High resolution spatial mapping of brominated pyrrole-2-aminoimidazole alkaloids distributions in the marine sponge Stylissa flabellata via MALDI-mass spectrometry imaging

A number of pharmacologically active brominated pyrrole-2-aminoimidazole (B-P-2-AI) alkaloids have been isolated from several families of marine sponges, including those belonging to the genus Stylissa. In the present study, MALDI mass spectrometry imaging (MALDI-imaging) was applied to determine the spatial distribution of B-P-2-AIs within 20 mm cross sections of S. flabellata. A number of previously characterised B-P-2-AIs were readily identified by MALDI-imaging and confirmed by MS-MS and NMR profiling. Unknown B-P-2-AIs were also observed. Discrete microchemical environments were revealed for several B-P-2-AIs including dibromophakellin which was localised within the external pinacoderm and internal network of choanoderm chambers. Additionally, dibromopalau'amine and konbu'acidin B were also found to be confined to the choanoderm, while sceptrin was found to be highly abundant within the mesohyl. Further brominated compounds of unknown structure were also observed to have distinct localisation in both choanoderm chambers and the pinacoderm. These findings provide insights into the chemical ecology of S. flabellata, as most B-P-2-AIs were found on highly exposed surfaces, where they may act to prevent pathogens, predation and/or biofouling. Moreover this study demonstrates the power of MALDI-imaging to visualise the location of a range of metabolites in situ and to characterise compounds by MS-MS directly from intact specimens without the need for extraction. These methodologies facilitate selective targeting of micro-regions of sponge to screen for symbiotic microbial candidates or genes that may be involved in the production of the correlated compounds, and may represent a change in paradigm for natural product drug development.