Phosphorus and nitrogen in a monomictic freshwater lake: Employing cyanobacterial bioreporters to gain new insights into nutrient bioavailability

It is an uncontested paradigm that an adequate supply of the macronutrients nitrogen (N) and phosphorus (P) is critical for maintaining phytoplankton primary production in aquatic ecosystems; it has also been suggested that there is an optimal total N: total P ratio for this globally significant process. This ratio, normally assessed by chemical determination of the major dissolved N and P species, poses a dilemma: do chemical measurements actually reflect the bioavailable fraction of these nutrient pools? Accurate determination of the various N and P species and their fluxes into phytoplankton cells is notoriously difficult. To provide a possible solution to this difficulty, we engineered strains of the cyanobacterium Synechococcus sp. strain PCC 7942 that 'report' on N and P bioavailability via a bioluminescent signal. These strains were used to quantify, for the first time, bioavailable concentrations of these essential macronutrients in a freshwater lake. Only a small fraction (0.01-1%) of the chemically determined P may actually be bioavailable to this unicellular cyanobacterium and, by inference, to the phytoplankton community in general. In contrast, bioavailable N comprises most of the dissolved N pool. Consequently, bioavailable N: P ratios based on these assays are higher then those based on chemical determinations, indicating that P limitation in Lake Kinneret is more extensive then previously thought.