Diversity and Spatial Distribution of Chromophytic Phytoplankton in the Bay of Bengal Revealed by RuBisCO Genes (rbcL)

Laxman Pujari,Jun Sun,Min Wang,Jinjun Kan,Nan Li,Xiaomei Shang,Chun Zhou,Xingzhou Wang,Chao Wu,Guicheng Zhang

doi:10.3389/fmicb.2019.01501

Abstract

Phytoplankton are the basis of primary production and play important roles in regulating energy export in marine ecosystems. Compared to other regions, chromophytic phytoplankton are considerably understudied in the Bay of Bengal (BOB). Here, we investigated community structure and spatial distribution of chromophytic phytoplankton in the BOB by using RuBisCO genes (Form ID rbcL). High throughput sequencing of rbcL genes revealed that diatoms, cyanobacteria (Cyanophyceae), Pelagophyceae, Haptophyceae, Chrysophyceae, Eustigamatophyceae, Xanthophyceae, Cryptophyceae, Dictyochophyceae, and Pinguiophyceae were the most abundant groups recovered in the BOB. Abundances and distribution of diatoms and Pelagophyceae were further verified using quantitative PCR analyses which showed the dominance of these groups near the Equator region (p < 0.01) where upwelling was likely the source of nutrients. Further, redundancy analysis demonstrated that temperature was an important environmental driver in structuring distributions of Cyanophyceae and dominant chromophytic phytoplankton. Morphological identification and quantification confirmed the dominance of diatoms, and also detected other cyanobacteria and dinoflagellates that were missing in our molecular characterizations. Pearson’s correlations of these morphologically identified phytoplankton with environmental gradients also indicated that nutrients and temperature were key variables shaping community structure. Combination of molecular characterization and morphological identification provided a comprehensive overview of chromophytic phytoplankton. This is the first molecular study of chromophytic phytoplankton accomplished in the BOB, and our results highlight a combination of molecular analysis targeting rbcL genes and microscopic detection in examining phytoplankton composition and diversity.

Highlights

Characterization of primary productivity in oceans have been mainly focused in euphotic zones (Sigman and Hain, 2012)
The main objectives of this study were: (1) to investigate community composition of chromophytic phytoplankton based on high throughput sequencing of research based on functional genes (rbcL) genes, and (2) to understand their spatial distribution in the Bay of Bengal (BOB) and how they respond to environmental gradients
chlorophyll a (Chl-a) concentrations showed a similar trend as salinity and temperature: increased from northern to the southern region of the BOB (p < 0.05) (Figures 2a–c)

Summary

Introduction

Characterization of primary productivity in oceans have been mainly focused in euphotic zones (Sigman and Hain, 2012). Marine phytoplankton are important primary producers in euphotic zones and carry out more than half of the photosynthetic carbon fixation in oceans (Falkowski et al, 1998). Molecular methods have been applied to decipher detailed phytoplankton communities including using small subunit ribosomal RNA genes (18S rRNA) (Read et al, 2014; Needham et al, 2018). Because of their evolutionarily conserved nature and multiple copies, rRNA genes may not provide high resolution and sometimes fail to identify organisms at the species or even at the genus level (Pochon et al, 2013; Fletcher et al, 2017). Functional genes are involved in major element metabolisms (e.g., carbon and nitrogen) and provide a broad view of both genetic and functional diversity of phytoplankton, which are potentially linked to biogeochemical cycling in oceans

Objectives

Methods

Results

Discussion

Conclusion