Community Structure of Eukaryotic Phytoplankton in Wetland of Golmud River and Its Lower Reaches and Relative Environmental Factors

Abstract

Identification of the community structure of phytoplankton is an important link in studying the structure and function of aquatic ecosystems. However, research on the community structure of phytoplankton in the Golmud area is very scarce at present. To explore the composition of phytoplankton in the Golmud area, eukaryotic phytoplankton and environmental parameters were sampled in September 2020. The composition and diversity of the eukaryotic phytoplankton community were determined by microscopic observation and Illumina high-throughput sequencing technology. The results showed that a total of 40 species of eukaryotic phytoplankton from 4 phyla were identified by microscopic observation, and 154 species of eukaryotic phytoplankton from 7 phyla were obtained by high-throughput sequencing, mainly Bacillariophyta and Chlorophyta. The dominant species were Dunaliella sp., Teleaulax sp., Parvodinium mixtum, and Lindavia radiosa. The eukaryotic phytoplankton density in summer was 0.09–12.08 × 10⁵ cells/L, the total biomass was 0.002–0.55 mg/L, and the concentration of chlorophyll-a was 0.00–0.91 μg/L. Multiple α diversity indices showed that the diversity of water in different salinities was in the order of freshwater > brackish > saline. The β diversity results showed that the eukaryotic phytoplankton community composition was more similar in water with the same salinity. The redundancy analysis results of water with different salinities showed that environmental factors susc as salinity, total phosphorus concentration, and dissolved oxygen affected the eukaryotic phytoplankton community structure, among which salinity had the greatest influence.

1. Introduction

As the primary producers in the water ecosystem, phytoplankton is the beginning of the material cycle and energy flow in the water ecosystem [1,2]. Located in the first link in the food chain, the type and quantity of phytoplankton are also closely related to the number of biological populations in the next link in the food chain [3,4]. In addition, the species and density of phytoplankton can also be used as indicators of water quality [5,6]. Therefore, studying the composition and structure of phytoplankton is an important link to studying the structure and function of aquatic ecosystems.

The influencing factors of the phytoplankton community structure are complex, and its dynamic evolution is usually a result of the joint action of multiple environmental factors at different times and in different spaces [7,8]. Previous studies have shown that the compositional characteristics and community diversity of phytoplankton communities are affected by multiple environmental factors in water bodies. For example, factors, such as water temperature, nutrient concentration [9], salinity [10], and electrical conductivity [11], can all affect eukaryotic phytoplankton communities’ structure. With the ability to quickly and accurately reveal information about some unknown taxa and better understand the ecological factors that affect their distribution patterns [12], high-throughput sequencing technologies are increasingly used to determine the species composition in communities [13,14,15]. The diversity index mainly includes species diversity on three spatial scales of α, β and γ diversity [16,17,18]. The eukaryotic phytoplankton diversity in this study was mainly judged using the α and β diversity indices since only the diversity within and between habitats was discussed in this study and did not involve large-scale regional diversity.

“Golmud” is a Mongolian transliteration that means “a place with dense rivers”. Golmud City is affiliated to the Haixi Mongolian and Tibetan Autonomous Prefecture of Qinghai Province, which is located in the central and western parts of Qinghai Province and the hinterland of the Qinghai-Tibet Plateau. As the second largest river in the Qaidam Basin, the Golmud River provides a large amount of freshwater recourses and precious records for studies, such as climate change, glaciers, and hydrology [19]. However, few studies have investigated the composition and structure of the eukaryotic phytoplankton community and the effects of environmental factors on phytoplankton diversity in the Golmud River Basin so far. This study not only analyzed the eukaryotic phytoplankton community structure characteristics of the Golmud River and the downstream wetlands but also analyzed its correlation with environmental factors, aiming to identify the structure of the eukaryotic phytoplankton community and provide basic data for the study of the structure and function of the water ecosystem in this region.

2. Materials and Methods

Sampling and environmental factors were measured. In September 2020, eukaryotic phytoplankton and environmental parameters were sampled at 10 sampling sites with different salinity in the Golmud. These samples were identified as saline (DBX01, DBX03), freshwater (DBX09, DBX10), or brackish (DBX02, DBX04–DBX08) according to the water salinity. The specific location of the sampling point is shown in Figure 1 (Deteailed information of sample sites in Table S1). Environmental factors, such as pH, dissolved oxygen (DO), water temperature (Temp), electrical conductivity (EC), total dissolved solution (TDS), and salinity (Sal), were measured with a YSI multi-parameter water quality analyzer. Total phosphorus (TP), total nitrogen (TN), ammonium (NH4-N, mg/L), and chemical oxygen demand (COD) were measured according to Analytical Methods for the Detection of Water and Wastewater (Fourth Edition).

DNA extraction and Illumina sequencing were performed. DNA was extracted using the OMEGA water kit. PCR amplification was performed using the universal primers of the eukaryotic 18S rDNA V4 region. The PCR products were sequenced using an Illumina Miseq platform.

For the sequencing analyses, QIIME2.0 and Usearch v11 were used for splicing, quality control, and OTU segmentation of the sequencing data. Pearson correlation analysis was used to test the correlation between different environmental factors. The alpha diversity indices ACE, Chao1, Shannon, and Simpson; Pielou indices; and Bray–Cruits indices were calculated in R Studio using the vegan package. Canoco 5.0 was used for trend correspondence analysis (detrended correspondence analysis, DCA). In this study, the DCA value was 1.97, and the redundancy components analysis (RDA) method was selected. PcoA clustering and RDA ranking were calculated in R studio using the vegan package and the plotting of the correlation charts was performed in R studio using the ggplot2 package.

3. Results

3.1. Results of Microscopic Observation

In total, 40 species from 4 eukaryotic planktonic phyla were identified in this survey. Among them, Bacillariophyta (27 species) and Chlorophyta (10 species) were the most abundant, and 2 species were identified in Cryptophyta and 1 species in Dinophyta. The concentration of eukaryotic phytoplankton chlorophyll-a was 0.00–0.91 μg/L, the density was 0.09–12.08 × 10⁵ cells/L, and the total biomass was 0.002–0.55 mg/L (Table S2). Species with a dominance degree greater than 0.02 were selected as the dominant species. The main dominant species in saline was Dunaliella sp. (dominance: 0.972), the main dominant species in brackish water were Teleulax sp. (dominance: 0.319) and Parvodinium mixtum (dominance: 0.137), and the main dominant species in freshwater were Lindavia radiosa (dominance: 0.573) and Synedra sp. (dominance: 0.554).

3.2. Results of High-Throughput Sequencing

After being compared with the Genbank database, 154 species of eukaryotic phytoplankton were obtained from 7 phyla, including Bacillariophyta, Chlorophyta, Ochrophyta, Cryptophyta, Dinophyta, Streptophyta, and Haptophyta. The total OTU abundance and the proportion of OTU abundance of each phylum are shown in Figure 2. Overall, Chlorophyta, Bacillariophyta, and Cryptophyta were more abundant. Species with a relative OTU abundance of more than 10% at each site were defined as the dominant species (Table S3). The dominant species in saline was Dunaliella sp. The dominant species in brackish water were Dunaliella sp., Tetraselmis cordiformis, Chlamydomonas sp., Cocconeis euglypta, Nitzschia bergii, Pelagodinium sp., Cryptophyceae sp., and Spumella sp. The dominant species in freshwater areas were Asulcocephalium miricentonis, Dinophyceae sp., Lindavia radiosa, Microglena monadina, and Rhodomonas minuta.

3.3. Analysis of Alpha and Beta Diversity Index

The larger the Shannon index and the Simpson index value, the higher the species diversity of the sample. The results of the alpha diversity index (Table S4) and the high-throughput sequencing results showed that the number of OTUs in each sampling site in the Golmud wetland was 63–154, the ACE index was 59.26–130.88, the Chao1 index was 57.26–130.58, the Shannon index was 0.18–3.40, the Simpson index was 0.05–0.94, and the Pielou index was 0.04–0.67. All indices were the highest at DBX10 and the lowest at DBX03. The α diversity of eukaryotic phytoplankton in waters with different salinity in the Golmud showed the following order: freshwater > brackish > saline.

The results of the principal co-ordinate analysis showing differences in the eukaryotic phytoplankton community between the different samples based on the distance matrix are shown in Figure 3. The algal community clustering results showed that the total interpretation degree of the first and second principal axes was 42.89%. The closer the sample points on the coordinate map, the greater the similarity of the species composition between the communities. Therefore, it can be concluded that DBX01 and DBX03 were highly similar, DBX09 and DBX10 were highly similar, and the remaining six samples showed high similarities. Specifically, water bodies with different salinities clustered into three categories.

3.4. Correlation between the Phytoplankton Community and Environmental Factors

The environmental parameters are shown in Table S5. The Pearson correlation test was used to screen the environmental factors, and five environmental factors, including Temp, DO, COD, TP, and Sal, were retained for subsequent analysis. The results of the redundancy analysis showed that the first and second axes explained 63.84% and 16.93%, respectively (Figure 4). The main environmental factors affecting the eukaryotic plant community structure in the Golmud and downstream wetlands were salinity, total phosphorus concentration, and dissolved oxygen, among which the salinity had the highest degree of influence.

4. Discussion

A total of 40 species of eukaryotic phytoplankton in 4 phyla were identified by microscopic observation, including Bacillariophyta, Chlorophyta, Cryptophyta, and Dinophyta, among which Bacillariophyta and Chlorophyta were dominant. This is consistent with previous research results on the phytoplankton community structure in the Golmud River in autumn [20]. The chlorophyll-a concentration of eukaryotic phytoplankton was less than 1 μg/L, the density was in the degree of 10⁴–10⁵ cells/L, and the total biomass was 0.002–0.55 mg/L, indicating the oligotrophic water characteristic in the Golmud wetland.

A total of 154 species of eukaryotic phytoplankton from 7 phyla were detected by high-throughput sequencing. Overall, the abundances of Chlorophyta, Bacillariophyta, and Cryptophyta were dominant, which is consistent with the microscopic observations. Both methods indicated that Dunaliella sp. is the dominant species in the saline area, Teleaulax sp. and Parvodinium mixtum in the brackish water area, and Lindavia radiosa in the freshwater area. This is due to Dunaliella sp.’s ability to tolerate a wide salinity range of about 0.2–35% NaCl [21], and can thus adapt to a high-salinity environment. Therefore, Chlorophyta algae have an advantage in the saline area. In brackish waters, Parvodinium mixtum was found in a habitat with an altitude of 1089–1784 m and a water temperature of 6–9 °C [22], which is close to the habitat in this study, suggesting that Parvodinium mixtum are algae that only survive at high altitudes and in cold water. The dominant species in the freshwater area was Lindavia radiosa. This is because this sample site is a reservoir in the upper reaches of the Golmud River, and Lindavia radiosa is a planktonic algae that grows in still water bodies, such as reservoirs. Due to the particle size of some algae being too small to be observed under an optical microscope, the number of quantitatively observed species was less than the number of high-throughput sequencing species. Previous studies have shown that the identification of species should not only rely on traditional morphological methods but molecular data should also be used if possible [23].

According to the alpha diversity index, species diversity was highest in freshwater areas and lowest in saline areas. This is because an increase in the water salinity places phytoplankton under increased stress due to high osmotic pressure. At this time, cells tend to lose water, shrink, and die. Only phytoplankton that can absorb some low-molecular-weight substances from the environment to adjust their osmotic pressure can survive [24]. Previous studies have shown that the β diversity may be affected by environmental factors, such as matrix heterogeneity [25], the nitrogen-to-phosphorus ratio [26], pH, and salinity [27]. The results of the β-diversity index in this study showed that the 10 samples were clustered into 3 parts according to the differences in salinity, indicating that the β-diversity of eukaryotic phytoplankton in the Golmud River Basin is related to salinity. In high-altitude areas, such as the Qinghai-Tibet Plateau, environmental factors, such as salinity and nutrients, have a significant impact on the microbial diversity and community structure [28,29,30]. The RDA analysis results of this study also showed that the environmental factors affecting eukaryotic phytoplankton in the study area include salinity, total phosphorus concentration, and dissolved oxygen, among which salinity is the main environmental driving factor and has the highest impact on the diversity of eukaryotic phytoplankton in the Golmud River Basin.

5. Conclusions

In summary, this study preliminarily obtained the composition of the eukaryotic phytoplankton community in the Golmud River and downstream wetlands, thus providing basic data for the monitoring, management, and protection of water ecosystems in this region.

(1)

Bacillariophyta and Chlorophyta were dominant phyla in the Golmud area. The density of the eukaryotic phytoplankton in this area was 0.09–12.08 × 10⁵ cells/L and the biomass was 0.002–0.55 mg/L.

(2)

Dunaliella sp. was the dominant species in saline water, Teleaulax sp. and Parvodinium mixtum were the dominant species in brackish water, and Lindavia radiosa was dominant in freshwater. Moreover, Parvodinium mixtum only exists in high-altitude and cold-water areas.

(3)

Differences in the eukaryotic phytoplankton community structure between water bodies with different salinity were identified. Salinity influences the alpha and beta diversity of phytoplankton communities.