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. 2018 Dec 4;16(12):486.
doi: 10.3390/md16120486.

Algal Oxylipins Mediate the Resistance of Diatoms against Algicidal Bacteria

Nils Meyer  1 , Johanna Rettner  2 , Markus Werner  3 , Oliver Werz  4 , Georg Pohnert  5
Affiliations

Algal Oxylipins Mediate the Resistance of Diatoms against Algicidal Bacteria

Nils Meyer et al. Mar Drugs. .

Abstract

Algicidal bacteria can lyse microalgal blooms and trigger shifts within plankton communities. Resistant algal species can escape lysis, and have the opportunity to dominate the phytoplankton after a bacterial infection. Despite their important function in ecosystem regulation, little is known about mechanisms of resistance. Here, we show that the diatom Chaetoceros didymus releases eicosanoid oxylipins into the medium, and that the lytic algicidal bacterium, Kordia algicida, induces the production of several wound-activated oxylipins in this resistant diatom. Neither releases nor an induction occurs in the susceptible diatom Skeletonema costatum that is lysed by the bacterium within a few days. Among the upregulated oxylipins, hydroxylated eicosapentaenoic acids (HEPEs) dominate. However, also, resolvins, known lipid mediators in mammals, increase upon exposure of the algae to the algicidal bacteria. The prevailing hydroxylated fatty acid, 15-HEPE, significantly inhibits growth of K. algicida at a concentration of approximately 1 µM. The oxylipin production may represent an independent line of defense of the resistant alga, acting in addition to the previously reported upregulation of proteases.

Keywords: HEPE hydroxylated eicosapentaenoic acid; HETE hydroxylated eicosatetraenoic acid; algicidal bacteria; diatoms; induced chemical defense; oxylipins; plankton; resolvins.

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Conflict of interest statement

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
Growth of Chaetoceros didymus and Skeletonema costatum (mean ± SD, n = 3) monitored as in vivo chlorophyll a fluorescence. Values are given in relative fluorescence units (RFU), over time, for control cultures and cultures treated with Kordia algicida.
Figure 2
Figure 2
Extracellular concentrations of oxylipins and free eicosapentaenoic acid (EPA) in C. didymus culture medium (mean ± SD, n = 3). Panels depict EPA (A), hydroxylated eicosapentaenoic acids (HEPEs), namely 15-HEPE (B) and 18-, 12-, 11-, and 5-HEPE (C), as well as 17-hydroxydocosahexanoic acid (17-HDHA) and resolvin (Rv)E1 (D). Statistically significant differences (unpaired t-test) at day 4 between control cultures and cultures treated with Kordia algicida are marked with * (p < 0.05).
Figure 3
Figure 3
Oxylipins detected 10 min after cell disruption of C. didymus. Depicted is the cellular production (mean ± SD, n = 7) of 5 different hydroxylated eicosapentaenoic acids (HEPEs) (A), as well as 17-hydroxydocosahexaenoic acid (HDHA) and resolvin (Rv)E3 (B) after wounding of C. didymus at day 4. Statistically significant differences (unpaired t-test) between control cultures and cultures treated with K. algicida are marked with ** (p < 0.01) and *** (p < 0.001).
Figure 4
Figure 4
Concentration–response curve for 15-HEPE inhibition of Kordia algicida growth (mean ± SD, n = 5). Growth was monitored after 6 h in the presence of different concentrations of 15-hydroxylated eicosapentaenoic acid (HEPE) and normalized to the control at 0 µg/mL (no added HEPE). Statistically significant differences (unpaired t-test) between control cultures and cultures treated with K. algicida are marked with *** (p < 0.001).
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