Molecular Biomarkers for Phosphorus Stress in the Marine Cyanobacterium Crocosphaera
The marine cyanobacterium Crocosphaera significantly contributes to nitrogen fixation and primary production in tropical and subtropical oceans. These regions are often characterized by low phosphorus (P) concentrations that may constrain growth and nitrogen fixation in Crocosphaera. Molecular markers are commonly used as diagnostic tools to detect when microorganisms are under P stress and to better identify the conditions influencing P limitation. Prior to this work, diagnostic biomarkers for P stress in Crocosphaera had yet to be characterized. To identify possible biomarkers, select gene transcripts were evaluated under P stress in cultures of the model strain, WH8501, a small-cell ecotype of Crocosphaera. Transcript levels of two genes were significantly higher: pstS, which codes for the high-affinity P-binding protein, and arsB, coding for the arsenite efflux protein. These genes were up-regulated at discrete stages of the P stress response, with pstS being an early indicator of P limitation and arsB transcripts detected through P starvation. Interestingly, pstS transcripts exhibited a diel expression pattern that was detected in P replete conditions. To further evaluate the efficacy of pstS as a biomarker for P stress in Crocosphaera, changes in its transcript levels were assessed in the presence of various phosphorus sources: inorganic (DIP) and organic (DOP), as well as changing light levels. While pstS transcripts were suppressed by all P sources tested, the response was more sensitive to DIP than DOP. Additionally, pstS was induced by high light levels, independent of the P stress response. Although pstS is a commonly used indicator of P stress, these results suggest that it is not a reliable indicator for Crocosphaera. To evaluate P stress in natural assemblages, biomarkers were used to target environmental populations in the North Pacific. pstS and arsB identified P stress in Crocosphaera, and an alkaline phosphatase gene (phoX) targeted another nitrogen-fixing cyanobacterium, Trichodesmium. Detection of these biomarkers suggest that P is an important driver of Crocosphaera abundance, especially large cell ecotypes, while Trichodesmium abundance is not as negatively impacted by low P concentrations. Future studies with biomarkers differentiating between ecotypes of Crocosphaera will help determine the controls on P limitation between strains.