Allelopathy in Cyanobacteria

The commercial availability of certain toxin standards has allowed researchers to examine the physiological mechanisms of allelopathy by cyanobacteria. The best known examples are from studies on microcystins, which affect plants and aquatic algae by interfering with protein phosphatases in a manner similar to their effect on vertebrate enzymes (Babica et al. 2006). However, there is evidence that microcystins can also promote the formation of reactive oxygen species (ROS) in photoautotrophs, which can cause extensive damage to cellular membranes and enzymes (Babica et al. 2006 Leflaive and Ten-Hage 2007). 

Neurotoxins, such as saxitoxin and anatoxin-a, have been implicated in mediating competitive interactions between toxic cyanobacteria and other photoautotrophs, but few studies have explicitly examined the allelopathic effects of these compounds (e g., Kearns and Hunter 2001). Although it is reasonable to assume that these compounds bind to algal and cyanobacterial sodium channels in a similar fashion as in vertebrate neurons, support for this hypothesis is currently lacking. 

2 Allelopathic Effects of A. flos-aquae on a Motile Alga 

One role of cyanobacterial allelochemicals may be to alter the motility and distribution of competing photoautotrophs. In a recent study, Kearns and Hunter (2001) examined the effects of toxic metabolites from the filamentous cyanobacterium A. flos-aquae on a unicellular phytoplankton species, Chlamydomonas rein-hardtii. A. flos-aquae synthesizes both microcystins as well as anatoxins, providing the authors with an ecologically relevant opportunity to assess the individual and combinatorial effects of these toxins on an alga. 

Cells of C. reinhardtii were exposed to cell-free filtrates from A. flos-aquae, pure microcystin-LR or anatoxin-a, or combinations of the toxins. Both the position of the cells and the chlorophyll-a concentration of the cultures were observed for 12 days. Exposure to crude extracts as well as to combinations of the toxins significantly decreased chlorophyll levels in the cultures. Furthermore, these compounds were all capable of paralyzing the algae and thus promoted the settlement of C. reinhardtii cells. One intriguing aspect of this dynamic interaction is the separate finding that C. reinhardtii may actually induce toxin synthesis in A. flos-aquae (Kearns and Hunter 2000), essentially signaling its own demise. 

---