Factors affecting alkaline phosphatase production in lake water and relation with phosphorus dynamics

Abstract

　Alkaline phosphatase (AP) in lake water is an important enzyme that hydrolyzes dissolved organic phosphorus. In Lake Suwa (Nagano Prefecture, Japan), the concentration of bioavailable inorganic phosphorus in the water has decreased by 1/7 over the past 40 years, indicating that the utilization of organic phosphorus by AP has become a significant process for organisms. However, few research studies have been conducted on AP and organic phosphorus in Lake Suwa. In this study, to clarify the dynamics of organic phosphorus mediated by AP in Lake Suwa, we fractionated AP and measured AP activity (APA) by AP producers in four water bodies, mainly in Lake Suwa. We also analyzed the relationships between AP activity and phosphorus or carbon concentrations in the photic and aphotic layers.
　In both the photic and aphotic layers, most AP was derived from phytoplankton, suggesting that phytoplankton-derived AP has a large influence on the mineralization of organic phosphorus. From the results of the analysis using the APA/Chlorophyll-a ratio (APA/Chl-a), AP production by phytoplankton might have been stimulated by the particulate phosphorus concentration in the photic layer of the lake. Further, AP from the photic layer settled in the aphotic layer together with phytoplankton. We speculated that AP supplied from the photic layer could also be involved in the mineralization of organic phosphorus in the aphotic layer. On the other hand, bacteria produced AP to obtain not only phosphorus but also carbon, and as a result, might supply inorganic phosphorus to phytoplankton in the photic layer.