The Jekyll-and-Hyde chemistry of Phaeobacter gallaeciensis

Abstract

Emiliania huxleyi, an environmentally important marine microalga, has a bloom-and-bust lifestyle in which massive algal blooms appear and fade. Phaeobacter gallaeciensis belongs to the roseobacter clade of α-Proteobacteria, the populations of which wax and wane with that of E. huxleyi. Roseobacter are thought to promote algal growth by biosynthesizing and secreting antibiotics and growth stimulants (auxins). Here we show that P. gallaeciensis switches its secreted small molecule metabolism to the production of potent and selective algaecides, the roseobacticides, in response to p-coumaric acid, an algal lignin breakdown product that is symptomatic of aging algae. This switch converts P. gallaeciensis into an opportunistic pathogen of its algal host.

Figure 1

Effect of pCA on secondary metabolites produced by P. gallaeciensis. (a) Structures of phenylacetic acid (1), TDA (2), its valence tautomer (3), pCA (4) and pCA-HSL (5). (b) HPLC-MS profile of the EtOAc extract of P. gallaeciensis BS107 cultures 72 h after inoculation in the absence (black trace) and presence (red trace) of 1 mM pCA. The starred peaks contain the 430 nm absorption feature typical for roseobacticides; their structures have yet to be determined. Inset, UV-visible absorbance spectrum of 6. (c) Crystal structure of 6 solved to 0.82 Å resolution. (d) Structures of 6 and 7 and their numbering schemes. (e) Dose response of 6 as a function of [pCA]. Each point is the average of two independent measurements. The red line is a dose-response fit and yields an EC₅₀ of 0.79 ± 0.03 mM (see Supplementary Methods for details).

Figure 2

A proposed biosynthesis for roseobacticides. Addition of the enolate of phenylacetyl CoA to the C2 of tropone gives 8, which after lactonization and release of CoA yields the [8+2] annulation product of the starting substrates (9). Oxidation, followed by 1,8-addition of MeSH to 10 and another oxidation gives 7. Phenylacetyl CoA is derived from 1, which along with tropone and MeSH have been shown to be produced by P. gallaeciensis BS107. 6 may be obtained from a similar reaction scheme starting with the addition of p-hydroxyphenylacetyl CoA to tropone.

Figure 3

Activity of 6 against E. huxleyi and C. muelleri. (a–c) Light micrographs of E. huxleyi after exposure to methanol (solvent control) for 24 hr (a) or to 3.5 µM 6 for 12 hr (b) or 24 hr (c). (d–e) Light micrographs of C. muelleri after 24 h exposure to methanol (solvent control, d) or to 3.5 µM 6 (e). Panels a–e contain live samples viewed by light microscopy and are typical of these experiments; scale bar = 2 µm.

Figure 4

Proposed working model for the interaction between E. huxleyi and P. gallaeciensis. In the mutualistic phase (green), E. huxleyi is healthy and provides DMSP and a biofilm surface to P. gallaeciensis, which in turn produces algal growth promoters and antibiotics (1–3) to protect E. huxleyi. The interaction turns pathogenic (red) when E. huxleyi senesces and releases the algal breakdown product 4, which induces P. gallaeciensis to produce the potent algaecides 6 and 7.