Cyanobacteria vs green algae: which group has the edge?

An examination of protist diversity in serpentinization-hosted ecosystems of the Samail Ophiolite of Oman

In the Samail Ophiolite of Oman, the geological process of serpentinization produces reduced, hydrogen rich, hyperalkaline (pH > 11) fluids. These fluids are generated through water reacting with ultramafic rock from the upper mantle in the subsurface. On Earth's continents, serpentinized fluids can be expressed at the surface where they can mix with circumneutral surface water and subsequently generate a pH gradient (∼pH 8 to pH > 11) in addition to variations in other chemical parameters such as dissolved CO2, O2, and H2. Globally, archaeal and bacterial community diversity has been shown to reflect geochemical gradients established by the process of serpentinization. It is unknown if the same is true for microorganisms of the domain Eukarya (eukaryotes). In this study, using 18S rRNA gene amplicon sequencing, we explore the diversity of microbial eukaryotes called protists in sediments of serpentinized fluids in Oman. We demonstrate that protist community composition and diversity correlate significantly with variations in pH, with protist richness being significantly lower in sediments of hyperalkaline fluids. In addition to pH, the availability of CO2 to phototrophic protists, the composition of potential food sources (prokaryotes) for heterotrophic protists and the concentration of O2 for anaerobic protists are factors that likely shape overall protist community composition and diversity along the geochemical gradient. The taxonomy of the protist 18S rRNA gene sequences indicates the presence of protists that are involved in carbon cycling in serpentinized fluids of Oman. Therefore, as we evaluate the applicability of serpentinization for carbon sequestration, the presence and diversity of protists should be considered.

Preference of carbon absorption determines the competitive ability of algae along atmospheric CO2 concentration

Although many studies have focused on the effects of elevated atmospheric CO2 on algal growth, few of them have demonstrated how CO2 interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO2 concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO2 and HCO3 - could explain changes in competitiveness between algae species along the gradient of atmospheric CO2 concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO2 concentration was low, species with high affinity for both CO2 and HCO3 - (HCHH) had the highest competitiveness, followed by the species with high affinity for CO2 and low affinity for HCO3 - (HCLH), the species with low affinity for CO2 and high affinity for HCO3 - (LCHH) and the species with low affinity for both CO2 and HCO3 - (LCLH); when the CO2 concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO2 concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO2, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae.

Attenuation pathways of erythromycin and biochemical responses related to algal growth and lipid synthesis in a microalga-effluent system

Microalgal cultivation in municipal wastewater treatment plants (WWTPs) can realize the coupling of wastewater treatment and microalgae energy utilization, however, the residual antibiotics in effluents from WWTPs affect the growth of microalgae. In this study, green alga (Scenedesmus obliquus) cells were inoculated into the effluents to ascertain the attenuation pathways of erythromycin (ERY) and the biochemical responses of microalga in a microalga-effluent system. Results showed that hydrolysis, photolysis, and biodegradation (including bioadsorption) cause the attenuation of ERY in a microalga-effluent system, and the biodegradation (including bioadsorption) has the greatest removal rate (reaching a maximum of 57.87%), followed by hydrolysis (reaching a maximum of 34.13%), and photolysis (less than 5%) after five days. The photosynthetic pigment contents in cells of microalga decreased the most (by 35.66% for chlorophyll a), and the production of ROS was stimulated (by 33.75%) after five-day exposure to ERY at an initial concentration of 100 μg/L. Meanwhile, the activity of ribulose-1,5-biphosphate carboxylase (RuBPCase) decreased by 55.65%, and the activity of acetyl-CoA carboxylase (ACCase) increased by 55.65%. The ROS level, photosynthetic pigment content, and RuBPCase activity were extremely significantly correlated with each other (P < 0.01), indicating that exposure to ERY changed those biochemical responses related to the rate of photosynthesis of microalga, inhibiting the growth thereof. On the other hand, exposure to ERY increased lipid production by microalga through the induced ACCase activity.

Effects of rising atmospheric CO2 levels on physiological response of cyanobacteria and cyanobacterial bloom development: A review

Increasing atmospheric CO₂ concentration negatively impacts aquatic ecosystems and may exacerbate the problem of undesirable cyanobacterial bloom development in freshwater ecosystems. Elevated levels of atmospheric CO₂ may increase the levels of dissolved CO₂ in freshwater systems, via air-water exchanges, enhancing primary production in the water and catchments. Although high CO₂ levels improve cyanobacterial growth and increase cyanobacterial biomass, the impacts on their internal physiological processes can be more complex. Here, we have reviewed previous studies to evaluate the physiological responses of cyanobacteria to high concentrations of CO₂. In response to high CO₂ concentrations, the pressures of inorganic carbon absorption are reduced, and carbon concentration mechanisms are downregulated, affecting the intracellular metabolic processes and competitiveness of the cyanobacteria. Nitrogen and phosphorus metabolism and light utilization are closely related to CO₂ assimilation, and these processes are likely to be affected by resource and energy reallocation when CO₂ levels are high. Additionally, the responses of diazotrophic and toxic cyanobacteria to elevated CO₂ levels were specifically reviewed. The responses of diazotrophic cyanobacteria to elevated CO₂ concentrations were found to be inconsistent, probably because of differences in other factors in experimental designs. Toxic cyanobacteria tended to be superior to non-toxic strains at low levels of CO₂; however, the specific effects of microcystin on the regulation require further investigation. Furthermore, the effects of increasing CO₂ levels on cyanobacterial competitiveness in phytoplankton communities and nutrient cycling in aquatic ecosystems were reviewed. High CO₂ concentrations may make cyanobacteria less competitive relative to other algal taxa; however, due to the complexity of natural systems and the specificity of algal species, the dominant positions of the cyanobacteria do not seems to be changed. To better understand cyanobacterial responses to elevated CO₂ levels and help control cyanobacterial bloom developments, this review has identified key areas for future research.

Changes in pH and dissolved inorganic carbon in water affect the growth, saxitoxins production and toxicity of the cyanobacterium Raphidiopsis raciborskii ITEP-A1

Raphidiopsis raciborskii is a widely distributed, potentially toxic cyanobacterium described as a tropical-subtropical species. However, its occurrence in temperate regions has been expanding. Understanding the environmental factors underlying the expansion and colonization success of Raphidiopsis has been the object of numerous studies. However, less is known regarding its responses to pH and inorganic carbon in water. Thus, the aim of the present study was to investigate the effects of changes in pH and dissolved inorganic carbon on growth and saxitoxins production in the strain R. raciborskii ITEP-A1. We incubated batch cultures with different unbuffered and buffered pH (neutral-acid and alkaline) and inorganic carbon availability (CO₂-rich air bubbling and the addition of NaHCO₃) to assess the effect of these factors on the growth, toxin production as well as saxitoxins composition of the cyanobacterium. The carbon concentrating mechanism (CCM) system of ITEP-A1 was also characterized by an in silico analysis of its previously sequenced genome. The growth and saxitoxins production of R. raciborskii were affected. The addition of sodium bicarbonate and air bubbling enhanced the growth of the cyanobacterium in alkaline pH. In contrast, saxitoxins production and relative toxicity were decreased. Moreover, significant changes in the cellular composition of saxitoxins were strongly related to pH changes. ITEP-A1 potentially expresses the low-flux bicarbonate transporter BicA, an efficient CCM which uptakes most of its carbon from HCO₃^-. Hence, increasing the diffusion of CO₂ in alkaline eutrophic lakes is likely to increase R. raciborskii dominance, but produce less toxic blooms.

Dynamic CO2 and pH levels in coastal, estuarine, and inland waters: Theoretical and observed effects on harmful algal blooms

Rising concentrations of atmospheric CO₂ results in higher equilibrium concentrations of dissolved CO₂ in natural waters, with corresponding increases in hydrogen ion and bicarbonate concentrations and decreases in hydroxyl ion and carbonate concentrations. Superimposed on these climate change effects is the dynamic nature of carbon cycling in coastal zones, which can lead to seasonal and diel changes in pH and CO₂ concentrations that can exceed changes expected for open ocean ecosystems by the end of the century. Among harmful algae, i.e. some species and/or strains of Cyanobacteria, Dinophyceae, Prymnesiophyceae, Bacillariophyceae, and Ulvophyceae, the occurrence of a CO₂ concentrating mechanisms (CCMs) is the most frequent mechanism of inorganic carbon acquisition in natural waters in equilibrium with the present atmosphere (400 μmol CO₂  mol^-1 total gas), with varying phenotypic modification of the CCM. No data on CCMs are available for Raphidophyceae or the brown tide Pelagophyceae. Several HAB species and/or strains respond to increased CO₂ concentrations with increases in growth rate and/or cellular toxin content, however, others are unaffected. Beyond the effects of altered C concentrations and speciation on HABs, changes in pH in natural waters are likely to have profound effects on algal physiology. This review outlines the implications of changes in inorganic cycling for HABs in coastal zones, and reviews the knowns and unknowns with regard to how HABs can be expected to ocean acidification. We further point to the large regions of uncertainty with regard to this evolving field.

Subtropical freshwater phytoplankton show a greater response to increased temperature than to increased pCO2

Global increases in atmospheric CO₂ and temperatures will impact aquatic systems, with freshwater habitats being affected. Some studies suggest that these conditions will promote cyanobacterial dominance. There is a need for a clearer picture of how algal species and strains within species will respond to higher temperatures and CO₂, especially in combination. This study examined two chlorophytes (Monoraphidium and Staurastrum), and two strains of the cyanobacterium Raphidiopsis raciborskii (straight S07 and coiled C03), to determine how the combination of higher temperature and CO₂ levels will affect their growth and maximum cell concentrations. Continuous cultures were used to compare the steady state cell concentrations at 28 °C and 30 °C, and CO₂ partial pressures (pCO₂), 400 and 750 ppm for all cultures, and in addition 1000 ppm at 28 °C for R. raciborskii strains. This study showed that, for all species, water temperature had a greater effect than higher pCO₂ on cell concentrations. There were clear differences in response between the chlorophyte species, with Monoraphidium preferring 28 °C and Staurastrum preferring 30 °C. There were also differences in response of the R. raciborskii strains to increasing temperature and pCO₂, with S07 having a greater increase in cell concentration. Genome analysis of R. raciborskii showed that the straight strain has five additional carbon acquisition genes (β-CA, chpY, cmpB, cmpD and NdhD4), indicative of increased carbon metabolism. These differences in the strains' response to elevated pCO₂ will lead to changes in the species population structure and distribution in the water column. This study shows that it is important to examine the effects of both pCO₂ and temperature, and to consider strain variation, to understand how species composition of natural systems may change under future climate conditions.

The dual function of the algal treatment: Antibiotic elimination combined with CO2 fixation

The study provided an algal treatment to achieve dual function with antibiotic elimination and CO₂ fixation simultaneously. Two widely used antibiotics, cefradine and amoxicillin were selected as the target compounds. First of all, we assessed the influence of light intensity on algal growth and antibiotic removal efficiency to obtain the optimal light intensity. Secondly, after the algal antibiotic treatment, the CO₂ capture capacities at varied CO₂ volume concentrations were assessed and compared. Significant improvement in the removal efficiency of cefradine occurred when CO₂ was added into the treatment. Change in the content of photosynthetic pigments and the activities of RuBisCO and carbonic anhydrase occurred as the algal responses to the treatment condition. Our results showed that Chlorella pyrenoidosa performed better than Microcystis aeruginosa in both the antibiotic removal efficiency and the CO₂ capture capacity. In the integrated algal treatment, the remove rate of antibiotic has been improved by 30.16% and at the same time, the CO₂ absorption rate has been promoted by 10.94%. Metabolite analyses also revealed the mechanism involved, which proved the crucial role of the algae in the biodegradation of the target antibiotic.