Towards the development of a new generation of whole-cell bioreporters to sense iron bioavailability in oceanic systems-learning from the case of Synechococcus sp. PCC7002 iron bioreporter.

Whole-cell bioreporters are genetically modified micro-organisms designed to sense bioavailable forms of nutrients or toxic compounds in aquatic systems. As they represent the most promising cost-efficient tools available for such purpose, engineering and use of bioreporters is rapidly growing in association with wide applicability. Bioreporters are urgently needed to determine phytoplankton iron (Fe) limitation, which has been reported in up to 30% of the ocean, with consequences affecting Earth's global carbon cycle and climate. This study presents a critical evaluation and optimization of the only Cyanobacteria bioreporter available to sense Fe limitation in marine systems (Synechococcus sp. PCC7002). The nonmonotonic biphasic dose-response curve between the bioreporters' signal and Fe bioavailability impairs an appropriate data interpretation, highlighting the need for new carefully designed bioreporters. Here, limitations under low Fe concentrations were related to cellular energy stress, nonlinear expression of the targeted promoter and siderophore expression. Furthermore, we provide critical standard criteria for the development of new Fe bioreporters. Finally, based on gene expression data under a range of marine Fe concentrations, we propose novel sensor genes for the development of new Cyanobacteria Fe bioreporters for distinct marine regions.

Bioavailability of trace metals to aquatic microorganisms: importance of chemical, biological and physical processes on biouptake.

An important challenge in environmental biogeochemistry is the determination of the bioavailability of toxic and essential trace compounds in natural media. For trace metals, it is now clear that chemical speciation must be taken into account when predicting bioavailability. Over the past 20 years, equilibrium models (free ion activity model (FIAM), biotic ligand model (BLM)) have been increasingly developed to describe metal bioavailability in environmental systems, despite the fact that environmental systems are always dynamic and rarely at equilibrium. In these simple (relatively successful) models, any reduction in the available, reactive species of the metal due to competition, complexation or other reactions will reduce metal bioaccumulation and thus biological effects. Recently, it has become clear that biological, physical and chemical reactions occurring in the immediate proximity of the biological surface also play an important role in controlling trace metal bioavailability through shifts in the limiting biouptake fluxes. Indeed, for microorganisms, examples of biological (transport across membrane), chemical (dissociation kinetics of metal complexes) and physical (diffusion) limitation can be demonstrated. Furthermore, the organism can employ a number of biological internalization strategies to get around limitations that are imposed on it by the physicochemistry of the medium. The use of a single transport site by several metals or the use of several transport sites by a single metal further complicates the prediction of uptake or effects using the simple chemical models. Finally, once inside the microorganism the cell is able to employ a large number of strategies including complexation, compartmentalization, efflux or the production of extracellular ligands to minimize or optimize the reactivity of the metal. The prediction of trace metal bioavailability will thus require multidisciplinary advances in our understanding of the reactions occurring at and near the biological interface. By taking into account medium constraints and biological adaptability, future bioavailability modeling will certainly become more robust.