Cross‐basin comparison of phosphorus stress and nitrogen fixation in Trichodesmium

Abstract

We investigated the phosphorus (P) status and N2 fixation rates of Trichodesmium populations from the North Pacific, western South Pacific, and western North Atlantic. Colonies of Trichodesmium were collected and analyzed for endogenous alkaline phosphatase (AP) activity using enzyme‐labeled fluorescence (ELF) and for nitrogenase activity using acetylene reduction. AP hydrolyzes dissolved inorganic phosphate (DIP) from dissolved organic phosphorus and is active in Trichodesmium colonies experiencing P stress. Across multiple stations in the subtropical North and South Pacific, there was low to moderate ELF labeling in Trichodesmium, although labeling was present in other taxa. In contrast, Trichodesmium ELF labeling in the North Atlantic ranged from low to high. Low ELF labeling corresponded with high DIP concentrations while high ELF labeling occurred only at North Atlantic stations with DIP concentrations #40 nmol L21, indicating that Trichodesmium was not experiencing dramatic P stress in the Pacific Ocean while P stress was evident in the western North Atlantic. However, nitrogenase activity was significantly higher in the P‐stressed western North Atlantic than in the Pacific Ocean (0.40‐1.30 compared to 0.01‐0.46 nmol C2H4 h−1 colony−1). These data underscore the differential basinlevel importance of P availability to Trichodesmium and suggest that factors other than P are constraining their N2 fixation rates in the Pacific.