New insights on the deep alkaline phosphatase paradox from a site in the Northeastern Pacific Ocean

Abstract

Extracellular hydrolytic enzymes are the main tools for microorganisms to access and degrade organic matter in the ocean. We investigated potential activities of four hydrolytic enzymes (alkaline phosphatase, beta-glucosidase, leucine aminopeptidase, chitinase) in the water column and surficial sediments at a deepwater site in the Northeastern Pacific Ocean (water depth: 2658 m). Our goal was to investigate the potential role of (re-)suspended particles as a source for enzyme activities in subsurface waters. Alkaline phosphatase (AP) dominated hydrolytic activities in subsurface waters and in sediments, reaching up to two orders of magnitude higher rates compared with the other three enzymes. Peak AP activities in the center of the oxygen minimum zone and in bottom waters were decoupled from concentrations of inorganic phosphorous (P_i), a pattern known as the deep AP paradox. A weak correlation between AP and bacterial abundances indicated that a fraction of AP was physically detached from their source cells (i.e., cell-free enzymes) and associated with particles either from surface waters (marine snow) or resuspended from the seafloor. We estimated that cell-free AP activities, which are decoupled from nutritional needs and oxidative processes of their source cells, contribute about 2%–9% of regenerated P_i in the bathypelagic Northeastern Pacific Ocean. (Re-)suspended particles as a source of cell-free AP thus play an important yet understudied role in deep-ocean elemental cycles.