Effect of phosphorus limitation on coccolith morphology and element ratios in Mediterranean strains of the coccolithophore Emiliania huxleyi

Abstract

Six Emiliania huxleyi strains isolated in the Mediterranean Sea were grown under phosphorus limitation and replete (control) conditions in batch culture. We tested the hypothesis that increasing phosphorus limitation, resulting from ocean warming, will affect coccolithophores and change their contribution to the PIC:POC ratios. Four strains were isolated in the oligotrophic Algerian basin (western Mediterranean), and two strains in the extremely oligotrophic and often phosphorus depleted Levantine basin (eastern Mediterranean). Samples for particulate inorganic carbon, PIC; particulate organic carbon, POC; particulate organic phosphorus, POP; and total particulate nitrogen, TPN, were taken at equal cell densities in phosphorus limited and control culture conditions (harvesting in exponential and stationary phases respectively). Different morphological features of coccoliths were analyzed. Phosphorus limitation was inferred from: 1. a decrease in growth rate between 22% and 34% compared to phosphorus replete cultures; 2. a reduction in final cell density from ca. 2×106cells/ml to ca. 8×104cells/ml, and 3. markedly lower POP quotas of the limited cells. Differences in the phosphate uptake machinery are suggested based on the fact that three western Mediterranean strains decreased the phosphate concentration in the culture media until the detection limit in the first 3 experimental days while 4–5days were necessary for the other strains. Cellular carbon quotas increased in all strains under phosphorus limitation in comparison to their respective control cultures. The increase in POC quotas ranged between 179% and 260% and in PIC quotas between 43% and 201%. TPN quotas increased under phosphorus limitation between 31% and 86% in five out of six strains. POC:TPN increased in all strains, which suggests a change in the molecular composition of the cells when exposed to phosphorus limitation. Strain specific responses were also observed in the PIC:POC ratio, which decreased in five strains and did not change in another. Phosphorus limitation did not lead to malformations. However, morphological changes of coccoliths occurred, also in a strain specific way, namely the percentage of over-calcified coccoliths was altered by P limitation, but we observed all signs in the responses among different strains. Nevertheless, this morphological feature does not reflect the changes in calcite production (PICp nor PIC:POC). Strain specific responses were not related to the isolation location, suggesting that the isolated western and eastern strains do not represent distinct eco-genotypes featuring different phosphorus usage strategies.