Cyanobacterial and eukaryotic algal odour compounds: signals or by-products? A review of their biological activity

Algal metabolites (produced by cyanobacteria or eukaryotic algae) are the primary cause of foul source-water odour. Industry has tended to treat algal volatile organic compounds (AVOCs) as metabolic waste, with a focus on their identification, sources and removal. Other, more proactive research has examined proximal abiotic triggers, yet AVOC outbreaks still remain highly unpredictable. An alternative and intriguing viewpoint suggests that, in fact, some AVOCs play active, functional roles in ecological processes and that these are key to their production dynamics; however, support for this premise remains scattered. This paper presents a synthesis of this topic, and evaluates evidence from freshwater, marine and terrestrial systems. It is concluded that (1) it is highly probable that some AVOCs are biologically active; and (2) this activity is largely dependent on their functional roles (as allelogens, pheromones or kairomones), and the environment or habitat of the producers and target organism(s). Activity levels of terpenoids such as geosmin and 2-methylisoborneol and nor-carotenoids are such that these compounds are likely to be ineffective in diffuse planktonic environments, but they may be functional at microscale levels in biofilm matrices. There is good evidence that some polyunsaturated fatty acids (PUFAs) or their derivatives (or both) are biologically active, depending on the mode and timing of cell lysis and subsequent PUFA peroxidation. This activity may account for the dichotomous role(s) of PUFAs in aquatic food webs, being essential resources at average sestonic levels, but toxins when concentrated in grazer digestive tracts. Alicyclic PUFA derivatives, which act as multifunctional semiochemicals among marine seaweeds, may play similar roles among freshwater periphyton. Little is known about the chemical ecology of biogenic sulphides in freshwater systems, but evidence that these compounds act as allelogens merits further research. Overall, these data are promising but often ambiguous, and demonstrate a fundamental need for more focused research on the chemical ecology of AVOCs in freshwater ecosystems.