Effects of intracellular structural associations on degradation of algal chloropigments in natural oxic and anoxic seawaters

To understand the effects of intracellular structural associations on degradation of algal chloropigments, we conducted a series of microcosm experiments by incubating Emiliania huxleyi cells (a marine haptophyte) in natural oxic and anoxic seawaters collected from a stratified water column in the Cariaco Basin. The incubated cell detritus were sequentially treated with two buffer solutions to separate pigment components into soluble and insoluble fractions. By using non-denaturing gel electrophoresis, several chlorophyll-complexes, free chlorophyll, and another unknown chlorophyll-containing component were further separated from the soluble fraction. The chlorophyll-complexes included those bound with high molecular weight core-proteins (CP-I and CP43+CP47) and low molecular weight polypeptides (LHC-I and LHC-II) in the cellular photosystems PS-I and PS-II. Overall pigment recovery from these fractions and gel bands was well equivalent to the total amount from direct acetone extraction of the cells. We followed the time-dependent concentration changes of chlorophyll-a (Chl-a), phaeophorbide-a (Ppb-a), and phaeophytin-a (Ppt-a) in all fractions and complexes to estimate the degradation rate constants of chloropigments in natural oxic and anoxic seawaters. Our experimental results demonstrated that the intracellular structural associations had important influences on degradation of chloropigments under different redox conditions. In general, total Chl-a degraded faster (∼4X) in oxic seawater than in anoxic seawater. However, the rate differences between oxic and anoxic conditions varied among the fractions and complexes. Degradation rate constants of Chl-a in soluble fraction were much higher (>10X) than those in insoluble fraction under both oxic and anoxic conditions. Chl-a bound with the complexes in PS-II appeared to be more reactive (∼2X) than that in PS-I under oxic conditions but the difference in degradation rate constants between two photosystems became indistinguishable under anoxic conditions. Variations of Ppb-a in different fractions and complexes during incubation showed different patterns, implying that cellular Chl-a could degrade through two different pathways: (1) internal degradation into Ppb-a within insoluble pool and polypeptide complexes; and (2) release first from protein complexes and followed by external degradation.