Macromolecular labelling patterns and inorganic nutrient limitation of a North Atlantic spring bloom

Abstract

Synthesis rates of macromolecular classes (protein, polysaccharide, neutral lipid, glycolipid and phospholipid) were measured using 14C incorporation to assess the physiological condition of phytoplankton during a spring bloon~ in the northeastern Sargasso Sea. The relative allocation of labelled photosynthate to polysaccharide increased with incubation irradiance, and declined overnight; opposite trends were found in the protein fraction. The rate of protein synthesis at night was a small fraction (0 to 23 '%I) of the day-time rate. Incubation irradiance and overnight metabolism had relatively little effect on total or class-specific lipid synthesis. lnorganic nutrients decreased to below analytical detection during the sampling period, with coincident declines in chlorophyll a concentrations. The relative allocation of photosynthate to protein was much reduced in the nutrient-depleted stations later on in the bloom, while the allocation to polysaccharide was increased. Total lipid synthesis was only slightly higher in the later stations, but the proportion of neutral lipid was significantly increased. The changes In macrornolecular labelling patterns were consistent with a physiological response to nutrient depletion by the phytoplankton, and a reduction of 50 % or more in specific growth rates.