A CAUTIONARY NOTE ON MEASURING PROTISTAN BACTERIVORY BY ACID LYSOZYME

Protistan bacterivory is considered important in aquatic ecosystems bu t difficult to measure. Two recently proposed enzymatic assays of prot istan bacterivory rely on lysozyme :hydrolysis of the beta(1-4) glycos idic bond between N-acetylglucosamine and the fluorogenic compound 4-m ethylumbelliferyl. We analyzed protists and metazoans for acid-lysozym e (L-ACID), and found that 5 of 6 protists and 20 of 21 (95%) metazoan genera had L-ACID. Protistan bacterivory estimates based on L-ACID ac tivity may be erroneous if micrometazoans are in analyzed samples, as is likely in benthic and littoral samples and without careful sample p rocessing. In addition, lysozymes (e.g., L-ACID, beta-N-acetylglucosam inidase) hydrolyze both peptidoglycan and chitin, meaning that bacteri vory may not be distinguished from chitin digestion by lysozyme analys es. Therefore, lysozyme-based analyses of bacterivory should be applie d only to samples that do not contain chitinous organisms, and perhaps predators of chitinous organisms. Finally, dissolved organic compound s interfered with fluorometric analysis of enzymatic substrate hydroly sis,: leading to an underestimate of L-ACID activity. Lysozyme-based a nalyses of protistan bacterivory developed for use on pelagic samples will be difficult to apply to some inland waters and benthic or littor al samples without precautions to exclude metazoans, chitinolytic enzy mes in organisms, and dissolved organic compounds.