THE INABILITY OF THE TEXAS "BROWN TIDE" ALGA TO USE NITRATE AND THE ROLE OF NITROGEN IN THE INITIATION OF A PERSISTENT BLOOM OF THIS ORGANISM

A planktonic alga similar in general morphology and pigments to Aureococcus anophagefferens Hargraves and Sieburth has caused persistent and ecologically damaging blooms along the south Texas coast. Experiments using 100 mu M NO sub(3) super(-), NO sub(2) super(-), and NH sub(4) super(+) demonstrated that the alga could not use NO sub(3) super(-) for growth but could use NO sub(2) super(-) and NH sub(4) super(+). Doubling iron or trace metal concentrations did not permit growth on NO sub(3) super(-). Chemical composition data for cultures grown in excess NO sub(3) super(-) or NH sub(4) super(+), respectively, were as follows: N/cell (0.88 vs. 1.3 pg), C:N ratio (25:1 vs. 6.4:1), C:chlorophyll a (chl a) (560:1 vs. 44:1), and chl a/cell (0.033 vs. 0.16 pg). These data imply that cells supplied with NO sub(3) super(-) were N-starved. Culture addition of 10 mM final concentration chlorate (a nitrate analog) did not affect the Texas isolate while NO sub(3) super(-)-utilizing A. anophagefferens was lysed, suggesting that the NO sub(3) super(-) reductase of the Texas isolate is nonfunctional. Rates of primary productivity determined during a dense bloom indicated that light-saturated growth rates were ca. 0.45/d, which is similar to maximum rates determined in laboratory experiments (0.58/d plus or minus 0.16). However, chemical composition data were consistent with the growth rate of these cells being limited by N availability (C:N 28, C:chl a 176, chl a/cell 0.019) Calculations based on a mass balance for nitrogen suggest that the bloom was triggered by an input of ca. 69 mu M NH sub(4) super(+) that resulted from an extensive die-off of benthos and fish.