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Applied and Environmental Microbiology, December 2007, p. 7844-7852, Vol. 73, No. 24
0099-2240/07/$08.00+0     doi:10.1128/AEM.01543-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Low Densities of Epiphytic Bacteria from the Marine Alga Ulva australis Inhibit Settlement of Fouling Organisms

Dhana Rao,^1,2 Jeremy S. Webb,^1,2, Carola Holmström,^1,2 Rebecca Case,^1,2 Adrian Low,^1,2 Peter Steinberg,^2,3 and Staffan Kjelleberg^1,2*

School of Biotechnology and Biomolecular Sciences,¹ Centre for Marine Bio-innovation, School of Biological, Earth and Environmental Sciences,² University of New South Wales, Sydney, New South Wales 2052, Australia³

Received 9 July 2007/ Accepted 15 October 2007

Bacteria that produce inhibitory compounds on the surface of marine algae are thought to contribute to the defense of the host plant against colonization of fouling organisms. However, the number of bacterial cells necessary to defend against fouling on the plant surface is not known. Pseudoalteromonas tunicata and Phaeobacter sp. strain 2.10 (formerly Roseobacter gallaeciensis) are marine bacteria often found in association with the alga Ulva australis and produce a range of extracellular inhibitory compounds against common fouling organisms. P. tunicata and Phaeobacter sp. strain 2.10 biofilms with cell densities ranging from 10² to 10⁸ cells cm^–2 were established on polystyrene petri dishes. Attachment and settlement assays were performed with marine fungi (uncharacterized isolates from U. australis), marine bacteria (Pseudoalteromonas gracilis, Alteromonas sp., and Cellulophaga fucicola), invertebrate larvae (Bugula neritina), and algal spores (Polysiphonia sp.) and gametes (U. australis). Remarkably low cell densities (10² to 10³ cells cm^–2) of P. tunicata were effective in preventing settlement of algal spores and marine fungi in petri dishes. P. tunicata also prevented settlement of invertebrate larvae at densities of 10⁴ to 10⁵ cells cm^–2. Similarly, low cell densities (10³ to 10⁴cells cm^–2) of Phaeobacter sp. strain 2.10 had antilarval and antibacterial activity. Previously, it has been shown that abundance of P. tunicata on marine eukaryotic hosts is low (<1 x 10³ cells cm^–2) (T. L. Skovhus et al., Appl. Environ. Microbiol. 70:2373-2382, 2004). Despite such low numbers of P. tunicata on U. australis in situ, our data suggest that P. tunicata and Phaeobacter sp. strain 2.10 are present in sufficient quantities on the plant to inhibit fouling organisms. This strongly supports the hypothesis that P. tunicata and Phaeobacter sp. strain 2.10 can play a role in defense against fouling on U. australis at cell densities that commonly occur in situ.

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* Corresponding author. Mailing address: School of Biotechnology and Biomolecular Sciences and Centre for Marine Bio-innovation, Biological Sciences Building, University of New South Wales, Sydney, NSW 2052, Australia. Phone: 61 (2) 9385 2102/2276. Fax: 61 (2) 9385 1779. E-mail: s.kjelleberg{at}unsw.edu.au

Published ahead of print on 26 October 2007.

Present address: School of Biological Sciences, University of Southampton, Southampton SO16 7PX, United Kingdom.

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Applied and Environmental Microbiology, December 2007, p. 7844-7852, Vol. 73, No. 24
0099-2240/07/$08.00+0     doi:10.1128/AEM.01543-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.