On day 57 of the experiment, two of the three nubbins of each colony in each tank were removed for other studies. On the final day of the experiment, the remaining nubbins were snap frozen in liquid nitrogen and transported on dry ice to a -80°C freezer. Tissue was removed from thawed nubbins with an airbrush, using a buffer at 4℃ (0.4 M NaCl, 0.05 M EDTA) and mechanically homogenised for ~10s. To determine chlorophyll, 0.5ml of slurry was centrifuged (4500 g, 5 minutes), and the pellet of algae was extracted in 100% acetone for 48 hours at -20℃. Absorbances of the resulting extracts were read in a glass 96 well plate (path length 6mm) at 663, 630 and 750 nm (Spectramax M2, Molecular Devices). Concentrations of Chlorophylls a and c2 were calculated by the equations of Jeffrey and Humphrey (1975).