Vertical distribution of enzymatically labile dissolved organic phosphorus in the phosphate-depleted western North Pacific
Fuminori Hashihama, Department of Ocean Sciences, Tokyo University of Marine Science and Technology
Recent nutrient studies using highly sensitive methods have revealed that unique domains with extremely low concentrations of phosphate (<10 nM) exist in the vast oligotrophic open ocean (Wu et al. 2000; Hashihama et al. 2009; Moore et al. 2009). In such phosphate-depleted domains, the growth of microorganisms is potentially limited by phosphorus (Sañudo-Wilhelmy et al. 2001). Extracellular alkaline phosphatase (AP) is a significant component of phytoplankton and bacteria, and plays a prominent role in the hydrolytic cleavage of dissolved organic phosphorus (DOP) (Hoppe 2003). AP-hydrolyzable labile DOP (LDOP) may be important for sustaining community productivity in the phosphate-depleted domains. However, its contents and dynamics in the oligotrophic ocean are poorly understood, because LDOP concentrations are consistently below the detection limit of conventional methods (Moutin et al. 2008).
Typical vertical profiles of LDOP, phosphate,
and chlorophyll florescence in the upper 200 m
of water column in the western subtropical
North Pacific. Data were collected at a station of
30°N, 137°E during boreal summer
In the NEOPS project, nanomolar concentrations of LDOP in the oligotrophic Pacific Ocean are being measured using a new sensitive method. This method was established by combining an enzymatic procedure and a nanomolar phosphate analytical system consisting of a gas-segmented continuous flow analyzer with a liquid waveguide capillary cell (Hashihama et al. 2013). Field application of this method revealed that the vertical distribution of LDOP was similar to that of phosphate in the upper 200 m of the water column in the western subtropical North Pacific. Both LDOP and phosphate concentrations tended to be extremely low (<20 nM) between the surface and the deep chlorophyll maximum layer (DCML) and to increase below the DCML (see Figure). High levels of LDOP and phosphate depletion above the DCML suggest that LDOP is actively hydrolyzed under phosphate-depleted conditions.
To further examine basin-scale dynamics of LDOP and associated parameters, we are now analyzing the samples collected by the NEOPS cruises, which were conducted in the extensive regions of the Pacific Ocean.
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