Isolation and characterization of halogenated monoterpenes in the investigation of the ecological relationship between Antarctic Plocamium cartilagineum and Paradexamine fissicauda

Plocamium cartilagineum is a widely occurring red algal species which plays a major role in structuring many benthic marine ecosystems found in the shallow waters of Antarctica [1]. This rhodophyte is known to produce many polyhalogenated monoterpenes in various relative abundances which have been linked through metabolomics with site-specificity, presumably driven by ecological interactions [1,2]. The cytotoxic nature of these secondary metabolites is thought to convey ecological relevance as feeding deterrents to sympatric algal consumers [3,4]. One remarkable exception to this trend is seen in the amphipod Paradexamine fissicauda, which has been shown to consume P. cartilagineum at a level unpalatable to many other associated amphipod species [5]. Further investigations have detected halogenated monoterpenes in the metabolome of specimens fed P. cartilagineum, while not in those fed non-chemically defended algae, with the former enjoying a lower probability of being eaten by fish, suggesting P. fissicauda is not only tolerating these chemical defenses, but sequestering halogenated monoterpenes for its own defense [5]. In order to gain a greater understanding of this relationship, numerous halogenated monoterpenes were isolated from P. cartilagineum specimens collected at Palmer Station in Antarctica for subsequent field-based feeding assays and further metabolomics studies. Metabolites which were shown to exist in the greatest abundance within P. cartilagineum extracts were targeted for subsequent HPLC purification and structural elucidation through 1D and 2D NMR as well as GC/MS analysis. During the course of this investigation several previously reported halogenated monoterpenes [6] were isolated and characterized while multiple new structures of the same class were also found.

Acknowledgements: We are grateful for financial support from the NSF (PLR-1341339) and the State of Florida for Center of Excellence funding of CDDI. We thank the field team members of S-022 and the staff of Antarctic Support Contract for logistical support, Edwin Rivera for his assistance in obtaining high quality NMR spectra, Laurent Calcul for his assistance with mass spec and Patrick Walther for his assistance in HPLC purification.

Keywords: Halogenated monoterpenes, Plocamium cartilagineum, Paradexamine fissicauda, secondary metabolite, chemical defense, chemical sequestration.

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