Journal
JOURNAL OF PHYCOLOGY
Volume 38, Issue 3, Pages 503-512Publisher
BLACKWELL PUBLISHING INC
DOI: 10.1046/j.1529-8817.2002.t01-1-01082.x
Keywords
ammonium; carbohydrate; dark N-assimilation; diel vertical migration; Heterosigma carterae; nitrate
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The capabilities of the diel vertically migrating flagellate Heterosigma carterae Hulburt for assimilating ammonium and nitrate into cell-N in light and in darkness were studied using cells of different N-status. Ammonium utilization in darkness, except by N-replete cells, attained >50% of use in the light. However, the capacity to use nitrate was poor in darkness, and less than 20% of nitrate-N that was taken up in darkness was then actually incorporated into cell-N. The ability to assimilate N in darkness improved as N-status (N:C) declined, concurrent with an increasing content of water-soluble carbohydrate. This carbohydrate was used to support dark N-assimilation. Cells held in darkness for over a day and that had halted nitrate-uptake were still capable of taking up ammonium. Furthermore, the act of taking up ammonium appeared to make available a source of C to support nitrate uptake that was previously unavailable. The implications of these results for the ecophysiology of this organism and for the construction of mathematical models of algal growth are considered.
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