The principle of inducible, chemical defence mechanisms is well known in terrestrial plants. The discovery of an activated, chemical defence mechanism in marine macroalgae [1] suggested the existence of equivalent grazer deterrence strategies in the aquatic environment. Therefore, we will investigate the DMSP-DMS-acrylic acid system for its role in algal defence.

Acrylic Acid

Most acrylic acid in the marine environment is assumed to originate from the breakdown of the algal osmolyte ß-dimethylsulphonio-propionate (DMSP). However, ß-alanine betaine, the quaternary ammonium analogue to DMSP, is another possible source.

The enzyme DMSP-lyase, found in algae and some bacteria, mediates the cleavage of DMSP to DMS and acrylic acid. This degradation reaction (1) increases during grazing, senescence and lysis of the algae.

(1)

Acrylic Acid Production

The macrophytes Enteromorpha sp. and Ulva sp., several dinoflagellates, Cryptomonas sp., Phaeocystis sp. and Emiliania huxleyi are all important DMSP producers.

Acrylic acid concentrations in the mucous layer of Phaeocystis colonies reach up to 6x10-3M, whereas those measured in the medium of senescent cultures are around 4x10-6M [2, 3]. Natural acrylic acid concentrations during bloom conditions range from 0.01 - 1.2x10-6M [4].

The antibiotically active concentration of acrylic acid is strongly dependent on the ambient pH and ranges from 10-5 - 10-2M [5, 6].

Indications for Toxic Effects of Acrylic Acid

In 1960, Sieburth detected acrylic acid and its antibiotic property in colonies of Phaeocystis sp.. Further indications for the toxic effects of this compound are the low bacterial numbers associated with actively growing Phaeocystis cultures and the dominance of this species during bloom conditions.

Grazing experiments report the discrimination of microzooplankton grazers against acrylic acid producing, high DMSP-lyase-activity prey [7]. Therefore, the role of DMSP as a non-toxic precursor in a grazing-activated defence system in phytoplankton and macroalgae is suggested.

Future Research

• Method development to measure natural, nanomolar acrylic acid concentrations
• Grazing experiments, focusing on acrylic acid and DMSP/cell concentrations and testing for the existence of inducible and activated defence mechanisms
• Investigation of avoidance reactions of grazers using acrylic acid gradients
• Acrylic acid inhibition tests with range of marine bacteria

References

[1] V. J. Paul and K. L. Van Alstyne, Journal of Experimental Marine Biology and Ecology, vol. 160, pp. 191-203, 1992.
[2] D. J. B. Noordkamp et al., Aquatic Microbial Ecology, vol. 16, pp. 45-52, 1998.
[3] D. J. B. Noordkamp et al., Journal of Sea Research, vol. 43, pp. 287-296, 2000.
[4] J. A. E. Gibson et al., Biological and Environmental Chemistry of DMSP and Related Sulfonium Compounds, R. P. Kiene et al., Ed. New York: Plenum Press, 1996, pp. 213-222.
[5] V. Andreoni et al., Annals of Microbiology, vol. 40, pp. 279-286, 1990.
[6] J. M. Sieburth, Science, vol. 132, pp. 676-677, 1960.
[7] G. V. Wolfe, Biological Bulletin, vol. 198, pp. 225-244, 2000.