Continuous underway measurement of microbial ectoenzyme activities in aquatic ecosystems

Microbial ectoenzyme activities in aquatic environments are important agents of polymer hydrolysis and indicators of the state of microbial carbon, nitrogen, or phosphorus nutrition. However, like most other biochemical and molecular measurements, ectoenzyme activities have been Limited to discrete water samples. We have developed a continuous underway method for measuring microbial enzyme activities using high-sensitivity fluorescent substrates. The system we developed consisted of a peristaltic proportioning pump, a temperature-controlled water bath, and a spectrofluorometer interfaced to a portable computer which controlled the fluorometer and logged the data. This method has been applied to alkaline phosphatase and to leucine aminopeptidase measurements in the surface waters of the Mississippi River plume and the Louisiana shelf, and alkaline phosphatase measurements in the surface waters of a Texas lake. This method will enable us to map the surface distributions of microbial enzyme activities on scales comparable to temperature, salinity, in vivo fluorescence, and other parameters which can be continuously mapped from a research ship while underway.