Microbial interactions associated with biofilms attached to Trichodesmium spp. and detrital particles in the ocean

Abstract

Quorum sensing (QS) via acylated homoserine lactones (AHLs) was discovered in the ocean, yet little is known about its role in the ocean beyond its involvement in certain symbiotic interactions. The objectives of this thesis were to constrain the chemical stability of AHLs in seawater, explore the production of AHLs in marine particulate environments, and probe selected behaviors which might be controlled by AHL-QS. I established that AHLs are more stable in seawater than previously expected and are likely to accumulate within biofilms. Based on this result, I chose to study AHL-QS in the bacterial communities inhabiting biofilms attached to Trichodesmium spp. and detrital (photosynthetically-derived sinking particulate organic carbon, POC) particles. These hot spots of microbial activity are primary sites of interaction between marine primary producers and heterotrophs and crucial components of the biological pump.Biofilm communities associated with Trichodesmium thiebautii colonies in the Sargasso Sea differed considerably from seawater microbial communities. In addition, there was no overlap between the communities associated with tuft and puff colonies. These results suggest that bacterial communities associated with Trichodesmium are not random; rather, Trichodesmium selects for specific microbial flora. Novel 16S rRNA gene sequences are present both in clone libraries constructed from DNA extracted from colonies of Trichodesmium spp. and in culture collections derived from wild and laboratory cultivated Trichodesmium spp., supporting the idea that the phycosphere of Trichodesmium is a unique microenvironment.Using high performance liquid chromatography-mass spectrometry, I demonstrated that bacteria isolated from Trichodesmium synthesize AHLs. In addition, I detected AHLs in sinking particles collected from a site off of Vancouver Island, Canada. AHLs were subsequently added to laboratory cultures of non-axenic Trichodesmium colonies and sinking POC samples. This is the first time AHLs have been detected in POC and indicates that AHL-QS was occurring in POC. Further, I showed that AHLs enhanced certain organic-matter degrading hydrolytic enzyme activities. My results suggest that AHL-QS is a factor regulating biogeochemically relevant enzyme activities on sinking POC and within the biofilms attached to Trichodesmium colonies and thereby may impact the timing and magnitude of biogeochemical fluxes in the ocean.

Quorum sensing control of phosphorus acquisition in Trichodesmium consortia

“…By contrast, the Erythrobacter isolates produced saturated (i.e., no 3-oxo group), long chain-length AHLs (e.g., tetradecane homoserine lactone (C14-HSL); Supplementary Table 1 and Supplementary Figure 1). The distribution of AHLs produced by the Erythrobacter isolates is similar to AHL distributions reported in other Alphaproteobacteria (Wagner-Döbler et al, 2005), and Alphaproteobacteria were recently found to be prevalent within Trichodesmium consortia by using cultivation-independent methods (Hmelo, 2010). Although the few AHL-producing Erythrobacter we isolated probably represented only a fraction of the AHLproducing epibionts within Trichodesmium colonies, Decho et al (2009) reported that the same saturated, long-chain AHLs were also prevalent in natural cyanobacterial mat communities.…”

“…Phylogeny of the epibionts was assessed using 16S rRNA gene sequences. Epibionts were grown overnight in seawater containing tryptone (1 g l À1 ), their DNA was extracted using standard methods, and 16S rRNA genes were amplified using the forward primer 27F (5 0 -AGAGTTTGATCMTGGCTCAG-3 0 ) and reverse primer 1492R (5 0 -TACGGYTACCTTGTTA CGACTT-3 0 ) as described (Hmelo, 2010). End sequencing was performed offsite either by MWG-Operon (Huntsville, AL, USA) or Agencourt Biosciences (Beverly, MA, USA).…”

“…End sequencing was performed offsite either by MWG-Operon (Huntsville, AL, USA) or Agencourt Biosciences (Beverly, MA, USA). Sequences were analyzed and phylogenetic trees constructed as described (Hmelo, 2010).…”

“…In the open ocean, Trichodesmium colonies contain a diversity of other types of cells (Paerl et al, 1989;Nausch, 1996;Sheridan et al, 2002;Hewson et al, 2009;Hmelo, 2010), including heterotrophic bacterial epibionts that, together with Trichodesmium, form a consortium that is highly adapted to the phosphorus-depleted conditions of the open ocean. Cell densities of epibionts in Trichodesmium colonies can be three orders of magnitude greater than cell densities in surrounding waters (Sheridan et al, 2002), and APase expression by these epibionts is frequently observed (Dyhrman et al, 2002;Hynes et al, 2009).…”

“…However, the distribution of APase activity among epibionts and Trichodesmium within colonies is highly variable even within the same environment (Dyhrman et al, 2002;Hynes et al, 2009), and the mechanisms driving the variability in this behavior are unknown. Proteobacteria related to those within Trichodesmium consortia (Hewson et al, 2009;Hmelo, 2010) are known to employ quorum sensing (QS; a cell-density dependent signaling system), based on acylated homoserine lactones (AHLs; Supplementary Figure 1) to coordinate their behavior (Wagner-Döbler et al, 2005;Case et al, 2008). Furthermore, it was recently shown that AHL-based QS is sustainable at the cell densities typical of Trichodesmium colonies (Hmelo and Van Mooy, 2009;Hmelo et al, in press).…”