Presence and regulation of alkaline phosphatase activity in eukaryotic phytoplankton from the coastal ocean: Implications for dissolved organic phosphorus remineralization

The biologically important constituents of the dissolved organic phosphorus (DOP) pool, their bioavailability, and their cycling in coastal systems are still poorly understood. Here we use the enzyme alkaline phosphatase as a metric of DOP bioavailability and track the activity of this enzyme in a coastal system. We observed alkaline phosphatase activity (APA) in the .0.2-mm size fraction of all surface samples tested during an Oregon coast cruise in August 2001. Although there was not a significant trend between APA and phosphate concentration in the data set as a whole, chlorophyll a–normalized APA was elevated at the station with the lowest dissolved inorganic phosphate (DIP) concentration. Activity was also elevated in nutrient-addition experiments in which nitrate amendments were used to force community drawdown of DIP. These data are consistent with phosphate regulation of APA. A cell-specific APA assay revealed that the percentage of diatoms with APA mimicked the trend in the hydrolytic rate, but such a trend was not observed for the dinoflagellates. Further, the percentage of dinoflagellate cells with APA was routinely higher than the percentage of diatom cells with activity. In nutrientaddition experiments designed to evaluate the regulation of APA, diatom taxa expressed APA less frequently than dinoflagellates, but they displayed a tighter regulation of the activity by DIP than dinoflagellates. The variability observed in the presence and regulation of APA in these eukaryotic phytoplankton indicates that DOP bioavailability is a potential driver of phytoplankton nutrition and species composition in the coastal ocean. Of the nutrient reservoirs in the ocean, the dissolved organic nutrient pools are increasingly recognized as important, as numerous studies have demonstrated the ability of phytoplankton to use dissolved organic phosphorus (DOP) and nitrogen (DON) as sources of phosphorus and nitrogen (e.g., DOP: Cembella et al. 1984a,b; DON: Antia et al. 1991). Not only are DOP and DON often available to phytoplankton, but their concentrations can exceed those of dissolved inorganic phosphate (DIP) and nitrogen (DIN) in some instances, particularly in the photic zone (Monaghan and Ruttenberg 1999; Karl and Björkman 2002; Ruttenberg and Dyhrman 2005). The influence of nutrient flux on primary production in the coastal ocean is a topic that is currently under intense scrutiny, as the coastal margins are recognized as critical sources of carbon export (Hales et al. 2005b). Yet the extent to which organic nutrients such as DOP are bioavailable and contribute to primary production in the coastal zone is still poorly understood. DOP bioavailability depends on its composition, the phytoplankton present, their physiology, and microbial regeneration processes, among a suite of variables. The molecular-level composition of DOP in seawater is poorly characterized, although recent studies on the high-molecular-weight DOP pool indicate a predominance of phosphonate (C-P) and phosphomonoester (C-O-P) bond classes (Clark et al. 1998; Kolowith et al. 2001). Of these two bond classes, the phosphomonoesters are typically considered more labile and available to primary producers, although recent genomic evidence indicates that phosphonates may be available to Synechococcus (Palenik et al. 2003). Phytoplankton employ various enzyme systems to access DOP compounds through hydrolytic cleavage of phosphate from DOP; most DOP compounds cannot be assimilated directly (Cembella et al. 1984a; Chróst 1991). Of the suite of enzymes that may be involved in the breakdown of DOP in seawater (e.g., diesterase, phytase, C-P lyase, 59 nucleotidase, and alkaline phosphatase), alkaline phosphatase is perhaps the best studied. Alkaline phosphatase can hydrolyze phosphomonoesters into bioavailable phosphate at alkaline pH (Kuenzler and Perras 1965). The enzyme is commonly regulated by phosphate supply, such that activity is increased when DIP drops below critical threshold levels (Cembella et al. 1984a,b; Jansson et al. 1988). Because of this property, alkaline phosphatase activity (APA) has been used to invoke P stress, or P limitation, in both marine and freshwater systems (Dyhrman and Palenik 1999; Beardall et al. 2001; Dyhrman et al. 1 Corresponding author ([email protected]). Acknowledgments We thank Sheean Haley, Tori Ziemann, Ellen Roosen, Mary Derr, and Christie Haupert for valuable technical assistance with sample collection and processing and also Pat Wheeler for data on chlorophyll a concentrations. Our thanks go to the Captain and crew of the RV Thompson and the PIs of the COAST Program. Funds for this work were provided by the National Science Foundation (NSF) Chemical Oceanography Program (OCE 0119134). Limnol. Oceanogr., 51(3), 2006, 1381–1390 E 2006, by the American Society of Limnology and Oceanography, Inc.