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.

Exponentially growing algal cultures were incubated with a suspension of K. algicida at a final OD550 of 0.01. These conditions were sufficient to lyse S. costatum within three days, while C. didymus was resistant and grew with similar kinetics as the control.

This pattern is consistent with previous investigations addressing the infection of diatoms with K. algicidal. By incubating with a bacterial density of OD 0.01, we achieved a sufficiently slow lysis of S. costatum, allowing for examination of potential induced responses.

Therefore, we adapted a lipidomics approach, initially developed to survey C18 to C22 fatty acidderived oxylipins from diatoms. After centrifugation of the cultures (1700 g, 15 min, 4 °C), a careful filtration (pore size 1.2 µm) of the supernatant was conducted on glass fiber filters to avoid cell disruption.

Extraction of the resulting filtrate with reversed phase C18 solid phase extraction cartridges gave samples for liquid chromatography/mass spectrometry (LC/MS). Quantifiable amounts of oxylipins in the pg mL−1 to ng mL−1 range were detected.

Given the fact that algae build up elevated concentrations of primary and secondary metabolites in the phycosphere (a diffusionlimited zone around the cells), the local concentrations surrounding the cells might be substantially higher. In the medium of the resistant C. didymus, a clear accumulation of extracellular oxylipins was observed, with a concomitant increase of free EPA