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Changes in lipid content and fatty acid (FA) composition in decomposing mangrove leaves of Bruguiera gymnorrhiza (L.) Lamk and Kandelia candel (L.) were investigated in a subtropical mangrove forest on Okinawa Island, Japan (26.5°N, 128°E) by field experiments for 18 wk (July to November 2000), using yellow senescent leaves, and compared with FAs in the green leaves and mangrove sediments. We tested the hypothesis that changes in FA composition during decomposition can indicate the state of leaf decay and periods of high and low microbial activity, and that bacteria may rapidly degrade polyunsaturated fatty acids (PUFAs). During decay, FA composition in the yellow leaves changed in 2 wk from predominantly saturated FAs to monounsaturated FAs, and to the more branched FAs typical of bacteria, and lipid and N increased due to microbial colonization. However, the microbial decomposition of leaves did not alter the concentrations of long-chain fatty acids (LCFAs), suggesting that these vascular plant-markers remain unchanged in mangrove detritus and surface sediments for more than 4 mo. Furthermore, bacteria did not degrade PUFAs as we had hypothesized, indicating that during decomposition of mangrove leaves, bacteria tend to conserve PUFAs (as they do nitrogen), thus enriching the detritus with nutrients. Comparison of ω3 and ω6 PUFAs between the 2 species showed that nutritional quality varies greatly with the state of the leaf material, increasing through time in B. gymnorrhiza and decreasing through time in K. candel.