Phosphorus acquisition by Chlamydomonas acidophila under autotrophic and osmo-mixotrophic growth conditions

Acid Tolerant and Acidophilic Microalgae: An Underexplored World of Biotechnological Opportunities

Despite their large number and diversity, microalgae from only four genera are currently cultivated at large-scale. Three of those share common characteristics: they are cultivated mainly autotrophically and are extremophiles or tolerate “extreme conditions.” Extreme growth conditions aid in preventing contamination and predation of microalgae, therefore facilitating outdoor cultivation. In search for new extremophilic algae suitable for large-scale production, we investigated six microalgal strains able to grow at pH below 3 and belonging to four genera; Stichococcus bacillaris ACUF158, Chlamydomonas acidophila SAG 2045, and Chlamydomonas pitschmannii ACUF238, Viridiella fridericiana ACUF035 and Galdieria sulphuraria ACUF064 and ACUF074. All strains were cultivated autotrophically at light intensity of 100 and 300 μmol m−2 s−1 and pH between 1.9 and 2.9. The autotrophic biomass productivities were compared with one of the most productive microalgae, Chlorella sorokiniana SAG 211-8K, grown at pH 6.8. The acid tolerant strains have their autotrophic biomass productivities reported for the first time. Mixotrophic and heterotrophic properties were investigated when possible. Five of the tested strains displayed autotrophic biomass productivities 10–39% lower than Chlorella sorokiniana but comparable with other commercially relevant neutrophilic microalgae, indicating the potential of these microalgae for autotrophic biomass production under acidic growth conditions. Two acid tolerant species, S. bacillaris and C. acidophila were able to grow mixotrophically with glucose. Chlamydomonas acidophila and the two Galdieria strains were also cultivated heterotrophically with glucose at various temperatures. Chlamydomonas acidophila failed to grow at 37°C, while G. sulphuraria ACUF64 showed a temperature optimum of 37°C and G. sulphuraria ACUF74 of 42°C. For each strain, the biomass yield on glucose decreased when cultivated above their optimal temperature. The possible biotechnological applications of our findings will be addressed.

Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment

Threats posed to humans - including environmental pollution, water scarcity, food shortages, and resource crises drive a new concept to think about wastewater and its treatment. Wastewater is not only a waste but also a source of energy, renewable and/or non-renewable resources, including water itself. The nutrient in wastewater should not only be removed but also need to be upcycled. Microalgae based wastewater treatment has attracted considerable interests because algae have the potential to efficiently redirect nutrients from wastewater to the accumulated algal biomass. Additionally, microalgae are commercialized in human consumption and animal feed owing to their high content of essential amino and fatty acids, vitamins, and pigments. The whole process establishes a circular economy, totally relying on the ability of microalgae to uptake and store nutrients in wastewater, such as carbon (C), nitrogen (N), and phosphorus (P). It makes the study of the mechanisms underlying the uptake and storage of nutrients in microalgae of great interest. This review specifically aims to summarize C, N, and P metabolisms in microalgae for a better understanding of the microalgae-based wastewater treatment from the nutrient uptake pathway, and examine the key physiological factors or the operating conditions related to nutrient metabolisms that may affect the treatment efficiency. At last, I discuss the potential approaches to enhance the overall treatment performance by adjusting the critical parameters for C, N, and P metabolisms.

Understanding How Microorganisms Respond to Acid pH Is Central to Their Control and Successful Exploitation

Microbes from the three domains of life, Bacteria, Archaea, and Eukarya, share the need to sense and respond to changes in the external and internal concentrations of protons. When the proton concentration is high, acidic conditions prevail and cells must respond appropriately to ensure that macromolecules and metabolic processes are sufficiently protected to sustain life. While, we have learned much in recent decades about the mechanisms that microbes use to cope with acid, including the unique challenges presented by organic acids, there is still much to be gained from developing a deeper understanding of the effects and responses to acid in microbes. In this perspective article, we survey the key molecular mechanisms known to be important for microbial survival during acid stress and discuss how this knowledge might be relevant to microbe-based applications and processes that are consequential for humans. We discuss the research approaches that have been taken to investigate the problem and highlight promising new avenues. We discuss the influence of acid on pathogens during the course of infections and highlight the potential of using organic acids in treatments for some types of infection. We explore the influence of acid stress on photosynthetic microbes, and on biotechnological and industrial processes, including those needed to produce organic acids. We highlight the importance of understanding acid stress in controlling spoilage and pathogenic microbes in the food chain. Finally, we invite colleagues with an interest in microbial responses to low pH to participate in the EU-funded COST Action network called EuroMicropH and contribute to a comprehensive database of literature on this topic that we are making publicly available.

Decreased phosphorus incorporation explains the negative effect of high iron concentrations in the green microalga Chlamydomonas acidophila

The green microalga Chlamydomonas acidophila is an important primary producer in very acidic lakes (pH 2.0-3.5), characterized by high concentrations of ferric iron (up to 1 g total Fe L^-1) and low rates of primary production. It was previously suggested that these high iron concentrations result in high iron accumulation and inhibit photosynthesis in C. acidophila. To test this, the alga was grown in sterilized lake water and in medium with varying total iron concentrations under limiting and sufficient inorganic phosphorus (Pi) supply, because Pi is an important growth limiting nutrient in acidic waters. Photosynthesis and growth of C. acidophila as measured over 5 days were largely unaffected by high total iron concentrations and only decreased if free ionic Fe³⁺ concentrations exceeded 100 mg Fe³⁺ L^-1. Although C. acidophila was relatively rich in iron (up to 5 mmol Fe: mol C), we found no evidence of iron toxicity. In contrast, a concentration of 260 mg total Fe L^-1 (i.e. 15 mg free ionic Fe³⁺ L^-1), which is common in many acidic lakes, reduced Pi-incorporation by 50% and will result in Pi-limited photosynthesis. The resulting Pi-limitation present at high iron and Pi concentrations was illustrated by elevated maximum Pi-uptake rates. No direct toxic effects of high iron were found, but unfavourable chemical Pi-speciation reduced growth of the acidophile alga.

Microalgae-bacteria biofilms: a sustainable synergistic approach in remediation of acid mine drainage

Microalgae and bacteria offer a huge potential in delving interest to study and explore various mechanisms under extreme environments. Acid mine drainage (AMD) is one such environment which is extremely acidic containing copious amounts of heavy metals and poses a major threat to the ecosystem. Despite its extreme conditions, AMD is the habitat for several microbes and their activities. The use of various chemicals in prevention of AMD formation and conventional treatment in a larger scale is not feasible under different geological conditions. It implies that microbe-mediated approach is a viable and sustainable alternative technology for AMD remediation. Microalgae in biofilms play a pivotal role in such bioremediation as they maintain mutualism with heterotrophic bacteria. Synergistic approach of using microalgae-bacteria biofilms provides supportive metabolites from algal biomass for growth of bacteria and mediates remediation of AMD. However, by virtue of their physiology and capabilities of metal removal, non-acidophilic microalgae can be acclimated for use in AMD remediation. A combination of selective acidophilic and non-acidophilic microalgae together with bacteria, all in the form of biofilms, may be very effective for bioremediation of metal-contaminated waters. The present review critically examines the nature of mutualistic interactions established between microalgae and bacteria in biofilms and their role in removal of metals from AMDs, and consequent biomass production for the yield of biofuel. Integration of microalgal-bacterial consortia in fuel cells would be an attractive emerging approach of microbial biotechnology for AMD remediation.

Microalgae-bacteria biofilms: a sustainable synergistic approach in remediation of acid mine drainage

Microalgae and bacteria offer a huge potential in delving interest to study and explore various mechanisms under extreme environments. Acid mine drainage (AMD) is one such environment which is extremely acidic containing copious amounts of heavy metals and poses a major threat to the ecosystem. Despite its extreme conditions, AMD is the habitat for several microbes and their activities. The use of various chemicals in prevention of AMD formation and conventional treatment in a larger scale is not feasible under different geological conditions. It implies that microbe-mediated approach is a viable and sustainable alternative technology for AMD remediation. Microalgae in biofilms play a pivotal role in such bioremediation as they maintain mutualism with heterotrophic bacteria. Synergistic approach of using microalgae-bacteria biofilms provides supportive metabolites from algal biomass for growth of bacteria and mediates remediation of AMD. However, by virtue of their physiology and capabilities of metal removal, non-acidophilic microalgae can be acclimated for use in AMD remediation. A combination of selective acidophilic and non-acidophilic microalgae together with bacteria, all in the form of biofilms, may be very effective for bioremediation of metal-contaminated waters. The present review critically examines the nature of mutualistic interactions established between microalgae and bacteria in biofilms and their role in removal of metals from AMDs, and consequent biomass production for the yield of biofuel. Integration of microalgal-bacterial consortia in fuel cells would be an attractive emerging approach of microbial biotechnology for AMD remediation.

Morphological and ultrastructural characterization of the acidophilic and lipid-producer strain Chlamydomonas acidophila LAFIC-004 (Chlorophyta) under different culture conditions

Chlamydomonas acidophila LAFIC-004 is an acidophilic strain of green microalgae isolated from coal mining drainage. In the present work, this strain was cultivated in acidic medium (pH 3.6) under phototrophic, mixotrophic, and heterotrophic regimes to determine the best condition for growth and lipid production, simultaneously assessing possible morphological and ultrastructural alterations in the cells. For heterotrophic and mixotrophic treatments, two organic carbon sources were tested: 1 % glucose and 1 % sodium acetate. Lipid content and fatty acid profiles were only determined in phototrophic condition. The higher growth rates were achieved in phototrophic conditions, varying from 0.18 to 0.82 day^-1. Glucose did not result in significant growth increase in either mixotrophic or heterotrophic conditions, and acetate proved to be toxic to the strain in both conditions. Oil content under phototrophic condition was 15.9 % at exponential growth phase and increased to 54.63 % at stationary phase. Based on cell morphology (flow cytometry and light microscopy) and ultrastructure (transmission electron microscopy), similar characteristics were observed between phototrophic and mixotrophic conditions with glucose evidencing many lipid bodies, starch granules, and intense fluorescence. Under the tested conditions, mixotrophic and heterotrophic modes did not result in increased neutral lipid fluorescence. It can be concluded that the strain is a promising lipid producer when grown until stationary phase in acidic medium and under a phototrophic regime, presenting a fatty acid profile suitable for biodiesel production. The ability to grow this strain in acidic mining residues suggests a potential for bioremediation with production of useful biomass.

Interspecific competition in phytoplankton drives the availability of essential mineral and biochemical nutrients

The underlying mechanisms and consequences of competition and diversity are central themes in ecology. A higher diversity of primary, producers often results in higher resource use efficiency in aquatic and terrestrial ecosystems. This may result in more food for consumers on one hand, while, on the other hand, it can also result in a decreased food quality for consumers; higher biomass combined with the same availability of the limiting compound directly reduces the dietary proportion of the limiting compound. Here we tested whether and how interspecific competition in phytoplankton communities leads to changes in resource use efficiency and cellular concentrations of nutrients and fatty acids. The measured particulate carbon: phosphorus ratios (C:P) and fatty acid concentrations in the communities were compared to the theoretically expected ratios and concentrations of measurements on simultaneously running monocultures. With interspecific competition, phytoplankton communities had higher concentrations of the monounsaturated fatty acid oleic acid and also much higher concentrations of the ecologically and physiologically relevant long-chain polyunsaturated fatty acid eicosapentaenoic acid than expected concentrations based on monocultures. Such higher availability of essential fatty acids may contribute to the positive relationship between phytoplankton diversity and zooplankton growth, and may compensate limitations by mineral nutrients in higher trophic levels.

Ammonium removal from anaerobically treated effluent by Chlamydomonas acidophila

Several batch culture studies were carried out to evaluate an anaerobically treated effluent as a low-cost growth medium for the microalga Chlamydomonas acidophila and to study the effectiveness of the microalga in removing NH4-N from the effluent. An initial decrease in the effluent pH to 3 was required for adequate growth of C. acidophila and removal of NH4-N. Growth of the microalgae was inhibited at high light intensity (224μmolphotonsm(-2)s(-1) at the surface of the vessels). However, the growth was not greatly affected by the high solid content and turbidity of the effluent. The microalga was able to grow in media containing NH4-N at concentrations of up to 1000mgL(-1) (50% of effluent) and to remove 88mg of NH4-NL(-1) in 10days. C. acidophila therefore appears a promising agent for the removal of NH4-N from anaerobically treated effluents.

Independent colimitation for carbon dioxide and inorganic phosphorus

Simultaneous limitation of plant growth by two or more nutrients is increasingly acknowledged as a common phenomenon in nature, but its cellular mechanisms are far from understood. We investigated the uptake kinetics of CO(2) and phosphorus of the algae Chlamydomonas acidophila in response to growth at limiting conditions of CO(2) and phosphorus. In addition, we fitted the data to four different Monod-type models: one assuming Liebigs Law of the minimum, one assuming that the affinity for the uptake of one nutrient is not influenced by the supply of the other (independent colimitation) and two where the uptake affinity for one nutrient depends on the supply of the other (dependent colimitation). In addition we asked whether the physiological response under colimitation differs from that under single nutrient limitation.We found no negative correlation between the affinities for uptake of the two nutrients, thereby rejecting a dependent colimitation. Kinetic data were supported by a better model fit assuming independent uptake of colimiting nutrients than when assuming Liebigs Law of the minimum or a dependent colimitation. Results show that cell nutrient homeostasis regulated nutrient acquisition which resulted in a trade-off in the maximum uptake rates of CO(2) and phosphorus, possibly driven by space limitation on the cell membrane for porters for the different nutrients. Hence, the response to colimitation deviated from that to a single nutrient limitation. In conclusion, responses to single nutrient limitation cannot be extrapolated to situations where multiple nutrients are limiting, which calls for colimitation experiments and models to properly predict growth responses to a changing natural environment. These deviations from single nutrient limitation response under colimiting conditions and independent colimitation may also hold for other nutrients in algae and in higher plants.

The expression of a carbon concentrating mechanism in Chlamydomonas acidophila under variable phosphorus, iron, and CO2 concentrations

The CO(2) acquisition was analyzed in Chlamydomonas acidophila at pH 2.4 in a range of medium P and Fe concentrations and at high and low CO(2) condition. The inorganic carbon concentrating factor (CCF) was related to cellular P quota (Q(p)), maximum CO(2)-uptake rate by photosynthesis (V(max,O2)), half saturation constant for CO(2) uptake (K(0.5)), and medium Fe concentration. There was no effect of the medium Fe concentration on the CCF. The CCF increased with increasing Q(p) in both high and low CO(2) grown algae, but maximum Q(p) was 6-fold higher in the low CO(2) cells. In high CO(2) conditions, the CCF was low, ranging between 0.8 and 3.5. High CCF values up to 9.1 were only observed in CO(2)-limited cells, but P- and CO(2)-colimited cells had a low CCF. High CCF did not relate with a low K(0.5) as all CO(2)-limited cells had a low K(0.5) (<4 μM CO(2)). High C(i)-pools in cells with high Q(p) suggested the presence of an active CO(2)-uptake mechanism. The CCF also increased with increasing V(max,O2) which reflect an adaptation to the nutrient in highest demand (CO(2)) under balanced growth conditions. It is proposed that the size of the CCF in C. acidophila is more strongly related to porter density for CO(2) uptake (reflected in V(max,O2)) and less- to high-affinity CO(2) uptake (low K(0.5)) at balanced growth. In addition, high CCF can only be realized with high Q(p).

Effect of nutrients on growth and lipid accumulation in the green algae Dunaliella tertiolecta

Production of biofuel from algae is dependent on the microalgal biomass production rate and lipid content. Both biomass production and lipid accumulation are limited by several factors, of which nutrients play a key role. In this research, the marine microalgae Dunaliella tertiolecta was used as a model organism and a profile of its nutritional requirements was determined. Inorganic phosphate PO4(3-) and trace elements: cobalt (Co2+), iron (Fe3+), molybdenum (Mo2+) and manganese (Mn2+) were identified as required for algae optimum growth. Inorganic nitrogen in the form of nitrate NO3- instead of ammonium (NH4+) was required for maximal biomass production. Lipids accumulated under nitrogen starvation growth condition and this was time-dependent. Results of this research can be applied to maximize production of microalgal lipids in optimally designed photobioreactors.