyi (under low stress, adequate nutrients and high predation pressure) may be able to take advantage of the system and mimic this signal, fooling the predator to find more palatable prey. 

Diploid E. huxleyi, responsible for the large algal blooms, have been thought to be well defended against predators. However, this research suggests that there is little to no discernible induced defence. A simple idea for the lack of defence response could be down to chemicals being blocked by the calcium carbonate shell therefore limiting any signals picked up by the predator. Haploid induced defences on the other hand appeared to have a significant effect at reducing ingestion by S. acuminatum by up to 43%. This raises the question as to why the two life cycles still exist if one is clearly better at defending against predators. A simple explanation is haploid E. huxleyi are more prone to photoinhibition and therefore the diploid life cycle can maintain high growth rates in high nutrient conditions. This suggest that the two life cycles exist to take advantage of separate niches, one in oligotrophic and the other in nutrient rich waters.

The lack of an inducible defence in diploid E. huxleyi contradicts the idea of bloom forming E. huxleyi being well defended. This may be due to the lower need for a defence, with high inorganic carbon content from coccolith production, it may cause diploid E. huxleyi to have a lower nutritional value. Further research in this area may help to demonstrate the predators preferred life cycle of E. huxleyi through a simple choice test. Experiments in the field, sampling over the course of an E. huxleyi bloom may help identify how the diploid life cycle deter predators if not from an induced defence. The haploid life cycle also needs to be looked at in more detail, however, it is relatively hard to extract which remains a barrier to research. Exploring the separate life cycles in more detail will allow us to further our understanding of the key role they play in the environments and niches they inhabit, this is especially important for this species due to the large role they play in nutrient and carbon cycling in our waters.

Kolb, A. (2012). An inducible predator defense in the marine microalga Emiliania huxleyi (Prymnesiophyceae) is linked to its heteromorphic haploid-diploid life cycle.

http://cedar.wwu.edu/wwuet/211/ (Thesis)