Rates of nitrogen fixation by the colonial cyanobacteria, Trichodesmium spp., long considered the dominant diazotroph in the ocean, can be limited by iron and/or phosphorus. Although models have defined regions within the oceans where diazotrophs, especially Trichodesmium, are predicted to be limited by Fe, light, P or none of these factors, the boundaries of these regions are uncertain. Empirical validation of the models requires field surveys that are difficult to conduct without a rapid diagnostic for Fe or P limitation. Two PIs from the University of Southern California have identified several diagnostic genes for Fe and P limitation that are useful for quickly assessing the Fe and P status with natural Trichodesmium populations in space and time. These genes can also provide insights into the mechanism of Fe uptake. For instance, one of the Fe stress-induced genes is feoB which is predicted to be required for efficient uptake of Fe(II). Although common in facultatively anaerobic bacteria, its presence in open-ocean cyanobacteria is limited to Trichodesmium and another diazotroph, Crocosphaera. The PIs hypothesize that because diazotrophs have a higher Fe requirement due to N2 fixation than other cyanobacteria, they may utilize feoB, perhaps in conjunction with an exogenous or endogenous reductase mechanism, to access additional reservoirs of Fe within the dissolved Fe pool in seawater.
The PIs will study the expression of Fe and P diagnostic probes within the western Pacific warm pool to determine if Trichodesmium is Fe or P limited in this region. On an already funded cruise from New Zealand to Hawaii in 2007, they anticipate encountering significant Fe gradients along N-S and E-W transects that span the range over which Trichodesmium becomes Fe-limited. Measurements of Fe and Fe-speciation will be made, including application of novel methods to extend current detection limits for Fe speciation to sub-nanomolar levels. Temporal aspects of the relationship between Fe speciation parameters and the Fe stress diagnostics will also be studied by following the characteristic Trichodesmium abundance maximum at the Hawaii Ocean Time Series Station (HOT) that occurs in the late summer.
One component of the Broader Impacts of this proposal is the validation of current global ocean model predictions. This project will also develop a large dataset of Fe concentration measurements in an important, yet understudied regime of the ocean. Furthermore, this project will fund the research for two graduate students and include the participation of undergraduates from small, 4-year colleges on cruises.
