Elevated alkaline phosphatase activity in a phosphate‐replete environment: Influence of sinking particles

Alkaline phosphatase activity (APA) is traditionally a proxy for phosphate (DIP)‐limitation because it is induced by DIP‐limited microbes to access the labile ester fraction of the organic phosphorus (OP) pool. Here, we present multi‐year summertime depth distributions of APA and enzyme kinetics in the DIP‐replete Celtic Sea. Our findings support the cumulating body of evidence that APA has a potentially widespread role in OP remineralization through the water column. APA and V max were positively correlated with depth and DIP, with total APA being threefold higher below (0.93 ± 0.32 nM P h −1 ) compared to above the thermocline (0.30 ± 0.24 nM P h −1 , p < 0.001). Separation of particles by sinking speed demonstrated that APA was eightfold higher on fast sinking ( F fast ) particles compared to slow sinking particles ( F slow ; p < 0.05). When normalized to particulate organic carbon (POC) and bacterial production (BP), APA POC and APA BP associated with F fast (0.76 ± 0.10 nmol P μ mol C −1 h −1 , 21.13 ± 2.2 nmol P nmol C −1 , respectively) were fourfold and 25‐fold higher compared to the combined APA associated with dissolved plus suspended ( F susp ) and F slow fractions (0.19 ± 0.06 nmol P μ mol C −1 h −1 and 0.84 ± 0.23 nmol P nmol C −1 , respectively). We postulate that this may reflect enhanced ectoenzyme activity associated with bacteria colonizing particle surfaces and/or release by zooplankton via faecal pellet excretion. Knowledge of the disparity between APA and BP associated with particle and dissolved phases is required to accurately define the O 2 : P ratio of regenerated P derived from sinking particles as a result of AP‐facilitated remineralization.