Interaction between Raphidiopsis raciborskii and rare bacterial species revealed by dilution-to-extinction experiments

Role of microalgae-bacterial consortium in wastewater treatment: A review

In the global effort to reduce CO2 emissions, the concurrent enhancement of pollutant degradation and reductions in fossil fuel consumption are pivotal aspects of microalgae-mediated wastewater treatment. Clarifying the degradation mechanisms of bacteria and microalgae during pollutant treatment, as well as regulatory biolipid production, could enhance process sustainability. The synergistic and inhibitory relationships between microalgae and bacteria are introduced in this paper. The different stimulators that can regulate microalgal biolipid accumulation are also reviewed. Wastewater treatment technologies that utilize microalgae and bacteria in laboratories and open ponds are described to outline their application in treating heavy metal-containing wastewater, animal husbandry wastewater, pharmaceutical wastewater, and textile dye wastewater. Finally, the major requirements to scale up the cascade utilization of biomass and energy recovery are summarized to improve the development of biological wastewater treatment.

Strategies for regulating the intensity of different cyanobacterial blooms: Insights from the dynamics and stability of bacterioplankton communities

The occurrence of cyanobacterial blooms is increasing in frequency and magnitude due to climate change and human activities, which poses a direct threat to drinking water security. The impacts of abiotic and biotic factors on the development of blooms have been well studied; however, control strategies for different bloom intensities have rarely been explored from the perspective of the dynamics and stability of bacterioplankton communities. Here, a network analysis was used to investigate the interactions and stability of microbial communities during different periods of R. raciborskii bloom in an inland freshwater lake. The abundance and diversity of rare taxa were significantly higher than that of abundant taxa throughout the bloom cycle. At the pre-bloom (PB) stage, microbial interactions among the different bacterial groups were weak but strongly negatively correlated, indicating low robustness and weak disturbance resistance within the community. However, community stability was better, and microbial interactions became more complicated at the high-bloom (HB) and low-bloom (LB) stages. Interestingly, rare taxa were significantly responsible for community stability and connectivity despite their low relative abundance. The Mantel test revealed that Secchi depth (SD), orthophosphate (PO43--P), and dissolved oxygen (DO) were significantly positively correlated with abundant taxa, rare taxa and PB. DO was significantly positively correlated with HB, intermediate taxa, and rare taxa, while water temperature (WT), N/P and total nitrogen (TN) were significantly positively correlated with LB, abundant taxa, intermediate taxa, and rare taxa. These findings suggest that reducing the PO43--P concentration at the PB stage may be an effective approach to preventing the development of R. raciborskii blooms, while regulating rare taxa at the HB and LB stages may be a key factor in controlling R. raciborskii blooms.

Coupling of cylindrospermopsin and pho-harboring Verrucomicrobia supports the formation of Raphidiopsis blooms in low-phosphorus waters

Cylindrospermopsin (CYN) can induce phytoplankton community to secrete alkaline phosphatase (ALP), which is one of the important strategies for the bloom-forming cyanobacterium Raphidiopsis to thrive in extremely low-phosphorus (P) waters. However, how bacterioplankton community, another major contributor to ALPs in waters, couples to Raphidiopsis through CYN, and the role of this coupling in supporting the dominance of Raphidiopsis in nature remain largely unknown. Here, we conducted microcosm experiments to address this knowledge gap, using a combination of differential filtration-based and metagenomics-based methods to identify the sources of ALPs. We found that, compared with algal-derived ALPs, bacteria-derived ALPs exhibited a more pronounced and sensitive response to CYN. This response to CYN was enhanced under low-P conditions. Interestingly, we found that Verrucomicrobia made the largest contribution to the total abundance of pho genes, which encode ALPs. Having high gene abundance of the CYN-sensing PI3K-AKT signaling pathway, Verrucomicrobia's proportion increased with higher concentrations of CYN under low-P conditions, thereby explaining the observed increase in pho gene abundance. Compared with other cyanobacterial genera, Raphidiopsis had a higher abundance of the pst gene. This suggests that Raphidiopsis exhibited a greater capacity to uptake the inorganic P generated by ALPs secreted by other organisms. Overall, our results reveal the mechanism of CYN-induced ALP secretion and its impact on planktonic P-cycling, and provide valuable insights into the role of CYN in supporting the formation of Raphidiopsis blooms.

Eutrophication increases the similarity of cyanobacterial community features in lakes and reservoirs

Eutrophication of inland waters is a mostly anthropogenic phenomenon impacting aquatic biodiversity worldwide, and might change biotic community structure and ecosystem functions. However, little is known about the patterns of cyanobacterial community variations and changes both on alpha and beta diversity levels in response to eutrophication. Here, we investigated cyanobacterial communities sampled at 140 sites from 59 lakes and reservoirs along a strong eutrophication gradient in eastern China through using CPC-IGS and 16S rRNA gene amplicon sequencing. We found that taxonomic diversity increased, but phylogenetic diversity decreased significantly along the eutrophication gradient. Both niche width and niche overlap of cyanobacteria significantly decreased from low- to high-nutrient waterbodies. Cyanobacterial community distance-decay relationship became weaker from mesotrophic to hypereutrophic waterbodies, while ecological uniqueness (i.e., local contributions to beta diversity) tended to increase in high-nutrient waterbodies. Latitude and longitude were more important in shaping cyanobacterial community structure than other environmental variables. These findings suggest that eutrophication affects alpha and beta diversity of cyanobacterial communities, leading to increasingly similar community structures in lakes and reservoirs with a higher level of eutrophication. Our work highlights how cyanobacterial communities respond to anthropogenic eutrophication and calls for an urgent need to develop conservation and management strategies to control lake eutrophication and protect freshwater biodiversity.

Relic DNA obscures DNA-based profiling of multiple microbial taxonomic groups in a river-reservoir ecosystem

Numerous studies have investigated the spatiotemporal variability in water microbial communities, yet the effects of relic DNA on microbial community profiles, especially microeukaryotes, remain far from fully understood. Here, total and active bacterial and microeukaryotic community compositions were characterized using propidium monoazide (PMA) treatment coupled with high-throughput sequencing in a river-reservoir ecosystem. Beta diversity analysis showed a significant difference in community composition between both the PMA untreated and treated bacteria and microeukaryotes; however, the differentiating effect was much stronger for microeukaryotes. Relic DNA only resulted in underestimation of the relative abundances of Bacteroidota and Nitrospirota, while other bacterial taxa exhibited no significant changes. As for microeukaryotes, the relative abundances of some phytoplankton (e.g. Chlorophyta, Dinoflagellata and Ochrophyta) and fungi were greater after relic DNA removal, whereas Cercozoa and Ciliophora showed the opposite trend. Moreover, relic DNA removal weakened the size and complexity of cross-trophic microbial networks and significantly changed the relationships between environmental factors and microeukaryotic community composition. However, there was no significant difference in the rates of temporal community turnover between the PMA untreated and treated samples for either bacteria or microeukaryotes. Overall, our results imply that the presence of relic DNA in waters can give misleading information of the active microbial community composition, co-occurrence networks and their relationships with environmental conditions. More studies of the abundance, decay rate and functioning of nonviable DNA in freshwater ecosystems are highly recommended in the future.

Spatiotemporal diversity and community structure of cyanobacteria and associated bacteria in the large shallow subtropical Lake Okeechobee (Florida, United States)

Lake Okeechobee is a large eutrophic, shallow, subtropical lake in south Florida, United States. Due to decades of nutrient loading and phosphorus rich sediments, the lake is eutrophic and frequently experiences cyanobacterial harmful algal blooms (cyanoHABs). In the past, surveys of the phytoplankton community structure in the lake have been conducted by morphological studies, whereas molecular based studies have been seldom employed. With increased frequency of cyanoHABs in Lake Okeechobee (e.g., 2016 and 2018 Microcystis-dominated blooms), it is imperative to determine the diversity of cyanobacterial taxa that exist within the lake and the limnological parameters that drive bloom-forming genera. A spatiotemporal study of the lake was conducted over the course of 1 year to characterize the (cyano)bacterial community structure, using 16S rRNA metabarcoding, with coincident collection of limnological parameters (e.g., nutrients, water temperature, major ions), and cyanotoxins. The objectives of this study were to elucidate spatiotemporal trends of community structure, identify drivers of community structure, and examine cyanobacteria-bacterial relationships within the lake. Results indicated that cyanobacterial communities within the lake were significantly different between the wet and dry season, but not between periods of nitrogen limitation and co-nutrient limitation. Throughout the year, the lake was primarily dominated by the picocyanobacterium Cyanobium. The bloom-forming genera Cuspidothrix, Dolichospermum, Microcystis, and Raphidiopsis were highly abundant throughout the lake and had disparate nutrient requirements and niches within the lake. Anatoxin-a, microcystins, and nodularins were detected throughout the lake across both seasons. There were no correlated (cyano)bacteria shared between the common bloom-forming cyanobacteria Dolichospermum, Microcystis, and Raphidiopsis. This study is the first of its kind to use molecular based methods to assess the cyanobacterial community structure within the lake. These data greatly improve our understanding of the cyanobacterial community structure within the lake and the physiochemical parameters which may drive the bloom-forming taxa within Lake Okeechobee.