Parallel assessment of marine autotrophic picoplankton using flow cytometry and chemotaxonomy

Seasonal Dynamics of Photosynthetic Picoeukaryotes in a Monsoon-Influenced Tropical Bay: a Flow Cytometric and Chemotaxonomic Approach

The composition and ecology of photosynthetic picoeukaryotes (PPE) are essential for understanding microbial food web functioning. We hypothesize that the simultaneous use of flow cytometry (FCM) and high-performance liquid chromatography (HPLC) tools will aid in discerning the dominant PPE groups contributing to abundance and biomass under prevailing environmental conditions. The PPE seasonal community abundance and pigment biomass were investigated from a southwest monsoon-influenced tropical bay from June 2015 to May 2016. A size-fractionated (<3 µm) approach using FCM and HPLC revealed five and six PPE groups, respectively. Picocryptophytes dominated the PPE biomass under varied environmental conditions, whereas picodiatoms contributed substantially, being abundant under turbulent, low-temperature, nutrient (NO₃^-, SiO₄^4-)-enriched conditions. The picochlorophytes dominated the community numerically. The relatively higher abundance and biomass of picoprasinophytes and a positive correlation with NO₃^- and NH₄⁺ imply proliferation under higher nutrient concentrations. The least contributors to biomass were dinoflagellates and picoprymnesiophytes. The relatively larger cell size of picocryptophytes and picodiatoms resulted in higher cumulative biomass, signifying their role in the microbial food web. Our study proposes incorporation of additional indicator pigments in algorithms used to estimate coastal picophytoplankton contribution to total phytoplankton biomass to avoid discrepancies.

Potential influence of overwintering benthic algae on water quality

Overwintering benthic algae not only directly impact drinking water safety, but also affect the algae recruitment in warm spring seasons. Thus, understanding the characteristics of overwintering benthic algae can provide scientific references for formulating preventative strategies of reasonable water resource. However, they have received less attention. In this study, the spatiotemporal variation of benthic algae and their harmful secondary metabolites were studied from autumn to summer in Qingcaosha Reservoir. Benthic algae (picophytobenthos accounting for 55.42%) had a high biomass during overwintering, and the groups of overwintering benthic algae included pico-Cyanobacteria, pico-Cryptomonas, pico-Chlorophyta, pico-Diatoms, Cyanobacteria, Chlorophyta, Cryptomonas and Diatoms, which were consistent with the planktonic algae species in warm seasons. In oligotrophic or mesotrophic water bodies, micronutrients of iron and manganese were key nutrient factors influencing the biomass of benthic algae. Furthermore, picophytobenthos were important potential contributors of harmful secondary metabolites. The content of microcystins, anatoxin-a, geosmin and 2-methylisoborneol in sediment were 15.75 µg/kg·FW, 48.16 µg/kg·FW, 3.91 ng/kg·FW, and 11.76 ng/kg·FW during winter, which had potential to be released into water bodies to impact water quality. These findings indicate that water quality monitoring programs need to consider sediment in winter as a potential source of toxins and preventative measures to prevent excessive proliferation of algae should be implemented in winter.

Factors influencing the pigment composition and dynamics of photoautotrophic picoplankton in shallow eutrophic lakes

Photoautotrophic picoplankton (0.2–2 μm) can be a major contributor to primary production and play a significant part in the ecosystem carbon flow. However, the understanding about the dynamics of both eukaryotic and prokaryotic components of picoplankton in shallow eutrophic freshwater environments is still poor. Very few studies in these ecosystems reveal the taxonomic composition of picoeukaryotes. The main objective of this study was to investigate the seasonal dynamics of phototrophic picoplankton with the emphasis on the eukaryote community composition in a large shallow, eutrophic lake of the northern temperate zone (Lake Võrtsjärv). Phytoplankton pigments were employed to determine the taxonomic composition of photoautotrophic picoplankton. We found out that photoautotrophic picoplankton constitutes an important part of the phytoplankton community in Lake Võrtsjärv and its contribution can be highly variable (from ~9.3% to ~39%) in different years. The eukaryotic photoautotrophic picoplankton was dominated by diatoms followed by chrysophytes and other minor groups. Picoeukaryotes were prevailing in low-light conditions and low temperatures as their predominance in the picoplankton community was tightly linked to the presence or absence of ice cover. Ice cover strongly suppressed the growth of picocyanobacteria. Total phosphorus, turbidity and metazooplankton abundance had a clear relationship with photoautotrophic picoplankton chlorophyll a.

Seasonality of Coastal Picophytoplankton Growth, Nutrient Limitation, and Biomass Contribution

Picophytoplankton in the Baltic Sea includes the simplest unicellular cyanoprokaryotes (Synechococcus/Cyanobium) and photosynthetic picoeukaryotes (PPE). Picophytoplankton are thought to be a key component of the phytoplankton community, but their seasonal dynamics and relationships with nutrients and temperature are largely unknown. We monitored pico- and larger phytoplankton at a coastal site in Kalmar Sound (K-Station) weekly during 2018. Among the cyanoprokaryotes, phycoerythrin-rich picocyanobacteria (PE-rich) dominated in spring and summer while phycocyanin-rich picocyanobacteria (PC-rich) dominated during autumn. PE-rich and PC-rich abundances peaked during summer (1.1 × 10⁵ and 2.0 × 10⁵ cells mL^–1) while PPE reached highest abundances in spring (1.1 × 10⁵ cells mL^–1). PPE was the main contributor to the total phytoplankton biomass (up to 73%). To assess nutrient limitation, bioassays with combinations of nitrogen (NO₃ or NH₄) and phosphorus additions were performed. PE-rich and PC-rich growth was mainly limited by nitrogen, with a preference for NH₄ at >15°C. The three groups had distinct seasonal dynamics and different temperature ranges: 10°C and 17–19°C for PE-rich, 13–16°C for PC-rich and 11–15°C for PPE. We conclude that picophytoplankton contribute significantly to the carbon cycle in the coastal Baltic Sea and underscore the importance of investigating populations to assess the consequences of the combination of high temperature and NH₄ in a future climate.

Picophytoplankton identification by flow cytometry and high-throughput sequencing in a clean reservoir

Understanding picophytoplankton variations that play important roles in the material circulation and energy flow are critical to assessing overall status of waterbody, especially for clean reservoirs which remain a relatively stable community structure and high species diversity due to lower nitrogen and phosphorus nutrients. However, their response to key environmental factors and tightly acting microbial remains poorly understood. Traditional quantification methods are limited, such as chlorophyll-a, turbidity and microscope. There are still many defects with present molecular analysis. In this study, a flow cytometric analysis and high-throughput sequencing combination methodology was developed and tested on clean water from a reservoir, by a monthly dynamic for a vegetative period April-September in 2019 to improve the accuracy of dynamic monitoring for the picophytoplankton system. More species of Pico-Cyanobacteria and Pico-Eukaryotes were discovered. The increased percentage of pigment compounds from 8.2% to 76.3% proves the effective reduce of heterotrophic disturbing and enrichment of target populations. Picophytoplankton that was previously neglected due to their low relative abundance has once again entered the scope of our eyes. Phytoplankton were divided into three categories. The first one was the highly abundant and frequently present taxa, the second one was the low-abundance but highly-transient population, and the third one was the low abundance and stable group. Synechococcus, Emiliania, Tetraselmis and Thalassiosira were dominant picophytoplankton and displayed obvious temporal and spatial distribution characteristics. Pico-PE rich Cyanobacteria and Nano-Eukaryotes with high transience abnormally increased in summer. Temperature, ammonia-N, nitrate-N, turbidity and total nitrogen were most influencing factors, while some picophytoplankton with special physiological structure showed distinct competitive advantages in the microbial community. As for the off-flavor compounds, the concentration of 2-methylisoborneol and geosmin were high even 66.7% and 20.8% of the samples exceeded their olfactory threshold. Chrysochromuina, Planktothrix and Microcystis might be the potential producers.

Determining temporal and spatial distribution of autotrophic picoplankton community composition through HPLC-pigment method and flow cytometry in the central Bohai Sea (China)

The temporal-spatial distribution of marine autotrophic picoplankton (APP) in the central Bohai Sea was investigated in April (spring), June (early summer), August (summer), and October (autumn) in 2015 through a combination of HPLC-pigment method and flow cytometry. Flow cytometry results showed that APP was composed of Synechococcus (Syn) and pico-eukaryotes (PEUKs). The lowest average abundances of Syn and PEUKs was obtained in April. Afterward, the average APP density substantially increased, and Syn dominated the total cell abundances. Although generally outnumbered by Syn, PEUKs were the larger contributor to total APP carbon biomass (>52%) in all the cruises, except in August, when Syn bloomed. Compared with the cytometric method, HPLC-pigment CHEMTAX revealed a more sophisticated diversity of APP community. In April, diatoms were the main contributor to pico-Chl a, whereas prasinophytes became the main contributor in June and October. Syn bloom was evidenced by CHEMTAX, which revealed that it contributed 69.3% of Chl a in August. Redundancy analysis suggested that temperature was the main factor influencing the distribution of APP. Moreover, nutrients and their structures had some effects, which depended on different APP groups in the area. The accordance between CHEMTAX and cytometric method was evaluated through correlation analysis. A significantly positive correlation between cell abundance and CHEMTAX-derived Chl a was observed for Syn in August and PEUKs in June. Nevertheless, further study is needed owing to the observed discrepancies between the methods.

A Facile and Sensitive DNA Sensing of Harmful Algal Blooms Based on Graphene Oxide Nanosheets

Gene detection has important applications in biology, biomedical engineering, clinical, environmental, and marine fields. Rapid, sensitive, and selective recognition of specific genes is essential in practical applications. In this study, we describe a facile and sensitive DNA sensing platform for specific and quantitative detection of Heterosigma akashiwo, which is one of the causative agents of red tides. Fast and sensitive detection is achieved by using chemically synthesized graphene oxide (GO) nanosheets. Probe DNA is designed according to the specific DNA fragments of harmful algae and labeled with fluorescent molecules FAM (fluorescein-based dye). GO nanosheet solution is made, in which the strong interaction between FAM-labeled probe and GO nanosheets keeps them in close proximity, facilitating the fluorescence quenching of the fluorophore by GO nanosheets. In the presence of a complementary target DNA, the FAM-labeled DNA probe and the target DNA hybridize and desorb from the surface of GO nanosheets, resulting in restoration of fluorophore fluorescence. The concentration of target DNA fragments is analyzed by the fluorescence intensity at ~ 520 nm with emission wavelength of 480 nm. The sensitive detecting platform achieved stable measurement of 1 pM specific genes from Heterosigma akashiwo. Our GO nanosheet-based DNA-sensing platform performs fast and sensitive detection of trace amount of DNA, and enables quantitative recognition of harmful algae, which has promising applications in real-time monitoring in the marine environment of red tide generative dynamics, allowing effective control, particularly in relation to marine aquaculture.

Phylogenetic Responses of Marine Free-Living Bacterial Community to Phaeocystis globosa Bloom in Beibu Gulf, China

Phaeocystis globosa blooms are recognized as playing an essential role in shaping the structure of the marine community and its functions in marine ecosystems. In this study, we observed variation in the alpha diversity and composition of marine free-living bacteria during P. globosa blooms and identified key microbial community assembly patterns during the blooms. The results showed that the Shannon index was higher before the blooming of P. globosa in the subtropical bay. Marinobacterium (γ-proteobacteria), Erythrobacter (α-proteobacteria), and Persicobacter (Cytophagales) were defined as the most important genera, and they were more correlated with environmental factors at the terminal stage of P. globosa blooms. Furthermore, different community assembly processes were observed. Both the mean nearest relatedness index (NRI) and nearest taxon index (NTI) revealed the dominance of deterministic factors in the non-blooming and blooming periods of P. globosa, while the bacterial communities in marine waters after the blooms tended to be controlled by stochastic factors. Our findings revealed that the assembly of the bacterial community in marine P. globosa blooms is a complex process with mixture effects of marine microbiomes and environmental parameters.