A big role for small molecules in mediating Emiliania huxleyi – Roseobacter interactions

• Author / Creator

  Labeeuw, Leen

• Microalgae are a diverse group of photosynthetic microorganisms that have complex relationships with their surrounding bacteria, which are often controlled by the exchange of bioactive molecules. Emiliania huxleyi is a ubiquitous marine microalga, forming massive blooms, driving the marine carbon pump and biogeochemical cycles like the sulphur cycle. Bacterial symbionts of this alga include an abundant group of α-proteobacteria known as roseobacters. These bacteria are known to containing a variety of secondary metabolites that may benefit an algal symbiont; for example, certain members produce a growth-promoting plant hormone, indole-3-acetic acid (IAA) or tropodithietic acid (TDA) that acts to chemically defend their host from further bacterial colonisation. However, at least one species of the roseobacters can switch to produce a pathogenic effect as the alga enters late-stage growth, secreting algaecides called roseobacticides. These are formed in response to a lignin intermediate, p-coumaric acid (pCA). This duality of mutualism and pathogenicity seems to be controlled by the various bioactive molecules released by the algae and bacteria. This thesis seeks to investigate the interactions between algae and their selected members of their microbiome using controlled laboratory conditions and by doing so, further our understanding of their natural interactions and find novel methods to enhance algae processing for industrial purposes, such as biofuels. Although pCA has been shown to be linked to aging microalgae, the role it plays in the physiology and/or ecology of these organisms is unclear, as is the molecular pathway used to create this compound. Lignin, one of the innovations of land plants, has been found in various algae, raising questions about the evolution of the lignin biosynthetic pathway. To determine the taxonomic distribution of the lignin biosynthesis genes, all publicly available genomes of algae were screened. Genes associated with p-coumaryl alcohol (H-monolignol) biosynthesis were found widely present in algae, and therefore postulated to have evolved long before the transition of photosynthetic eukaryotes to land. The original function of this lignin precursor is therefore unlikely to have been related to water transport. Lignin intermediates are shown to have an antimicrobial action against common marine bacteria, suggesting an early role in the biological defence of some unicellular and multicellular algae. Roseobacticides production by the roseobacter Phaeobacter gallaeciensis was stimulated after addition of a lignin intermediate, pCA. However, pCA depressed the levels of the antimicrobial tropodithietic acid (TDA) that is normally produced by the bacterium, thereby providing less protection to the alga and P. gallaeciensis itself against other bacteria. P. gallaeciensis accelerates senescence and selectively kills one cell type from E. huxleyi, the coccolith producer, while leaving the bald strain alive. Indole-3-acetic acid (IAA) is an important auxin influencing plant development, but the production in algae has been contentious. Screening the tryptophan dependent pathway revealed that the biosynthetic potential for IAA is present in various algal groups, especially in E. huxleyi. Addition of L-tryptophan to E. huxleyi stimulated IAA production, but only in the coccolith-bearing strain. Conversely, addition of exogenous IAA only elicited a physiological response in the bald cell type. A roseobacter Ruegeria sp. R11, previously shown to produce IAA, co-cultured with L-tryptophan and both cell types of E. huxleyi produced less IAA than the axenic coccolith cell type culture similarly induced. This suggests that IAA plays a novel role signalling between different E. huxleyi cell types, rather than between a bacteria and its algal host. In order to determine a possible commercial application of these findings, the growth and lipid yield of these bacteria and bioactives were measured against E. huxleyi, another haptophyte Isochrysis sp., as well as the chlorophyte Dunaliella tertiolecta. Only R11 showed early promise in stimulating the lipid content of the green alga, leading to potential industrial applications. Despite their small size, unicellular organisms such as the microalga E. huxleyi are capable of a complex set chemical interactions, both interspecies (e.g. E. huxleyi – roseobacter) and even intraspecies (e.g. E. huxleyi coccolith bearing – bald cell types). These interactions and the bioactives that mediate them could be important drivers in shaping the ecology, life history, and bloom-bust interaction of this microalga in the marine environment.

• Subjects / Keywords

  • Roseobacter
  • Algae
  • Phytohormone
  • IAA
  • Emiliania huxleyi
  • Lignin

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3QN5ZS35

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.