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

Authors


Osnat Gillor, Zuckerberg Institute for Water Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel.
E-mail: gilloro@bgu.ac.il

Summary

1. 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.

2. 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.

3. 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.

4. 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.

Ancillary