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In both freshwater and marine ecosystems, phytoplankton are the most dominant primary producers, contributing substantially to aquatic food webs. Algicidal bacteria that can associate to microalgae from the phytoplankton have the capability to control the proliferation and even to lyse them. These bacteria thus play an important role in shaping species composition in pelagic environments. In this review, we discuss and categorise strategies used by algicidal bacteria for the attack on microalgae. We highlight the complex regulation of algicidal activity and defence responses that govern alga-bacteria interactions. We also discuss how algicidal bacteria impact algal physiology and metabolism and survey the existing algicidal metabolites and enzymes. The review illustrates that the ecological role of algicidal bacteria is not yet fully understood and critically discusses the challenges in obtaining ecologically relevant data.

Algicidal bacteria can lyse microalgal blooms and trigger shifts within plankton communities. Resistant algal species can escape lysis, and have the opportunity to dominate the phytoplankton after a bacterial infection. Despite their important function in ecosystem regulation, little is known about mechanisms of resistance. Here, we show that the diatom Chaetoceros didymus releases eicosanoid oxylipins into the medium, and that the lytic algicidal bacterium, Kordia algicida, induces the production of several wound-activated oxylipins in this resistant diatom. Neither releases nor an induction occurs in the susceptible diatom Skeletonema costatum that is lysed by the bacterium within a few days. Among the upregulated oxylipins, hydroxylated eicosapentaenoic acids (HEPEs) dominate. However, also, resolvins, known lipid mediators in mammals, increase upon exposure of the algae to the algicidal bacteria. The prevailing hydroxylated fatty acid, 15-HEPE, significantly inhibits growth of K. algicida at a concentration of approximately 1 µM. The oxylipin production may represent an independent line of defense of the resistant alga, acting in addition to the previously reported upregulation of proteases.

The recovery of Aeromonas spp. from the unchlorinated water supply for a Western Australian city of 21,000 people was monitored at several sampling points during a period of 1 year. Membrane filtration techniques were used to count colonies of Aeromonas spp., coliforms, and Escherichia coli in water sampled before entry to service reservoirs, during storage in service reservoirs, and in distribution systems. Aeromonas spp. were identified by subculture on blood agar with ampicillin, oxidase tests, and the use of Kaper medium and then were tested for production of enterotoxins and hemolysins. During the same period, two-thirds of all fecal specimens sent for microbiological examination were cultured on ampicillin-blood agar for Aeromonas spp. Recovery of Aeromonas spp. from water supplies at distribution points correlated with fecal isolations and continued during autumn and winter. Coliforms and E. coli were found most commonly in late summer to autumn. This pattern differs from the summer peak of Aeromonas isolations both from water and from patients with Aeromonas spp.-associated gastroenteritis in Perth, Western Australia, a city with a chlorinated domestic water supply. Of the Aeromonas strains from water, 61% were enterotoxigenic, and 64% produced hemolysins. Aeromonas spp. have frequently been found in water (12, 16, 19), including chlorinated and unchlorinated drinking water (7, 17, 20). Exposure to water contaminated with Aeromonas spp. has been reported to precede some human Aerom0onas infections (8) which are particularly hazardous in patients with impaired immunity. In patients without immunological abnormality, Aeromonas spp. have been implicated in diarrheal disease (22). Many strains of Aeromonas spp. produce exotoxins which include enterotoxins, cytotoxins, and hemolysins. Strains capable of producing these possible virulence factors have been isolated from water (16, 17) and may be a source of human enteric disease (16). In the present study, we have investigated the unchlorinated piped domestic water supply of a country center in Western Australia in relation to the distribution of fecal isolation of Aeromonas spp. at the same time in the same area. MATERIALS AND METHODS Source of water samples. During the period from December 1981 to December 1982, water samples were collected at about weekly intervals from the domestic water supply of a country center, population 21,000, in Western Australia. All water came from underground sources. At each source, raw water was treated by sedimentation and rapid sand filtration but was not chlorinated. Treated water was stored in unroofed service reservoirs and entered the distribution system without further treatment. Water was sampled after treatment but before entry to service reservoirs (level 1), during storage in service reservoirs (level 2), and in the distribution systems beyond service reservoirs (level 3). Atmospheric temperatures were * Corresponding author.

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