Quorum sensing: Gram-negative bacteria are positive for Gracilaria dura carpospore liberation. 

Macroalgae release extracellular substances, which provides nutrition for a variety of microorganisms. Bacteria are the most abundant of microbial groups on macroalgae and are found on the external surfaces and internal tissues of macroalgae. Bacterial communities have shown to impact the growth, morphogenesis, and reproduction on the macroalgae: Ulva and Gracilaria spp. – which are the most common macroalgae found across global coastlines. 

Microbial communities do not live in isolation, but rather rely on intercellular interactions to thrive (Johnson et al., 2016). For bacteria, quorum sensing is a method of intercellular communication. Communication is coordinated by the detection and production of signal molecules called autoinducers; autoinducers regulate gene expression in response to bacterial population density. These signal molecules are chemically diverse, but the most widespread are N-acyl homoserine lactones (AHLs), which are found in Gram-negative bacteria. Gram-positive bacteria do not use AHL systems but rely on peptide signals instead, which are highly species-specific.  

Bacterial quorum sensing can impact the development of macroalgae species, which may include: facilitation in the settlement of zoospores, root stimulation, root and shoot growth, root nodule formation, pathogenic resistance, and even plant defence. Additionally, Authors note that prior studies found associations between AHLs on red algae Acrochaetium spp. for carpospore liberation, but knowledge was limited. The study by Singh et al. (2015) aimed to investigate the impact of bacterial quorum sensing on carpospore liberation for green and red macroalgae.  

The study analysed bacterial species abundances and presence of autoinducers from epiphytic and endophytic bacteria on Ulva and Gracilaria spp. Samples were collected from two different locations (Indian coastal areas) and three seasons. Additionally, AHL-containing culture filtrates were measured from cystocarp plantlets of Gracilaria dura, to analyse carpospore liberation. Bacterial isolates were identified by 16S rRNA gene amplification and sequencing. AHL production and separation was measured by a process of inoculation and centrifugation. AHL identification was measured by liquid chromatography electrospray ionization mass/mass spectrometry and electrospray ionization mass spectrometry.  

Gammaproteobacteria occurred across all samples, seasons, and locations. This group is thought to be highly responsible for the formation of biofilms, which can determine the production of quorum sensing-mediated genes. Of the bacteria analysed, seven isolates were found to produce AHLs in G. dura; this includes Shewanella algae, which produced five different types of AHLs alone. The study found that carpospore liberation increases with the relative increase in concentration of C4- and C6-HSL, up to 10 µm. However, C8-, C10-, 3-oxo-C12-HSL, and culture filtrates of Gram-positive bacterium Bacillus flexus showed no influence on carpospore liberation.  

It appears that in this study, the short acyl chain molecules enhanced carpospore liberation when compared to longer acyl chain molecules. Short acyl chain molecules appear to have higher diffusion and solubility rates in water, which allows them to be actively taken up into plant roots and transported through shoots. 

Overall, quorum sensing efficiency does not only benefit bacterial phenotypes, but has an impact on the reproductive output of macroalgal species. The findings of this study infer that the diffusion ability, stability, and availability of AHLs are important factors that help to function carpospore liberation from G. dura. 

Referenced material:  

Johnson, W.M., Soule, M.C.K., and Kujawinski, E.B. (2016) “Evidence for quorum sensing and differential metabolite production by a marine bacterium in response to DMSP.” The ISME journal, 10(9), pp.2304. 

Article reviewed: 

Singh, R.P., Baghel, R.S., Reddy, C.R.K., and Jha, B. (2015) “Effect of quorum sensing signals produced by seaweed-associated bacteria on carpospore liberation from Gracilaria dura.” Frontiers in plant science, 6, pp.117.