Estimating carbon, nitrogen, protein, and chlorophyll <i>a</i> from volume in marine phytoplankton

The size of 30 small (2–60 µ m) phytoplankton species was examined with a microscope and a Coulter Counter before and after fixation. Acid Lugol’s iodine caused cells to shrink immediately. The shrinkage effect was constant for concentrations of 1–10% Lugol’s iodine (in seawater). For optically measured cells fixed in 2% Lugol’s iodine, volume of live cells = 1.33 × (volume of fixed cells). Coulter Counter and optically measured volumes did not agree. For live cells, optical cell volume = 1.24–2.04 × (Coulter Counter determined volume); this difference is likely due to inaccurate volume measurements of non‐spherical cells by the Coulter Counter and by inaccurate microscopy resulting from optical distortions (errors of &lt;0.5 µ m in cell dimensions). Cell quota estimates were presented following the relation y = ax b , where x = optically measured cell volume ( µ m 3 ), y = any cell constituent (pg cell −1 ), and a and b are constants. The constants a and b were 0.109 and 0.991 for carbon, 0.0172 and 1.023 for nitrogen, 0.043 and 1.058 for protein, and 0.00428 and 0.917 for Chl a. Our relation of carbon to volume differs from other literature values, in which there is no consensus. Our data can be used to determine carbon, nitrogen, protein, and Chl a estimates from field material that has been fixed with Lugol’s iodine, observed live, optically measured, or Coulter Counter measured; however, the variability in published data suggests that any of these estimates will have a large potential error.