Bacterial response to blooms dominated by diatoms and Emiliania huxleyi in nutrient-enriched Mesocosms

The bacterial response to two algal blooms dominated by the marine coccolithophoridEmiliania huxleyiand the marine diatomsSkeletonema costatumandLeptocylindricus danicusin nutrient-enriched mesocosms was observed. Bacterial abundance was determined using epifluorescent microscopy and bacterial activity using3H-thymidine and3H-leucine incorporation. There were significant differences in the bacterial response to the two blooms with the maxima in bacterial activity associated with theE. huxleyibloom occurring almost synchronously with the maxima inE. huxleyiabundance and the maxima in bacterial activity associated with the diatom bloom occurring about 1 week after the maxima in diatom abundance. It is suggested that this results from the greater tendency ofE. huxleyito release simple monomeric organic compounds compared toS. costatum. Levels of bacterial3H-leucine incorporation relative to bacterial3H-thymidine incorporation were significantly higher in the diatom dominated bloom compared to theE. huxleyidominated bloom. It is suggested that this is associated with the large scale exudation of complex polypeptides byS. costatum. In both enclosures bacterial3H-thymidine incorporation peaked rapidly relative to bacterial3H-leucine incorporation implying a bacterial demand for inorganic nitrogen at the peak of bacterial activity. Consequently, at the peak of theE. huxleyibloom algae may have been competing with bacteria for inorganic nitrogen, whereas during the diatom dominated bloom this competition probably did not occur until the diatom bloom had begun to subside. Following the collapse of both blooms the bacterial community regained its pre-bloom status as a small source of ammonium.