• bacterial galls;
  • gas chromatography–selective ion-monitoring mass spectrometry;
  • indole-3-acetic acid;
  • Prionitis lanceolata;
  • red algal hormones

Indole-3-acetic acid (IAA), a plant hormone necessary for terrestrial plant growth and development, was detected and quantified in the marine red alga Prionitis lanceolata Harvey (Halymeniaceae, Gigartinales, Rhodophyta) using gas chromatography–selective ion-monitoring mass spectrometry (GC-SIM-MS). This allowed comparison of free IAA levels between the algal thallus and eubacterially induced galls on this alga characterized by abnormal algal growth and cell division and extensive, intercellular microbial proliferation. The levels of free IAA in the P. lanceolata thallus averaged 2.5 (±1.1) ng·g−1 fresh wt. Free IAA levels in galls were more variable, ranging from ca. 4 to 39 (8.3 ± 10.9) ng·g−1 fresh wt, but were significantly higher overall (P= 0.0022). The identity of the IAA in this marine florideophycean alga was confirmed by full scan GC-MS analysis of both galls and thalli. The levels of free IAA in P. lanceolata were two to three orders of magnitude higher than those observed previously in the Rhodophyta. The origin of elevated IAA levels in P. lanceolata galls is unknown because it is possible that this compound is produced by either the gall-inducing bacterial symbiont or the host alga.