The micro organism communities which colonized on fresh blade,decomposing blade and detritus of kelp Laminaria japonica were analyzed using fatty acid biomarkers.Results showed that the total crude lipid contents of decomposing blade and kelp detritus were significantly higher than those in fresh kelp blade,whereas monounsaturated and polyunsaturated fatty acids contents were less than those in fresh blade.Bacterial biomarkers 18∶1(n-7)and odd/branched fatty acids were more abundant in decomposing tissue and detritus than fresh kelp,indicating the colonization of bacteria in the kelp decomposing process.The diatom biomarker 16∶1(n-7)/16∶0 ratio exhibited the highest in kelp detritus,and it was also higher in decomposing tissue than in fresh blade,indicating the adherence of large amount of diatoms.No protozoa fatty acid biomarker(DHA)was detected in fresh kelp blade; however,it existed in decomposing tissue and detritus at a relative concentration of 0.65% and 1.57% respectively,indicating the colonization of protozoa.Due to the colonization of complex micro organism community,decomposing kelp blade and kelp detritus showed different carbon and nitrogen components.Fresh kelp blade had a high C/N ratio of 17.32,but in decomposing tissue and detritus,the organic nitrogen content increased remarkably,which resulted in the decline of the C/N ratio to 6.61 and 6.48.Results confirmed that kelp detritus is a mixture of decomposing kelp debris,diatoms,bacteria and protozoa.Through the colonization of different kinds of micro organisms and biodegradation,kelp debris became organic nitrogen abundant and more nutritious,hence much more suitable as the food source of other consumers in a kelp-based polyculture area.