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 NO3-, NO2- and NH4+ demonstrated that the alga could not use NO3- for growth but could use NO2- and NH4+. Doubling iron or trace metal concentrations did not permit growth on NO3-. Chemical composition data for cultures grown in excess NO3- or NH4+ respectively, were as follows: N.cell(-1)(0.88 vs. 1.3 pg), C:N ratio (25:1 vs. 6.4:1), C:chlorophyll a (chi a) (560:1 vs. 44:1), and chi a.cell(-1) (0.033 vs. 0.16 pg). These data imply that cells supplied with NO3- were N-starved. Culture addition of 10 mM final concentration chlorate (a nitrate analog) did not affect the Texas isolate while NO3--utilizing A. anophagefferens was lysed, suggesting that the NO3- 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(-1), which is similar to maximum rates determined in laboratory experiments (0.58 d(-1) +/- 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, chi a.cell(-1) 0.019). Calculations based on a mass balance for nitrogen suggest that the bloom was triggered by an input of ca. 69 mu M NH4+ that resulted from an extensive die-off of benthos and fish.