Cyanophage (viral) control of toxic cyanobacterial abundance and dispersal

Abstract

Globally, cyanobacterial blooms are increasing along with their adverse impacts on human heath. Cyanobacterial toxins are now more often appearing in a wide range of environments from drinking water supplies to the seafood industry. Yet factors that control their occurrence, bloom formation and collapse are still not clear. Could this be because natural cyanophage (viruses specific to cyanobacteria) are exerting an underlying natural control in addition to the physico-chemical factors? Here we test whether viruses from two Australian freshwater lakes (drinking water supplies) could control the abundance of two toxic species of cyanobacterium — Cylindrospermopsis raciborskii and Microcystis aeruginosa. These cyanobacteria were selectively isolated from the two lakes. Microscopy confirmed the resulting culture were single cyanobacterial species. Filtration (through a 0.2 µm filter) was used to isolate viral communities from each lake sample, chloroform washed and stored in the dark at 4° C until their host populations were growing in culture. Then the natural lake viral cocktails were incubated with each cyanobacteria host growing under optimum conditions. C. raciborskii cells abundance decreased by 86% in 5 days, while the number of viruses in the culture increased stepwise. The cyanophage replication time was 21 h, with an average burst size of 64 viruses cell−1. TEM showed this cyanophage to be in the Siphoviridae family of viruses. C. raciborskii is a filamentous cyanobacterium. The cyanophage would often lyse only a single cell in a filament and so split it into smaller viable fragments. This process would help disperse the host in the wild. Hence this cyanohpage influences both the abundance and distribution of C. raciborskii.

For Microcystis aeruginosa it took 6 d for its abundance to decrease by 95%. The density of the cyanophage was positively correlated with the rate of M. aeruginosa cell lysis (r2 = 0.95). The cyanophage replication time was 11.2 h, with an average burst size of 28 viruses per host cell. TEM showed that two types of virus were controlling the host abundance and both belonged to the Podoviridiae group (short tails) of viruses. In the lake, the number of these cyanophage was 5.6 x 104 . mL-1, representing 0.23% of the natural viral population of 2.46 x 107 mL-1. Our results showed that this cyanophage could be a major natural control mechanism of M. aeruginosa in the source drinking water supply.