Planktonic microbial eukaryotes in polar surface waters: recent advances in high-throughput sequencing

Marine microbial eukaryotes are important primary producers and play critical roles in key biogeochemical cycles.Recent advances in sequencing technology have focused attention on the extent of microbial biodiversity,revealing a huge,previously underestimated phylogenetic diversity with many new lineages.This technology has now become the most important tool to understand the ecological signifcance of this huge and novel diversity in polar oceans.In particular,high-throughput sequencing technologies have been successfully applied to enumerate and compare marine microbial diversity in polar environments.Here,a brief overview of polar microbial eukaryote diversity,as revealed by in-situ surveys of the high-throughput sequencing on 18S rRNA gene,is presented.Using these‘omic’approaches,further attention still needs to be focused on diferences between specifc locations and/or entire polar oceans and on bipolar comparisons of diversity and distribution.

Distribution of Protists in the Deep South China Sea Revealed by High-Throughput Sequencing

Protists(microbial eukaryotes)are indispensable members of the marine microbial food web.In recent years,organisms living in the deep sea(>1000 m water depth)have increasingly become the focus of research;however,studies on protistan assemblages are relatively scarce compared with their prokaryotic counterparts.In the present study,high-throughput sequencing of the hypervariable V9 region of the 18S rRNA gene was used to explore the community composition of protists in bathypelagic waters of the South China Sea.Based on the analysis of the alpha and beta diversities of 14 samples,we discovered:1、members belonging to Rhizaria,Alveolata,and Excavata were the dominant groups in terms of both relative sequence abundance and operational taxonomic unit(OTU)richness in all samples,although their relative contributions differed among different samples;2、cluster analysis showed that the distribution of protistan assemblages was related neither to the sampling location nor to the water depth,and other environmental factors might have caused the differences among the communities;3、phototrophs,including members of the Bacillariophyta,Bolidophyceae,Dictyochophyceae,Prasinophyceae,and Prymnesiophyceae,were detected in all samples,which indicated their contributions to the downward transportation via the biological pump and the potential presence of phagotrophy of these phototrophic cells in the deep ocean.

Characteristics of microbial community involved in early biofilms formation under the influence of wastewater treatment plant effluent

Effluents from wastewater treatment plants（WWTPs） containing microorganisms and residual nutrients can influence the biofilm formation. Although the process and mechanism of bacterial biofilm formation have been well characterized, little is known about the characteristics and interaction of bacteria, archaea and eukaryotes in the early colonization, especially under the influence of WWTP effluent. The aim of this study was to characterize the important bacterial, archaeal and eukaryotic species in the early stage of biofilm formation downstream of the WWTP outlet. Water and biofilm samples were collected 24 and 48 hr after the deposition of bio-cords in the stream. Illumina Miseq sequencing of the 16 S and 18 S rDNA showed that, among the three domains, the bacterial biofilm community had the largest alpha and beta diversity. The early bacterial colonizers appeared to be ＂biofilm-specific＂, with only a few dominant operational taxonomic units（OTUs） shared between the biofilm and the ambient water environment. Alpha-proteobacteria and Ciliophora tended to dominate the bacterial and eukaryotic communities, respectively, of the early biofilm already at 24 hr, whereas archaea played only a minor role during the early stage of colonization. The network analysis showed that the three domains of microbial community connected highly during the early colonization and it might be a characteristic of the microbial communities in the biofilm formation process where co-occurrence relationships could drive coexistence and diversity maintenance within the microbial communities.

Dynamics of Haptophytes(Eukaryota,Hacrobia)Revealed via Short-Term Sampling in the South China Sea

Haptophytes(Eukaryota,Hacrobia)play a crucial role in the energy budget and element cycling of diverse aquatic ecosystems due to their ability to engage in both phototrophic and mixotrophic nutritional modes.Nevertheless,there is a significant lack of knowledge regarding the short-term variations,such as diel dynamics,of their ecological features.During a short time frame in the summer of 2018,samples were collected from three distinct water layers in the South China Sea,including surface water,the deep chlorophyll maximum(DCM)layer,and 200 m depth.Fluorescence in situ hybridization coupled with tyramide signal amplification was used to quantify haptophyte cell abundance.Most haptophyte communities in all three water layers were composed of cells 2-5μm in size,while the proportion of cells<2μm increased with water depth.High-throughput sequencing of the V4 hypervariable regions of the SSU rRNA revealed that Chrysochromulina and Phaeocystis predominated the community,and the former was more abundant in the surface water and 200 m depth and the latter was more abundant in the DCM layer.Higher abundance of small cells(<2μm and 2-5μm)during the night was found compared to the day time,whereas large cells(5-10μm and 10-20μm)were more prevalent during the day time.The results of correlation analyses showed that haptophyte abundance was possibly impacted by both environmental biotic(heterotrophic nanoflagellates,heterotrophic bacteria,and viruses)and abiotic(temperature,salinity,and nutrients)factors.