Biological characterization of the nitrogen-fixing Rhizosolenia-Richelia symbiosis

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Nitrogen-fixation provides a key input of new nitrogen into oligotrophic, oceanic regions. Work over the past two decades has highlighted the role of Trichodesmium. More recently, the role of coccoid cyanobacteria as well as symbiotic associations of the filamentous cyanobacteria Richelia intracellularis with species of diatoms (Rhizosolenia and Hemiaulus) has received attention. Little is known of the growth rates, nutrient needs, chemical composition, or environmental tolerances of these diatom-diazotroph associations (DDAs). However, it is clear that DDAs are numerically important in some oceans and can play a major role in mediating new nitrogen inputs. Recent models have identified the need for species-specific parameters, but these are lacking for DDAs. In particular, temperature dependent properties require quantification for application to global warming scenarios. Laboratory studies of the Rhizosolenia -Richelia DDA are now possible due to the reproducible cultivation of this association. This four-year research program will quantify temperature and salinity effects on growth rates and NB2B-fixation rates. It will explore the role of silicate and phosphate (inorganic and organic) in controlling growth rates, chemical composition and NB2B-fixation through host-symbiont interactions. The mass accumulation of the DDAs in sediment traps as well as in the sedimentary record suggest DDAs are important vectors to depth. The potentially high sinking rates relative to Trichodesmium permit rapid export of new N and sequestration of C. This work will quantify settling rates under conditions of phosphate and silicate-limited growth and provide the first estimates of potential losses due to sinking. This program will provide the first broad characterization of a DDA and provide valuable input data for models. The laboratory data will be used in a pair of cruises to the N. Pacific Ocean to quantify rates and limitations of DDAs known to occur there. Work will focus on the response to phosphorus additions using DDA growth rates and NB2B fixation. Concurrent studies using net tows will quantify growth rates of Rhizosolenia-Richelia under natural conditions. DDA blooms are potential means to remove C and N quickly from the euphotic zone via mass sedimentation of the diatom host. Diatom remains in sediments suggest this is an important vector for sedimentary deposition. The auteocological work in this study will produce information important for interpreting how such events can occur. In addition, temperature tolerance studies will yield data useful for understanding how this DDA could respond to warming oceans. Broader Implications Symbioses are a fascinating part of biology in general, and this work will enrich the biological literature by providing the first systematic study of the biology of marine DDAs. The data will provide important input data for biological and paleooceanographic models examining Nitrogen fixation and carbon flux into the deep sea. This program will support two graduate students, and serve as the basis for several weekly radio programs and talks at a public seminar series.