Project: Phosphorus Bioavailability and its Effect on the Role of Trichodesmium in Elemental Cycling

The vast oligotrophic gyres of the world's ocean encompass roughly 60% of the global marine environment. Once thought to be biological deserts, recent research has determined that these regions may account for up to half of the total oceanic organic carbon export. In a society faced with the task of characterizing and predicting the behavior of our ecosystem under the stress of a changing environment, a thorough understanding of these vast marine biomes can move us toward a quantitative representation of the marine ecosystem that can adapt to environmental change. In this respect, the continuous observation and study of the North Pacific subtropical gyre (NPSG) over the past 15 years by the Hawaii Ocean time-series (HOT) program has provided an extensive record of oceanic biogeochemical dynamics. The annual cycle of this system is dominated by tight coupling between the processes of photosynthesis and respiration such that the majority of biologically produced carbon is recycled to the system. In contrast, net export of carbon in the NPSG occurs primarily during summer periods as a result of regular blooms of large, buoyant N2-fixing photoautotrophs. A fundamental trait of these bloom events is the observation of elevated dissolved and particulate N:P and C:P ratios, indicating that the biological system is shifted to a more intensely phosphorus (P) limited state during bloom events. While the occurrence of pulsed export events is well documented in this system, the physiological mechanisms driving the companion stoichiometric diversions remain poorly understood.

In this proposal, the investigators have identified three ecologically relevant physiological adaptations, which may quantitatively explain the ability of a key bloom-forming organism, Trichodesmium, to increase in biomass and abundance, alter stoichiometric ratios of dissolved and particulate pools and thus regulate the flow of elements and the magnitude of export in an otherwise nutrient starved marine environment. These adaptations are: 1) utilization of dissolved organic pools, 2) extreme variability of internal P quotas and 3) buoyancy control. With these physiological adaptations in mind, the objectives of this research are as follows: 1) To measure uptake and regeneration rates of soluble reactive P (SRP) and dissolved organic P (DOP) in natural Trichodesmium populations. 2) To obtain robust estimates of the plasticity of the relative cellular content and compartmentalization of P under different environmental conditions and to characterize how changes in P quotas affect organic production of particulate and dissolved organic carbon and nitrogen by Trichodesmium spp. 3) To test the hypothesis that buoyancy-mediated vertical migration of Trichodesmium colonies facilitates mining of the phosphocline and injection of DIP into the euphotic zone. 4) To utilize the results derived from the above research activities to assess and model the role of Trichodesmium in the flux of elements (C, N, and P) and regulation of pelagic ecosystem structure under different climate scenarios (i.e. under increased or decreased periods of water column stratification). Fulfillment of these objectives will be achieved via the integration of field, and laboratory research components.