Kinetics of cell death in the cyanobacterium Anabaena flos-aquae and the production of dissolved organic carbon

Kinetics of cell death and the production of dissolved organic carbon (DOC) were investigated in Anabaena flos-aquae (Lyngb.) Breb grown on three different N sources (N-2, nitrate, and ammonium) in a phosphorus (P)-limited chemostat. The fraction of live cells in the total population increased as growth rate increased with decreasing P limitation. Cell death was less in nitrate and ammonium media than in N-2. The specific death rate (gamma), when calculated as the slope of v(x)(-1) vs. D-1, where v(x) and D are live cell fraction (or cell viability) and dilution rate, respectively, was 0.0082 day(-1) in N-2 and 0.0042 day(-1) in nitrate. The slope of the plot in ammonium culture was not significant; however, the value of the live cell fraction was within the range for the NO3- culture. The fraction of live vegetative cells in N-2 culture was constant at all growth rates and the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts. Live heterocysts comprised 3.5% of the total cells at a growth rate of 0.25 day(-1) and increased to 6.3% at 0.75 day(-1) with the ratio of live heterocysts to live vegetative cells linearly increasing with growth rate. The fraction of live vegetative cells was invariant in nitrate cultures as in N-2 cultures. The live heterocysts fraction also increased with growth rate in nitrate cultures, along with the live heterocysts : live vegetative cells ratio, but the level was lower than in N-2 cultures. DOC released from dead cells increased inversely with growth rate in N-2 from 36.4% of the total DOC at a growth rate of 0.75 day(-1) to 54.1% at 0.25 day(-1). The contribution of cell death to the total DOC production in nitrate and ammonium media was significantly less than that under N-2. DOC from dead cells consisted mainly of high-molecular-weight compounds, whereas DOC excreted from live cells was largely of low molecular weight.