Molecular markers for <i>Richelia </i>symbionts of open ocean diatom symbioses

Decades of research has been devoted to understanding the occurrence, distribution, and activity of different nitrogen fixers in the global oceans. With this work we provide new methods for specific and sensitive detection of three biogeochemically important marine nitrogen fixing <i>Richelia</i> spp. symbionts of diatoms. The nine new quantitative polymerase chain reaction assays were experimentally tested in lab and field samples. The assays target three gene orthologs that when expressed can be indicative of metabolic activities between various <i>Richelia</i> spp.: phosphorus transport (<i>pstS</i>), iron transport (<i>exbB</i>), and a constitutively expressed gene for normalization <i>(rnpA</i>). Our results show that these assays are specific, can detect temporal dynamics in gene expression of lab cultures, and importantly are indicative of relative nutrient limitation (e.g., P, Fe) and are an improvement over the previously used <i>nifH</i> assays as estimations of abundance (DNA based) and metabolic activities (RNA based) were without cross-reaction.