Phytoplankton enhances inhibition of pathogen

Phytoplankton cultures are commonly used in aquaculture, where they are known as ‘green water’. The cultures are usually grown from natural seawater, and are added to larvae tanks to provide nutrition, improve water quality, and make the water less clear thereby reducing larval light stress. Most importantly they alter the bacterial composition of the water, leading to dominance by alphaproteobacteria and the Cytophaga-Flavobacterium cluster. A number of Roseobacter clade members (alphaproteobacteria), have been found to suppress growth of the fish pathogen Vibrio anguillarum. This paper aims to explore this inhibitory activity in the presence of the phytoplankton Nannochloropsis oculata, so that the benefits to aquaculture can be better understood.

Roseobacter clade strains were isolated from a culture of N. oculata, and identified using FISH and PCR. Six Roseobacter strains (Sulfitobacter sp. (two strains), Thalassobius sp., Stappia sp., Rhodobacter sp. and Antarctobacter sp.) were inoculated with V. anguillarum strain psh-9019 into three different types of media. After incubation at 20°C for 3 days, colonies were counted. Cultures of each strain on each media served as controls.

On the first medium, an organic nutrient-rich medium for heterotrophic marine bacteria (VNSS), the control V. anguillarum culture grew to 1.6x109 CFU/ml, and similar results were obtained for V. anguillarum cultured together with the Roseobacter strains. However, on the second medium, a phytoplankton culture medium (ESM), the viability of V. anguillarum decreased approximately ten-fold when cocultured with the Roseobacter strains.

The final medium was NCF, which contained substances excreted by the phytoplankton N. oculata grown in ESM. Control V. anguillarum CFUs were similar to the results obtained on ESM medium, but when cocultured with Roseobacter strains on NCF medium, V. anguillarum was completely eradicated after 5 days. These results strongly suggest that Roseobacter clade members can exhibit a much greater antibacterial effect upon V. anguillarum when cocultured with a phytoplankton species such as N. oculata.
The authors suggest that N. oculata may excrete substances such as dimethylsulfoniopropionate (DMSP) and amino acids which more optimally support growth of the Roseobacter isolates. They go on to say that under oligotrophic conditions mimicked by the ESM and NCF media, the Roseobacter strains are more efficient at obtaining small amounts of nutrients than V. anguillarum. The authors also believe that the Roseobacter strains directly inhibit the growth of the pathogen, with N. oculata releasing compounds which act as signaling molecules for communicating with Roseobacter isolates, affecting the inhibitory activity.

This paper has very extensive methods and results sections, which makes it quite complex at times. However it is one of the first to show that these particular Roseobacter strains isolated from phytoplankton cultures can affect growth of the fish pathogen V. anguillarum, and the investigation into the effects of coculture with N. oculata adds another dimension to disease control in aquaculture.

A review of: Sharifah, E.N. and Eguchi, M. (2011) ‘The phytoplankton Nannochloropsis oculata enhances the ability of Roseobacter clade bacteria to inhibit the growth of fish pathogen Vibrio anguillarum’, PLoS ONE 6(10): e26756. doi:10.1371/journal.pone.0026756.