The signal based relationship between the green seaweed Ulva and its indigenous bacterial community

This project has focused on the relationship between the green seaweed Ulva, commonly found in the intertidal zone of the UK coastline and its cognate bacterial community. It has previously been reported that motile Ulva zoospores are attracted to N-Acylhomoserine lactones (AHLs), signalling molecules utilised by Gram-negative bacteria in a density dependent form of cellular communication termed quorum sensing (QS) and produced by several biofilm dwelling species of marine bacteria. The species represented in the bacterial community associated with Ulva spp. were identified by generating a 16S rDNA phylogenetic clone library from bacterial DNA isolated from the surface of the seaweed. These data revealed that the majority of the population belonged to the Proteobacteria or Bacteroidetes phyla. In order to investigate whether QS signalling affected the rate of zoospore germination in addition to zoospore attraction, Ulva zoospores were settled and allowed to grow on synthetic AHLs, biofilms derived from AHL-producing model organisms and strains relevant to the Ulva epiphytic population which were shown to produce AHLs. Results from these experiments revealed that AHLs affected zoospore germination and the early growth of the Ulva germling as zoospores germinated and grown in the absence of AHLs were significantly longer than those germinated in the presence of AHLs. We therefore hypothesise that reduced germling growth in the presence of AHLs allows Ulva to obtain a healthy epiphytic bacterial community that is vital for the seaweed’s later development. Further understanding of Ulva growth biology could have potential applications in preventing marine biofouling by this genus of seaweed. This study progressed to characterise AHL production in a number of strains of Shewanella and Bacteroidetes bacteria, which, for differing reasons were deemed relevant to Ulva biology. Although data presented by this thesis showed AHL production in these bacterial groups, AHL synthase and response regulator sequences could not be identified in the published genome sequences from either Shewanella or the Bacteroidetes. This study also identified an AHL inactivating acylase enzyme in an environmental Shewanella isolate. This acylase, AacS, was shown to degrade a variety of synthetic AHLs and the AHLs produced by Yersinia pseudotuberculosis. This study has therefore increased the range of marine bacteria known to be producing AHLs, however the lack of AHL synthase and response regulator genes in the genomes of these bacteria leads to the conclusion that many marine bacteria possess novel, yet to be characterised AHL-mediated QS systems. Finally, this study screened a number of extracts from marine microalgae for compounds that act as agonists or antagonists to AHL-mediated QS. Although no AHL mimics were identified data presented by this thesis showed extracts to affect the luminescence produced in lux-based AHL bio-reporters in the presence of exogenously added signal, affect a number of QS regulated phenotypes in marine pathogens and effect QS regulated genes in the human pathogen Pseudomonas aeruginosa. As such, we hypothesise that these microalgae have the ability to produce quorum-quenching compound(s). Further characterisation of quorum-quenching compound(s) produced by microalgae may be beneficial in the bio-control of pathogenic bacteria in aquaculture and may act as candidates for novel antibiotics.