Biomarker evidence for the co-occurrence of three races (A, B and L) of Botryococcus braunii in El Junco Lake, Galápagos

In spite of the fact that individual races of Botryococcus braunii are widely distributed in lakes, reports of the co-existence of different chemical races are rare. Here we report biomarker evidence for the co-occurrence of three different races of B. braunii (A, B and L) in the water column and sediments of El Junco Lake, a freshwater crater lake at 700 m elevation on San Cristóbal Island, Galápagos. We also show the existence of lycopadienes and lycopatrienes in suspended particles and sediments. Biomarker indicators of the three races were: cis and trans C25–C31 n-alkadienes and a C29 triene for race A, a series of C34H58 botryococcenes for race B, and a C40H78 hydrocarbon, trans,trans-lycopadiene accompanied by minor amounts of isomers for race L. Epoxides derived from n-alkadienes and trans,trans-lycopadiene, specific to races A and L respectively, were also detected. Several previously unreported lycopadienes and lycopatrienes were identified in both the water column and sediment, and are believed to represent by-products and intermediates, respectively, of trans,trans-lycopadiene synthesis. Botryococcene and lycopadiene concentrations reached 16.7 and 0.8 mg/g dry sediment in near-surface sediments, and varied over more than three orders of magnitude, while n-alkadienes were present in trace quantities. Pyrolysates from the kerogen fraction of sediment contained lycopadiene and alkadiene related lipids along with chemically resistant polymers of B. braunii cell walls. Apparently n-alkadienes, botryococcenes and lycopadienes and -trienes can survive in oxic sediments for several decades, and the down core variation in these lipids likely reflects changes in lake hydrology, alternately favoring the bloom or near-extinction of B. braunii race B.