Fig. 4.

Effect of chitosan addition on growth of two Prochlorococcus strains as a function of light intensity. MIT9303 is a primary chitinase degrader and MIT9313 is a secondary degrader, missing the chitinase genes. Cultures were grown in Pro99 media with (purple) and without (black) added chitosan, at two different light intensities: 1.5 μmol photons m⁻² s⁻¹ (A) and 12 μmol photons m⁻² s⁻¹ (B), the former being growth-limiting for these strains as reflected in the difference in growth rates, μ, at the different light levels. Growth was monitored by bulk chlorophyll fluorescence and cell counts were measured using flow cytometry in cultures growing at 1.5 μmol photons m⁻² s⁻¹ (SI Appendix, Fig. S5). Error bars show SD between the three biological replicates. The average growth rate and associated SD (μ, in units day⁻¹) were calculated in exponential phase (marked with a black line) and is shown for each curve.