Independent iron and light limitation in a low-light-adapted Prochlorococcus from the deep chlorophyll maximum

Nina Yang,Feixue Fu,David A Hutchins,Seth G John,Nicholas J Hawco

doi:10.1038/s41396-020-00776-y

Abstract

Throughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups like Prochlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments with Prochlorococcus strain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon in Prochlorococcus MIT1214 cells ranged from 10–40 × 10−6 mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adapted Prochlorococcus, dFe must be recycled rapidly and at high efficiency.

Highlights

1234567890();,: 1234567890();,: Since its emergence several hundred million years ago, the Prochlorococcus genus has diversified into dozens of distinct subpopulations, but the main branches of this radiation are structured primarily by their growth at different irradiance [1]
Upregulation of the photosynthetic apparatus in low-light-adapted ecotypes compensates for dwindling light flux, contributing to the formation of a deep chlorophyll maximum (DCM) layer at the base of the euphotic zone [2]
As photosynthetic proteins represent a large pool of Fe in these cells, growth at low light is expected to increase

Summary

Present address

Incubations in the California Current have shown that diatoms and other eukaryotic phytoplankton at the DCM respond to increases in both Fe and light [7], but these taxa are less abundant in offshore waters. At Station ALOHA—a site that is broadly representative of the North Pacific Subtropical Gyre—most primary production in the DCM (100–125 m depth) is accomplished by low-light-adapted Prochlorococcus ecotypes [8, 9] whose Fe requirements have not been characterized. ALOHA, under a matrix of Fe and irradiance typical of the DCM (see Supplementary Materials and Methods), which follows the 0.5 mol photon m−2 day−1 isolume [10] and overlaps with peak abundance of LL1 Prochlorococcus (0.1–1 mol photon m−2 day−1 [9]).

B High light

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