NITROGEN BUDGETS AND DISSOLVED ORGANIC-MATTER CYCLING

Time series of biomass and nutrient levels in batch cultures of unicellular algae show widely unbalanced nitrogen budgets in some cases, with ca 0.6 to 3 times more dissolved inorganic nitrogen (DIN) consumed than particulate nitrogen (PN) produced. For those studies showing a balanced final budget, the discrepancy between DIN disappearance and PN increase does not appear to be random with time. Those trends imply excretion of dissolved organic nitrogen (DON), which can represent up to 75 % of the DIN uptake, in the early part of the incubation, followed by DON uptake when DIN is exhausted. For 4 out of 5 studies in which DON was measured, this variable could account for 91 to 100 % of the missing nitrogen in the final budget.