Biological processes result in the production of various trace gases including dimethyl sulphide (DMS), organohalogens and non-methane hydrocarbons. Chemical signalling has been predominantly studied in benthic organisms, where an exchange of information can be utilised by infochemicals such as sexual attractants (pheromones), predator associated chemicals (kairomones), or digestibility reducers and toxins (allomones). Many examples of infochemistry in the sea describe the interactions in coral reef ecosystems but our knowledge on the production and perception of infochemicals by oceanic plankton is very limited.

Are biogenic trace gases involved in chemical signalling?

It is possible that biogenic trace gases are infochemicals in trophic interactions. Some strains of the phytoplankton Emiliania huxleyi differ in their ability to enzymatically produce DMS from its precursor DMSP (Steinke et al. 1998) and microzooplankton feeds on strains with high DMS production at a reduced rate (Wolfe et al. 1997). Probably, the co-produced acrylate is an allomone that is harmful to grazers and some microzooplankton species avoid eating high acrylate-producers when alternative prey is available. Plankton predators often inspect the suitability of prey before ingestion. This mechanical handling can trigger the rapid release of infochemicals such as DMSP, DMS or acrylate and may signal that the selected item is unsuitable prey. The trace gases investigated in our group have the potential for signalling in such predator-prey interactions (bitrophic interactions) and may explain why species with high DMSP and DMS production (e.g. dinoflagellates or the haptophytes Emiliania huxleyi and Phaeocystis spp.) are successful in the competition with other phytoplankton.

From bitrophic to tritrophic interactions

In higher plants, the interaction between a plant and its predator (an herbivore) results in the release of volatile infochemicals. This assists the enemy of the herbivores (a carnivore) to detect and locate an infected plant. Such interactions between three different trophic levels (tritrophic interactions) are well studied in land plants, and model organisms, for example the plant Phaseolus lunatus, the herbivorous mite Tetranychus urticae and the carnivorous mite Phytoseiulus persimilis, have been used to characterise the release of volatiles and signal transduction pathways.

Since grazing of microzooplankton on phytoplankton increases the production of trace gases, it is possible that these signals can be exploited by carnivorous mesozooplankton such as copepods. These organisms live in a nutritionally dilute environment and chemical cues that help to find suitable prey would be beneficial for their survival.

Filter-feeding copepods create a laminar flow field that enables the copepods to scan relatively large volumes of water for suitable prey. Probably, the boundary layer surrounding small microzooplankton is enriched with DMS after uptake of DMS producing phytoplankton and will be distorted in this flow field. Once the infochemicals come in contact with chemical sensors on the first antennules of the copepod, a redirection of the flow field ensures capture and further inspection of the potential prey item.

Analogous to the role of gaseous infochemicals in land plants, biogenic gases may be involved in shaping the structure and functioning of marine trophic webs spanning microbial to geographical scales. The sensory behaviour of many marine organisms is understudied or unknown, but the ecophysiology of volatile infochemicals does provide a suitable framework to address chemical communication in the sea.

Figure 3 - Chemical signalling in the ocean: from microbial to geographical scales
Image © Glynn Gorick 2003