Elemental composition of marine Prochlorococcus and Synechococcus: Implications for the ecological stoichiometry of the sea

Abstract

The elemental composition of marine cyanobacteria is an important determinant of the ecological stoichiometry in low‐latitude marine biomes. We analyzed the cellular carbon (C), nitrogen (N), and phosphorus (P) contents of Prochlorococcus (MED4) and Synechococcus (WH8103 and WH8012) under nutrient‐replete and P‐starved conditions. Under nutrient‐replete conditions, C, N, and P quotas (femtogram cell21) of the three strains were 46 ± 4, 9.4 ± 0.9, and 1.0 ± 0.2 for MED4; 92 ± 13, 20 6 3, and 1.8 ± 0.1 for WH8012; and 213 ± 7, 50 ± 2, 3.3 ± 0.5 for WH8103. In P‐limited cultures, they were 61 ± 2, 9.6 ± 0.1, and 0.3 ± 0.1 for MED4; 132 ± 6, 21 ± 2, and 0.5 ± 0.2 for WH8012; and 244 ± 21, 40 ± 4, and 0.8 ± 0.01 for WH8103. P limitation had no effect on the N cell quota of MED4 and WH8012 but reduced the N content of WH8103. The cellular C quota was consistently higher in P‐limited than in nutrient‐replete cultures. All three strains had higher C: P and N: P ratios than the Redfield ratio under both nutrient‐replete and P‐limited conditions. The C:N molar ratios ranged 5–5.7 in replete cultures and 7.1–7.5 in P‐limited cultures; C: P ranged 121–165 in the replete cultures and 464–779 under P limitation; N: P ranged 21–33 in the replete cultures and 59–109 under P limitation. Our results suggest that Prochlorococcus and Synechococcus may have relatively low P requirements in the field, and thus the particulate organic matter they produce would differ from the Redfield ratio (106C : 16N : 1P) often assumed for the production of new particulate organic matter in the sea.