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 rbc L). High throughput sequencing of rbc L 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 rbc L genes and microscopic detection in examining phytoplankton composition and diversity.

Shifts in phytoplankton phenology were observed in the Strait of Malacca (SM) and Sunda Shelf (SS), which were speculated to be potentially related to global warming and climate anomaly events. Such interactions between phytoplankton structure and physico-chemical factors were less known in narrow straits. Therefore, the spatial distribution pattern and diversity of surface phytoplankton assemblage, local hydrology, and nutrient regimes were investigated over the SM and SS (South China Sea, SCS) during 2017 and 2018 pre-monsoon season (spring). Diatoms, dinoflagellates, and cyanobacteria were representatives of microphytoplankton in the survey area. Total phytoplankton abundance peaked near Singapore Strait (SGS) and diminished toward SS. From the lower ratio of diatoms to dinoflagellates (<3) in SS, we deduced lower carbon pump efficiency here. In agreement with the modeled results proposed previously, cold conditions (negative Indian Ocean Dipole, IOD) were more suitable for high diatom (especially centric forms) abundance, while warm scenarios (positive IOD/El Niño period as in 2017) seemed to favor dinoflagellates and/or cyanobacteria. Specifically, diatom proportion increased by 30% and dinoflagellate, cyanobacteria reduced by 71%, 75% in response to shifts of climate anomaly from 2017 cruise to 2018 cruise. This study between field microalgae and physical and chemical conditions would be helpful to launch large-scale climate model, biogeochemistry, and carbon cycling in future research.

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To examine the influence of mesoscale eddy on the natural phytoplankton community and its sinking rate changes, a comprehensive investigation cruise was carried out in the western South China Sea in autumn 2016. A total of 108 phytoplankton species were found, which belong to 54 phytoplankton genera; most of them were dominated by Dinophyta (54 genera), followed by Bacillariophyta (50 genera), Cyanophyta (3 genera), and Chrysophyta (1 genus). Bacillariophyta and Dinophyta were the main phytoplankton communities in the investigated sea area. The sinking rate of phytoplankton ranged from 0.12 to 3.17 m day–1, determined by the SETCOL method. The highest phytoplankton sinking rate was found in the 200-m water layer, followed by the DCM layer. No significant correlation was found between phytoplankton sinking rates and most of the environmental parameters during this cruise. At a similar time, we have carried out the estimation of carbon flux in the investigated sea area by using the sinking rate of phytoplankton, which showed that the carbon flux ranged from 2.41 × 10–6 to 0.006 mg C m–2 day–1; in addition, the maximum was at the 200-m layer. Phytoplankton community and sinking rate were significantly affected by the mesoscale eddy processes. The cold eddy could affect the community distribution of diatom and dinoflagellate, and the upwelling mainly affects the community of dinoflagellate. Both of them could contribute to a higher sedimentation rate of phytoplankton in the surface and DCM layers. Warm eddy could reduce the abundance of phytoplankton in the surface layer; simultaneously, the sinking rate of phytoplankton in the shallow water layer above 100 m is also reduced. These results can fill in the knowledge gap of mesoscale eddy processes in the study of phytoplankton community change and sinking rate; furthermore, it can provide insights into phytoplankton carbon and its implementation in further carbon sink.

Transparent exopolymer particles (TEP) are micron-grade (tens to hundreds µm in diameter) colloidal substances, currently defined as a class of transparent gel particles that can be stained by the specific dye of Alcian Blue (Alldredge et al., 1993; Passow, 2002b). TEP are capable of being formed biotically by extracellular excretion of plankton, or abiotically by the assemblage of dissolved precursor materials, i.e., dissolved polysaccharide (PCHO), which are mainly produced by various planktonic algae (Logan et al., 1995). Given