Ocean currents promote rare species diversity in protists

Deviation from neutral species abundance distributions unveils geographical differences in the structure of diatom communities

In recent years, the application of metagenomics techniques has advanced our understanding of plankton communities and their global distribution. Despite this progress, the relationship between the abundance distribution of diatom species and varying marine environmental conditions remains poorly understood. This study, leveraging data from the Tara Oceans expedition, tests the hypothesis that diatoms in sampled stations display a consistent species abundance distribution structure, as though they were sampled from a single ocean-wide metacommunity. Using a neutral sampling theory, we thus develop a framework to estimate the structure and diversity of diatom communities at each sampling station given the shape of the species abundance distribution of the metacommunity and the information of a reference station. Our analysis reveals a substantial temperature gradient in the discrepancies between predicted and observed biodiversity across the sampled stations. These findings challenge the hypothesis of a single neutral metacommunity, indicating that environmental differences substantially influence both the composition and structure of diatom communities.

Neutral Theory and Plankton Biodiversity

The biodiversity of the plankton has been interpreted largely through the monocle of competition. The spatial distancing of phytoplankton in nature is so large that cell boundary layers rarely overlap, undermining opportunities for resource-based competitive exclusion. Neutral theory accounts for biodiversity patterns based purely on random birth, death, immigration, and speciation events and has commonly served as a null hypothesis in terrestrial ecology but has received comparatively little attention in aquatic ecology. This review summarizes basic elements of neutral theory and explores its stand-alone utility for understanding phytoplankton diversity. A theoretical framework is described entailing a very nonneutral trophic exclusion principle melded with the concept of ecologically defined neutral niches. This perspective permits all phytoplankton size classes to coexist at any limiting resource level, predicts greater diversity than anticipated from readily identifiable environmental niches but less diversity than expected from pure neutral theory, and functions effectively in populations of distantly spaced individuals.

A Lagrangian model for drifting ecosystems reveals heterogeneity-driven enhancement of marine plankton blooms

Marine plankton play a crucial role in carbon storage, global climate, and ecosystem function. Planktonic ecosystems are embedded in patches of water that are continuously moving, stretching, and diluting. These processes drive inhomegeneities on a range of scales, with implications for the integrated ecosystem properties, but are hard to characterize. We present a theoretical framework that accounts for all these aspects; tracking the water patch hosting a drifting ecosystem along with its physical, environmental, and biochemical features. The theory resolves patch dilution and internal physical mixing as a function of oceanic strain and diffusion. Ecological dynamics are parameterized by an idealized nutrient and phytoplankton population and we specifically capture the time evolution of the biochemical spatial variances to represent within-patch heterogeneity. We find that, depending only on the physical processes to which the water patch is subjected, the plankton biomass response to a resource perturbation can vary in size up to six times. This work indicates that we must account for these processes when interpreting and modeling marine ecosystems and provides a framework with which to do so.

Shift of phytoplankton assemblages in a temperate lake located on the eastern route of the South-to-North Water Diversion Project

There remains no consensus on the effects of changes in the environment factors under the action of water diversions on phytoplankton communities. Herein the changing rules applying to phytoplankton communities subject to water diversion were unveiled based on long-term (2011-2021) time-series observations on Luoma Lake, located on the eastern route of the South-to-North Water Diversion Project. We found that nitrogen decreased and then increased, while phosphorus increased after operation of the water transfer project. Algal density and diversity were not affected by water diversion, while the duration of high algal density was shorter after water diversion. Phytoplankton composition had dramatic differences before and after water transfer. The phytoplankton communities exhibited greater fragility when they first experienced a human-mediated disturbance, and then they gradually adapted to more interferences and acquired stronger stability. We furthermore found the niche of Cyanobacteria narrowed while that of Euglenozoa widened under the pressure of water diversion. In addition to WT and DO, the main environmental factor before water diversion was NH₄-N, whereas the effect of NO₃-N and TN on phytoplankton communities increased after water diversion. These findings fill the knowledge gap as to the consequence of water diversion on water environments and phytoplankton communities.

Coalescent dynamics of planktonic communities

Planktonic communities are extremely diverse and include a vast number of rare species. The dynamics of these rare species is best described by individual-based models. However, individual-based approaches to planktonic diversity face substantial difficulties, due to the large number of individuals required to make realistic predictions. In this paper, we study the diversity of planktonic communities by means of a spatial coalescence model that incorporates transport by oceanic currents. As a main advantage, our approach requires simulating a number of individuals equal to the size of the sample one is interested in, rather than the size of the entire community. By theoretical analysis and simulations, we explore the conditions upon which our coalescence model is equivalent to individual-based dynamics. As an application, we use our model to predict the impact of chaotic advection by oceanic currents on biodiversity. We conclude that the coalescent approach permits one to simulate marine microbial communities much more efficiently than with individual-based models.

Bacteria, Protists, and Fungi May Hold Clues of Seamount Impact on Diversity and Connectivity of Deep-Sea Pelagic Communities

The topography and hydrography around seamounts have a strong influence on plankton biogeography. The intrinsic properties of various biological taxa inherently also shape their distribution. Therefore, it is hypothesized that different pelagic groups respond differently to effects of seamounts regarding their distribution and connectivity patterns. Herein, bacterial, protist, and fungal diversity was investigated across the water column around the Kocebu Guyot in the western Pacific Ocean. A higher connectivity was detected for bacteria than for protists and an extremely low connectivity for fungi, which might be attributed to parasitic and commensal interactions of many fungal taxa. The seamount enhanced the vertical connectivity of bacterial and protist communities, but significantly reduced protist connectivity along horizontal dimension. Such effects provide ecological opportunities for eukaryotic adaption and diversification. All the bacterial, protist, and fungal communities were more strongly affected by deterministic than stochastic processes. Drift appeared to have a more significant role in influencing the fungal community than other groups. Our study indicates the impact of seamounts on the pelagic community distribution and connectivity and highlights the mechanism of horizontally restricted dispersal combined with vertical mixing, which promotes the diversification of eukaryotic life.

A dynamic ancestral graph model and GPU-based simulation of a community based on metagenomic sampling

In this paper we present an ancestral graph model of the evolution of a guild in an ecological community. The model is based on a metagenomic sampling design in that a random sample is taken at the community, as opposed the taxon, level and species are discovered by genetic sequencing. The specific implementation of the model envisions an ecological guild that was founded by colonization at some point in the past that then potentially undergoes diversification by natural selection. Within the graph, species emerge and evolve through the diversification process and their densities in the graph are dynamic and governed by both ecological drift and random genetic drift, as well as differential viability. We employ the 3% sequence divergence rule at a marker locus to identify Operational Taxonomic Units. We then explore approaches to see if there are indirect signals of the diversification process, including population genetic and ecological approaches. In terms of population genetics, we study the joint site frequency spectrum of OTUs, as well its associated statistics. In terms of ecology, we study the species (or OTU) abundance distribution. For both we observe deviations from neutrality, which indicates that there may be signals of diversifying selection in metagenomic studies under certain conditions. The model is available as a GPU-based computer program in C/C++ and using OpenCL, with the long-term goal of adding functionality iterativelyto model large-scale eco-evolutionary processes for metagenomic data.

Effective Resource Competition Model for Species Coexistence

Local coexistence of species in large ecosystems is traditionally explained within the broad framework of niche theory. However, its rationale hardly justifies rich biodiversity observed in nearly homogeneous environments. Here we consider a consumer-resource model in which a coarse-graining procedure accounts for a variety of ecological mechanisms and leads to effective spatial effects which favor species coexistence. Herein, we provide conditions for several species to live in an environment with very few resources. In fact, the model displays two different phases depending on whether the number of surviving species is larger or smaller than the number of resources. We obtain conditions whereby a species can successfully colonize a pool of coexisting species. Finally, we analytically compute the distribution of the population sizes of coexisting species. Numerical simulations as well as empirical distributions of population sizes support our analytical findings.

Changes in protist communities in drainages across the Pearl River Delta under anthropogenic influence

Drainages in the Pearl River Delta urban agglomeration (PRDUA) host vital aquatic ecosystems and face enormous pressures from human activities in one of the largest urban agglomerations in the world. Despite being crucial components of aquatic ecosystems, the interactions and assembly processes of the protistan community are rarely explored in areas with serious anthropogenic disturbance. To elucidate the mechanisms of these processes, we used environmental DNA sequencing of 18S rDNA to investigate the influence of environmental factors and species interactions on the protistan community and its assembly in drainages of the PRDUA during summer. The protistan community showed a high level of diversity and a marked spatial pattern in this region. Community assembly was driven primarily by stochastic processes based on the Sloan neutral community model, explaining 74.28%, 75.82%, 73.67%, 74.40% and 51.24% of community variations in the BJ (Beijiang), XJ (Xijiang), PRD (Pearl River Delta), PRE (Pearl River Estuary) areas and in total, respectively. Meanwhile, environmental variables including temperature, pH, dissolved oxygen, transparency, nutrients and land use were strongly correlated with the composition and assembly of the protistan community, explaining 40.40% of variation in the protistan community. Furthermore, the bacterial community was simultaneously analysed by the 16S rDNA sequencing. Co-occurrence network analysis revealed that species interactions within bacteria (81.41% positive) or protists (82.80% positive), and those between bacteria and protists (50% positive and 50% negative) impacted the protistan community assembly. In summary, stochastic processes dominated, whereas species interactions and environmental factors also played important roles in shaping the protistan communities in drainages across the PRDUA. This study provides insights into the ecological patterns, assembly processes and species interactions underlying protistan dynamics in urban aquatic ecosystems experiencing serious anthropogenic disturbance.

Phytoplankton taxonomic and functional diversity patterns across a coastal tidal front

Oceanic physics at fine scale; e.g. eddies, fronts, filaments; are notoriously difficult to sample. However, an increasing number of theoretical approaches hypothesize that these processes affect phytoplankton diversity which have cascading effects on regional ecosystems. In 2015, we targeted the Iroise Sea (France) and evidenced the setting up of the Ushant tidal front from the beginning of spring to late summer. Seawater samples were taken during three sampling cruises and DNA-barcoding allowed us to investigate patterns of eukaryotic phytoplankton diversity across this front. First focusing on patterns of taxonomic richness, we evidenced that the front harbored a hotspot of eukaryotic phytoplankton diversity sustained throughout summer. We then detail the ecological processes leading to the formation of this hotspot by studying shifts in community composition across the Iroise Sea. Physical mixing mingled the communities surrounding the front, allowing the formation of a local ecotone, but it was cycles of disturbances and nutrient inputs over the front that allowed a decrease in competitive exclusion, which maintained a higher diversity of rare phytoplankton taxa. These processes did not select a specific ecological strategy as inferred by a trait approach coupled to our taxonomic approach. Instead the front favored higher richness within widespread strategies, resulting in functional redundancy. We detail how fine-scale ocean physics affect phytoplankton diversity and suppose that this interplay is a major control on regional ecosystems.