Distributions of iron, phosphorus and sulfur along trichomes of the cyanobacteria Trichodesmium

Abstract

The nonheterocystous cyanobacterium Trichodesmium fixes C and N concurrently during the light period in tropical and subtropical oceans. Synchrotron mapping of Fe, P and S in trichomes of Trichodesmium erythraeum Erhenberg IMS 101 (CCMP 1985) collected during exponential and senescent growth revealed that 16% of trichomes contained sections of up to 25 cells with ca. 2-fold elevated Fe and S but ca. 2-fold lowered P in comparison to neighboring trichome sections. The correlation between Fe and S in these trichomes was moderate to strongly positive (R > 0.35), while the correlation between Fe and P was moderate to strongly negative (R < 0.35). Higher Fe in theses trichome sections might indicate the presence of nitrogenase. Increase in S in conjunction with Fe is likely driven by other S-containing compounds in addition to Fe–S proteins. Furthermore, the concurrent increase in S and decrease in P in these Fe-rich trichome sections might indicate a switch from P- to S-containing compounds. Diurnal changes and growth phase-related differences in the correlation between Fe and P both point to Trichodesmium's ability to re-allocate elements depending on their physiological need. Concurrent P depletion and Fe and S enrichment in trichome sections is a strong indication that Trichodesmium is able to develop special trichome regions consisting of multiple cells with a unique chemical composition. Whether these cells are uniquely dedicated to N₂-fixation (i.e., diazocytes) is an open question.