Uncoupling of Cell Division and Pigment Synthesis During Manipulations of a Natural Diatom Population
The relationship between population growth and pigment synthesis was examined in Rehoboth Bay, Delaware, in August 1990 during a bloom dominated by small diatoms (5 μm). Changes in cell number and the concentration of chlorophylls and carotenoids were examined in diluted samples (9:1 mixture of <0.45 μm filtered:unfiltered seawater) and in undiluted samples (screened for >211 μm organisms). This manipulation allowed estimation of the grazing impact of microzooplankton on the growth rate of phytoplankton. However, at most, microzooplankton grazing could account for a 12% reduction in phytoplankton growth rate, from 3.00 to ≈ 2.65 d−1. Rather, varying rates of cell division and pigment synthesis were observed, indicating that there was nutrient limitation in undiluted samples. Cell division rate was only slightly affected by dilution, but rates of net pigment synthesis differed widely from estimates of cell-specific growth and loss rate. Chlorophyll a degradation was apparent in the undiluted light treatment, while in diluted samples, chlorophyll a accumulated at a rate nearly equal to cell division rate. Nutrient limitation affected the rates of chlorophyll a and c synthesis but not the synthesis rates of diatom carotenoids, fucoxanthin, and diadinoxanthin. Thus, a de-coupling in time of chlorophyll a synthesis from carotenoid synthesis and cell division complicated the use of taxon-specific pigments in assessing the effect of dilution on algal population dynamics, but provided insight into the nutrient status of the phytoplankton assemblage.