Production of organic matter by Trichodesmium IMS101 as a function of phosphorus source

Herein we present the results of a series of laboratory experiments conducted to delineate the relationship between dissolved organic phosphorus (DOP) utilization and organic matter production by the model diazotroph Trichodesmium IMS 101. The rate and ratio of Trichodesmium carbon (C) and nitrogen (N) production driven phosphorus (P) derived from select phosphonates and P monoesters were not found to be significantly different than growth rates based on dissolved inorganic phosphorus (DIP). Conversely, despite the observed similarities in C and N fixation rates between parallel DIP and DOP incubations, cultures grown solely on methylphosphonate (MPn) had reduced maximal P content per unit chlorophyll a or C relative to cultures grown on P monoesters or DIP. So while Trichodesmium IMS 101 appears to be able to maintain similar rates of growth on DIP and select DOP substrates, the P uptake efficiency and, hence, the P content per cell is greater for growth on DIP or tested P monoesters relative to growth on the phosphonate MPn. The primary conclusion arising from these results is that neither the observation of P monoester hydrolysis nor highly P-deficient organic matter or expression of phosphonate hydrolyzing genes can be interpreted to indicate P limitation of C or N fixation of Trichodesmium spp. under light-saturated, DIP-deficient conditions.