Measurement of ectoenzyme activities as an indication of inorganic nutrient imbalance in microbial communities

Assessment of nutrient limitation in microbial populations in marine enviro nments is an important topic. Existing methodologies generally demand long incubations in an experimental assay manipulating nutrients, We suggest a n ovel approach using the ratio between 2 ectoenzymatic activities, alkaline phosphatase (APA) and aminopeptidase (AMA), to evaluate nutrient Limitation of microbial communities in situ. APA is used as an indication of P limita tion and AMA of N limitation. The ratio bypasses the use of biomass estimat ions to calculate specific activities, which are error-prone and time-consu ming. Our results from enrichment experiments have shown that the ratio APA :AMA increases in P-deficient treatments, and decreases after inorganic P a ddition. After P-deficient nutrient addition (n treatments) the ratio APA:A MA increased to 90-230%, whereas following N+P balanced additions the ratio decreased to 30-70% of the initial value. These results demonstrate that t his approach is easy, quick and gives a general insight into the nitrogen v ersus phosphorus nutrient balance of the communities without the need for b iomass estimations or measurement of actual nutrient concentrations.