Ocean currents promote rare species diversity in protists

Abstract

Oceans host communities of plankton composed of relatively few abundant species and many rare species. The number of rare protists species in these communities, as estimated in metagenomic studies, decays as a steep power law of their abundance. The ecological factors at the origin of this pattern remain elusive. We propose that oceanic currents affect biodiversity patterns of rare species. To test this hypothesis, we introduce a spatially-explicit coalescence model able to reconstruct the species diversity in a sample of water. Our model predicts, in the presence of oceanic currents, a steeper power law decay of the species abundance distribution and a steeper increase of the number of observed species with sample size. A comparison of two metagenomic studies of planktonic protist communities in oceans and in lakes quantitatively confirms our prediction. Our results support that oceanic currents positively impact the diversity of rare aquatic microbes.Observations contradict this prediction and show that planktonic communities are very diverse (4-7).This violation of the basic principles of niche theory (8,9) has puzzled ecologists for decades (10) and has fostered numerous studies attempting to explain the diversity of plankton (11). One proposal is that variable environments offer more possibilities for specialization of ecological traits (5,(12)(13)(14)(15)(16)(17)(18). Another proposal is that oceanic currents can create barriers reducing competition among species, and therefore promoting species coexistence (19,20). Quantitative analyses also suggest that oceanic currents play a significant role in organizing large-scale diversity patterns (21,22), and that dispersal limitation contributes, alongside with niche specialization, to microbial biodiversity of oceans (23-27).DNA metabarcoding has allowed rapid and extensive measurements of the diversity of aquatic microbial communities, providing new means to study the ecological forces shaping planktonic communities.Metabarcoding studies have revealed that, beside commonly observed species, planktonic communities are characterized by a vast range of rare species. This so-called "rare biosphere" (28, 29) makes up the majority of planktonic species (25,30). The diversity of planktonic species can be quantified by the Species Abundance Distribution (SAD), defined as the frequency P (n) of species with abundance n in a sample. SADs of rare marine protists are qualitatively different from those of abundant species (31,32) and appears to follow a power law distribution P (n) ∝ 1/n α .(1)The exponent α varies significantly among samples, appears weakly correlated with environmental factors, and is significantly larger than 1 on average (33). Diversity patterns in other microbial communities, such as that of the human gut (34), are well described by a form of SAD following the Fisher log series, P (n) ∝ e −c n /n (35-37), as predicted by Hubbell's neutral model (36,38,39). For large communities, the parameter c is very small, so that the distribution is close to a powe...

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