Phytoplankton in oligotrophic oceans is generally dominated by the slow-growing Prochlorococcus. However, Prochlorococcus episodically showed high growth rates even in oligotrophic environments (Jiang et al., 2021, 2022). This contradiction could be explained by the diverse Prochlorococcus populations which could specifically adapt to the ambient environments and grow actively under certain conditions.

To figure out whether there are diverse Prochlorococcus populations in the surface oligotrophic oceans, we conducted a meridional cruise in the Eastern Indian Ocean (EIO, 16.5^oN to 20^oS). Although the dissolved inorganic nitrogen was consistently depleted (~58 nM), Prochlorococcus showed contrasting physiologies in the northern and southern EIO. The cells were relatively larger (diameter of 0.54 ± 0.04 μm) and had a higher cellular divinyl Chl a (Dv) in the northern (1.12 ± 0.25 pg cell^-1) than southern (0.46 ± 0.02 μm and 0.37 ± 0.09 pg Dv cell^-1). In terms of growth, Prochlorococcus in the northern showed a higher division rate but lower Dv production rate (0.70 ± 0.36 and 0.41 ± 0.23 d^-1) than southern (0.35 ± 0.05 and 1.34 ± 0.57 d^-1). Additionally, the enriched nutrients in incubation experiments (NH₄Cl and KH₂PO₄) did not significantly affect the division rate while increased the Dv production rate by three times (0.41 ± 0.23 to 1.24 ± 0.44 d^-1) in the northern. While in the southern EIO, the Dv production rate was not affected but the division rate was decreased by half (0.35 ± 0.05 to 0.16 ± 0.06 d^-1).

The results of metagenomic analysis also suggested diverse Prochlorococcus populations across the surface EIO. Although the Prochlorococcus phylotype composition determined by 16S rDNA and marker gene rpoC1 indicated the predominance of highlight-adapted Prochlorococcus ecotype II (> 80%) throughout the EIO, the genes related to nutrients utilization showed variations in the EIO. For instance, the nitrate/nitrite transport gene was less abundant in the northern EIO where nutrients are more possible to be supplied by physical events, while was more abundant in the southern where the water column is generally stratified with rare nutrients supply. Our results indicated the diverse Prochlorococcus populations in the EIO in terms of both physiologies and genomes. The current understanding regarding Prochlorococcus as a homogeneous population needs to be revised, to also consider the diverse strategies of Prochlorococcus adapted in different oligotrophic environments for dealing with ambient growth condition such as nutrients availability.