Integrating Stochastic and Deterministic Process in the Biogeography of N2-Fixing Cyanobacterium Candidatus Atelocyanobacterium Thalassa.

Liuyang Li,Chao Wu,Yuqiu Wei,Jun Sun,Danyue Huang,Changling Ding

doi:10.3389/fmicb.2021.654646

Abstract

UCYN-A is one of the most widespread and important marine diazotrophs. Its unusual distribution in both cold/warm and coastal/oceanic waters challenges current understanding about what drives the biogeography of diazotrophs. This study assessed the community assembly processes of the nitrogen-fixing cyanobacterium UCYN-A, developing a framework of assembly processes underpinning the microbial biogeography and diversity. High-throughput sequencing and a qPCR approach targeting the nifH gene were used to investigate three tropical seas: the Bay of Bengal, the Western Pacific Ocean, and the South China Sea. Based on the neutral community model and two types of null models calculating the β-nearest taxon index and the normalized stochasticity ratio, we found that stochastic assembly processes could explain 66–92% of the community assembly; thus, they exert overwhelming influence on UCYN-A biogeography and diversity. Among the deterministic processes, temperature and coastal/oceanic position appeared to be the principal environmental factors driving UCYN-A diversity. In addition, a close linkage between assembly processes and UCYN-A abundance/diversity/drivers can provide clues for the unusual global distribution of UCYN-A.

Highlights

Diazotrophs play a fundamental role in biogeochemical processes by providing bioavailable nitrogen (Sohm et al, 2011; Karl et al, 2012), which determines the primary production in the ocean (Postgate, 1982; Gruber and Sarmiento, 1997; Karl et al, 1997)
Thirty-six operational taxonomic units (OTUs) representing 23.81% of all sequences were clustered as UCYN-A1, and 51 OTUs contributing 72.30% of the dataset were affiliated to UCYN-A2
Our study provides the first example of research revealing both stochastic processes and deterministic processes driving the biogeography and diversity of marine UCYN-A communities

Summary

Introduction

Diazotrophs play a fundamental role in biogeochemical processes by providing bioavailable nitrogen (Sohm et al, 2011; Karl et al, 2012), which determines the primary production in the ocean (Postgate, 1982; Gruber and Sarmiento, 1997; Karl et al, 1997). A recent study has suggested that N2 fixation, instead of nitrate (NO3−) or ammonium (NH4+) utilization, is important for the UCYN-A/haptophyte symbiosis to meet their nitrogen demands under conditions of replete dissolved inorganic nitrogen (DIN) (Mills et al, 2020). These findings expanded our knowledge of the spatial distribution and scale of habitat preference of UCYN-A, which were different from the well-known cyanobacterium Trichodesmium and the diatom symbiont Richelia that prefer warm (Breitbarth et al, 2007) and oligotrophic seawater (Tyrrell, 1999). The question of whether symbionts with such self-sufficiency and ability to survive over a wide range of temperatures were less susceptible to nichebased selection, or instead they formed versatile ecotypes to adapt to various environments owing to niche differentiation, leads us to reconsider how the biogeography and diversity of UCYN-A are assembled

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