Coexistence of dominant marine phytoplankton species sustained by nutrient specialization

Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient 33 surface waters of the subtropical ocean, but the basis for their coexistence in these biomes is still unclear. 34 Here we combine in situ microcosm experiments and an ecological model to show that this coexistence 35 can arise from specialization in the uptake of distinct nitrogen (N) substrates. In field incubations, the 36 response of both Prochlorococcus and Synechococcus to nanomolar N amendments demonstrates N 37 limitation of growth in both populations, but Prochlorococcus showed a higher affinity to ammonium 38 whereas Synechococcus was more adapted to nitrate uptake. A simple ecological model demonstrates 39 that the differential nutrient affinity of these species can explain their coexistence. Phylogenetic analysis 40 of the presence of nitrate reductase and nitrite reductase further support the higher nitrate affinity of 41 Synechococcus compared to Prochlorococcus . Our study suggests that the evolution of differential 42 nutrient affinities is an important mechanism for sustaining coexistence of species under resource 43 competition.


In situ experiments.
To test the effect of increased nutrients at nano-molar level, we conducted five N 83 and phosphorus (P) addition (M1 to M5) and three    (Table S1) and more than 0.05 nM of DFe, (Table S2).

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Neither P nor Fe enrichment increased any of the tested phytoplankton groups including Synechococcus prefers NO3 -. Note that added NH4 + was depleted on the third day, leading to decreased populations in both organisms.

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Prochlorococcus showed significantly higher peak abundance in the NH4 + additions relative to the NO3 -100 additions in all five N and P bioassays (RM-ANOVA, p < 0.05) (Fig. 1, Fig. S1C, Table S3), indicating 101 a higher affinity to NH4 + than NO3 -. This affinity difference can be explained by the low electron 102 requirement to assimilate NH4 + , which has been previously shown in Prochlorococcus 21,22 both in 103 culture and in situ 15 . Although Prochlorococcus was considered to be incapable of NO3assimilation 22,23 , 104 recent studies have shown that especially some clades of the High Light (HL) ecotypes appear able to 105 assimilate NO3with lower growth rates compared to other nitrogen sources 21,24,25 . Since our experiment 106 Nitr Ammo Urea Phos factor(treat, levels = c("Nitr", "Ammo", "Urea", "Phos")) factor(time) Relative cell abundance (relative to control) Relative cell abundance (relative to control)

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Preferential grazing by zooplankton (e.g. nanoflagellates 31 ) may further ensure the coexistence of 148 these species [32][33][34][35] , and this has been the main explanation for the coexistence for these organisms.

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However, our study shows that, without reliance on zooplankton behavior, Prochlorococcus and

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The lower K value for NO3in Synechococcus than Prochlorococcus predicts that their relative 155 abundance will also depend strongly on the NO3 -/NH4 + ratio of the nutrient supply. We tested this model 156 prediction using field observations from the Hawaii Ocean Timeseries (HOT 36 ) in the subtropical North Further, the stimulation of Synechococcus photosynthetic and growth activity after 20 nM of NO3 -163 enrichment was later confirmed by in situ bioassay experiment 7 .

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Changes in the NO3 -/NH4 + supply ratio can be altered by ecological in addition to climatic changes.

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The low N environments dominated by these taxa are also prime habitats for N2 fixing diazotrophs, 194 which inject newly fixed NH4 + into surface waters. The presence of nitrogen fixing organisms that 195 provide NH4 + may favor Prochlorococcus, due to its lower K value for NH4 + , and as a result

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All washing procedures were carried out in an onshore class-1000 clean air room and plastic gloves 249 were worn during these operations.