Occurrence of unsaturated C25 highly branched isoprenoids (HBIs) in a freshwater wetland

Eight C25 highly branched isoprenoid (HBI) alkenes were detected in the freshwater wetland of the Florida Everglades and tentatively assigned as a C25 diene, three C25 trienes, two C25 tetraenes and two C25 pentaenes based on their mass spectra, retention index and literature reports. One diene and one triene were observed more frequently than the others. The HBIs were present in varying amount of up to 1000, 19,000, 780 and 150 ng/g dry weight in the periphyton, floc, surface soils and deeper soils (ca. 1900 AD), respectively. Compound specific carbon isotope analysis of the two most dominant HBIs (one diene and one triene) showed them to be highly depleted in 13C (δ13C −40.0‰ to −38.5‰) in freshwater floc, suggesting recycled CO2 produced from the decomposition of organic matter as an important C source in the biosynthesis of these compounds. HBIs were present across this freshwater wetland in greater diversity and abundance at locations with higher surface water N concentration and longer hydroperiod (inundation). Historical variation in the occurrence of the HBIs in soil cores from multiple slough and ridge environments was assessed. Their increased abundance after ca. 1960 AD in almost all cores suggests proliferation of their precursor biota (freshwater diatoms) over the past five decades.