The best offence is a good defence-Chemical defence in the microplankton.

Chemical defences are used by a variety of organisms in the natural world. They do this in order to eliminate agents of mortality, which are highly diverse (Strom et al., 2003). However, chemical defences are better studied in terrestrial ecosystems than in marine. This paper by Strom et al., 2003, looks at the use of chemical defence in the microplankton. Examples of chemical defences in planktonic ecosystems are known of, however the significance these defences in terms of ecosystem function has not been systematically explored. This is what Strom et al., aimed to explore.

This study collected samples of the bloom-forming, coccolithophorid E. hux, collected from the centre for the collection of marine phytoplankton. The samples were maintained at 15°C. The protist grazers were collected from local marine waters and stored at 12°C. The main aspect of the method from this paper was the measuring the ingestion rates and growth on E.huxleyi, as these were what would actually show whether E.huxleyi  displayed reduced grazing or not. These were estimated from rates of accumulation of E.huxleyi in the grazers food vacuoles. The E.huxleyi were removed and suspended with the protist grazers for 12-24 hours. The grazers were examined under blue light and assessed as either possessing or not possessing E.huxleyi cells. If cells were present, then the amount was counted. Ingestion rates were measured in 1 of 2 ways. Either by counting the number of E.huxleyi cells in the grazers, or if the grazers didn’t contain countable E.huxleyi then they were scored as feeding or non-feeding. Growth rates were also estimates by suspending additional samples and observing growth every 24 hours for 3 days.

Of the 6 tested protist grazer species, five showed lower feeding rates on E.huxleyi with high DMSP lyase activity than on strains with low DMSP lyase activity. This suggests that high levels of DMSP lyase promote reduced palatability of the algae. However, Strom et al., suggest that the nutritional value of the algae may still be a contributing factor, so can’t be ruled out. It is also hypothesised that reduced grazing on E.huxleyi is down to signals, rather than acute toxins. They put this down to 2 reasons; (i) Organisms don’t actually need to ingest the E.huxleyi in order to receive the cue not to eat them, (ii) Exposure to high lyase E.huxleyi cells doesn’t lead to harmful consequences in terms of ability to feed and grow on alternative phytoplankton prey. The reductions in grazing rate were almost always associated with high DMSP lyase activity in E.huxleyi. These reductions in grazing usually led to a reduction in growth rate of the grazers. So as a whole, DMSP lyase is very tightly associated with reductions in grazing rate.

Other conducted research has shown DMSP has a role as a feeding inhibitor. As a result it may be associated with chemical defence (Strom et al., 2003).

This research may have significant implications in such areas as phytoplankton blooms, as pointed out by strom et al. DMSP has been shown to be a feeding inhibitor and to reduce palatability and so may help species to form persistent blooms. This obviously has profound implications in areas such as harmful algal blooms. If algae can use DMSP to increase their resistance to grazers, could HABs become more frequent?  Though this paper provides evidence for the role of DMSP in reducing grazing in phytoplankton however it doesn’t really touch upon possible inferences that may be able to be made in natural communities. I feel the major limitation is the fact that nutritional value can’t be ruled out as a possible reason for the differences in feeding rates. Further work would be useful in determining the extent to which this is true. This would make the link between the DMSP lyase and reduced grazing on E.huxleyi far more solid.

Strom, S., Wolfe, G., Holmes, J., Stecher, H., Shimeneck, C., Lambert, S., Moreno, E.. (2003). Chemical defense in the microplankton I: Feeding and growth rates of heterotrophic protists on the DMS-producing phytoplankter Emiliania huxleyi. Limnology and Oceanography. 48, 217-229

Strom, S., Wolfe, G., Slajer, A., Lambert, S., Clough, J.. (2003). Chemical defense in the microplankton II: Inhibition of protist feeding by b-dimethylsulfoniopropionate (DMSP). limnology and Oceanography. 48, 230-237.