FIG 1 .

Bulk Prochlorococcus culture dynamics during the experiment. Prochlorococcus was grown either in pure culture or in coculture with Alteromonas. At the 24-h time point, the cultures were split and either maintained under diel light (13-h:11-h [13:11] light:dark) or moved into continuous darkness for 120 h. (A) Cell abundances. The Prochlorococcus exponential growth rates were 0.51 ± 0.03 day⁻¹ in coculture and 0.47 ± 0.02 day⁻¹ when grown alone. (B) Prochlorococcus photosynthetic quantum efficiency (Fv/Fm). (C) Prochlorococcus PSII effective absorption cross section (σPSII). (D) Relative red fluorescence per Prochlorococcus cell during the course of the experiment. The treatment of cells is indicated as follows: cells grown with light (white background) and cells grown in the dark (gray-shaded regions). The dashed vertical line indicates the time of the “expected” sunrise for the cultures kept in the dark. Values are means (± standard deviations [SD]) for three biological replicates. Details of statistical support for differences among cultures can be found in Table S2 in the supplemental material.