Mulholland, M. R. Chesapeake Biological Lab, University of Maryland Center for Environmental Science, email@example.com
Capone, D. R. Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, firstname.lastname@example.org
NITROGEN AND CARBON UTILIZATION BY TRICHODESMIUM SPP.
It is generally held that the cyanobacterium, Trichodesmium, relies entirely upon N2 fixation for their N requirement. However, N-based growth rate estimates in natural populations are generally substantially lower than those based on C, suggesting utilization of other N sources. Moreover, Trichodesmium releases combined N (as DON) and in blooms this can result, after regeneration, in elevated ambient NH4+ levels.
We examined the capacity of Trichodesmium to assimilate various forms of combined N (NH4+, NO3-, urea, and glutamate) at tracer levels. Our results indicate that in laboratory cultures of Trichodesmium IMS101 and NIBB1067 between 70 and 85% of the daily nitrogen uptake in cultures can be accounted for by combined N uptake (primarily NH4+). While the assimilated NH4+ ultimately derives from N2 fixation, NH4+ uptake can represent a substantial portion of N utilization. Rates of NH4+ uptake paralleled the concentration of NH4+ in the media. N2 fixation was restricted to the light period and accounted for the balance of total daily uptake. Both Trichodesmium NIBB1067 and field populations had a high affinity for NH4+.
Carbon incorporation is also being examined over a diel cycle in cultures of Trichodesmium IMS101 to determine whether measured N utilization satisfied the N demand estimated from rates of C fixation.
Our results suggest that in nature, Trichodesmium populations do utilize ambient pools of NH4+, possibly for net growth.
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