Microbial Extracellular Alkaline Phosphatase Revealed the Linkage Between Carbon and Phosphorus Cycles in the Deepest Ocean

We present the first dataset of microbial extracellular alkaline phosphatase activities (APA) and community compositions measured in the entire water column of the Challenger Deep (CD), Mariana Trench. We show that there existed two distinct P-APA regimes in the CD water column, reflecting different nutrient use strategies of the microbes; that in surface water (50 m) where microorganisms were P-limited (110 nmol L-1) and expressed extremely high APA (average 23.8 nmol h-1) for generating inorganic P, and that in the deep waters (1,000–10,918 m) which were P-replete (average 2.2 mmol L-1), yet microorganisms were carbon-starved and also expressed high levels of APA (average 17.0 nmol h-1). We suggest that microorganisms in the deep waters likely utilized a piggyback strategy to satisfy carbon requirements by expressing elevated APA to decompose dissolved organic phosphorus and concurrently, leading to high concentrations of P in the deep. Our results implicate the intimate connection and coupling between P- and C-cycling mediated by microbes in the hadal biosphere.