The organic matter present in the sediment of the hypereutrophic alkaline soda lagoon of Santa Olalla, Doñana National Park, SW Spain, is mainly composed of extractable and insoluble lipids. This sedimentary organic matter, derived from phytoplankton, has suffered an intense microbial reworking, and the insoluble, highly resistant biopolymers occurring in the cell envelope of cyanobacteria and in the outer cell wall layer of algae have accumulated. The presence of resistant biopolymers is evidenced by the characteristic n- α, ω alkadiene, n-alk-1-ene and n-alkane triplets originated upon analytical pyrolysis of the sediment, particulate matter and extracted humic acid. A similar pattern was also observed in the pyrolysate of the most resistant (non-hydrolysable, non-oxidable) part of soil humic acids, which probably represents the polymethylene moiety of biopolymers from terrestrial plants.