Nutrient regeneration mediated by extracellular enzymes in water column and interstitial water through a microcosm experiment

Abstract In coastal lakes the role of microorganisms in driving nutrientsregeneration at different water depths and in sediments is not yet fully understood. The dynamics of microbial (algal and bacterial) abundance and bacterial activities involved in organic matter transformation were measured, together with nutrientconcentrations, through a microcosm experiment set up using the oligotrophic Faro lake as a study model over a total period of 15 days and with a four-day frequency. Water column at different depths (surface, middle and bottom) and interstitial water obtained by sediment centrifugation were used in appropriate ratios (mixed 1:1 with surface waters) to fill 21-Litre plastic aquaria in order to simulate processes occurring in natural conditions. At early experimental period, the sharp decrease of dissolved organic nutrientsand the abundant production of leucine aminopeptidase (LAP) and alkaline phosphatase (AP) in correspondence with high phytoplanktonabundance in bottom and interstitial water reflected the relevance of organic nutrientsfor inorganic nutrientsregeneration and phytoplanktongrowth. Size fractionation of LAP and AP as well as the positive relationship between microbial compartments suggested that bacteria and phytoplanktonworked in close reciprocal synergy, and coupling of nitrogen and phosphorus regeneration, especially in bottom and interstitial waters, was observed. At later experimental period, the change in bacterial community, especially the increase of filamentous shaped cells, together with a simultaneous increase of protozoan abundance indicated that nutrientreplenishment made the microbial loopstructure more competitive. In oligotrophic conditions, such as those in Faro lake, organic nutrientenrichment of bottom and interstitial waters was associated with changes in the bacterial community, with consequent stimulation of extracellular enzymes to support phytoplanktongrowth. Nutrientavailability from microbial regeneration resulted in an increased complexity of the microbial loopstructure, with bacteria and phytoplanktonadopting specific strategies to respond to the changing environment.