Intense ectoenzyme activities associated with Trichodesmium colonies in the Sargasso Sea

Ectoenzyme activities of alkaline phosphatase (APA) and leucine aminopeptidase (LAP) associated with Trichodesmium, a globally significant dinitrogen (N2) fixer, were measured on cruises to the Bermuda Atlantic Time-Series (BATS) site. Rates associated with Trichodesmium were compared to microbial enzyme activities in natural seawater between August 1992 and November 1997. Colonies of Trichodesmium function as ‘microsites’ for ectoenzyme activity and express high rates of APA (4 to 66 nmol colony−1 h−1) and LAP (53 to 389 nmol colony−1 h−1). For Tricho desmium APA, substrate half-saturation constants (Km) exceeded surrounding seawater APA by a factor of 20 to 40 times. Overall, the ectoenzyme activity measured in puff shaped colonies of Trichodesmium did not differ significantly from the rates associated with the tuft morphology. Ectoenzyme activites measured in the cyanobacterial consortium did not vary as strongly seasonally as was observed in natural seawater. Elemental ratios of Trichodesmium colonies showed a molar C:N ratio around the Redfield stoichiometry (mean = 6, range 4 to 7) while the C:P ratios were much higher (mean = 513, range 163 to 1044). Calculated per volume seawater, measured uptake of phosphate and leucine were 4 to 6 orders of magnitude lower in Trichodesmium (1 to 5 fmol P l−1 h−1 and 0.1 to 3 fmol Leu l−1 h−1) than in microplankton (0.3 to 4 nmol P l−1 h−1 and 0.002 to 0.02 nmol Leu l−1 h−1). At peak abundance, Trichodesmium contributed a major part of total ectoenzyme activity in surface waters (81% of APA, 64% of LAP) suggesting that the diazotrophic colonies are significant sites of net nutrient regeneration. We propose that the high rates of regeneration associated with Trichodesmium colonies may accumulate dissolved nutrients during the course of bottle incubations, which leads to isotope dilution and an underestimation of P and N uptake in radiotracer studies with these cyanobacteria.