Investigating mixotrophic metabolism in the model diatom Phaeodactylum tricornutum

Algicidal bacteria trigger contrasting responses in model diatom communities of different composition

Algicidal bacteria are important players regulating the dynamic changes of plankton assemblages. Most studies on these bacteria have focused on the effect on single algal species in simple incubation experiments. Considering the complexity of species assemblages in the natural plankton, such incubations represent an oversimplification and do not allow making further reaching conclusions on ecological interactions. Here, we describe a series of co‐incubation experiments with different level of complexity to elucidate the effect of the algicidal bacterium Kordia algicida on mixed cultures of a resistant and a susceptible diatom. The growth of the resistant diatom Chaetoceros didymus is nearly unaffected by K. algicida in monoculture, while cells of the susceptible diatom Skeletonema costatum are lysed within few hours. Growth of C. didymus is inhibited if mixed cultures of the two diatoms are infected with the bacterium. Incubations with filtrates of the infected cultures show that the effects are chemically mediated. In non‐contact co‐culturing we show that low concentrations of the lysed algae support the growth of C. didymus, while higher concentrations trigger population decline. Complex cascading effects of algicidal bacteria have thus to be taken into account if their ecological role is concerned.

Ecotoxicity of the lipid-lowering drug bezafibrate on the bioenergetics and lipid metabolism of the diatom Phaeodactylum tricornutum

Pharmaceutical residues impose a new and emerging threat to the marine environment and its biota. In most countries, ecotoxicity tests are not required for all pharmaceutical residues classes and, even when mandatory, these tests are not performed using marine primary producers such as diatoms. These microalgae are among the most abundant class of primary producers in the marine realm and key players in the marine trophic web. Blood-lipid-lowering agents such as bezafibrate and its derivatives are among the most prescribed drugs and most frequently found human pharmaceuticals in aquatic environments. The present study aims to investigate the bezafibrate ecotoxicity and its effects on primary productivity and lipid metabolism, at environmentally relevant concentrations, using the model diatom Phaeodactylum tricornutum. Under controlled conditions, diatom cultures were exposed to bezafibrate at 0, 3, 6, 30 and 60 μg L^-1, representing concentrations that can be found in the vicinity of discharges of wastewater treatment plants. High bezafibrate concentrations increased cell density and are suggested to promote a shift from autotrophic to mixotrophic metabolism, with diatoms using light energy generated redox potential to breakdown bezafibrate as carbon source. This was supported by an evident increase in cell density coupled with an impairment of the thylakoid electron transport and consequent photosynthetic activity reduction. In agreement, the concentrations of plastidial marker fatty acids showed negative correlations and Canonical Analysis of Principal coordinates of the relative abundances of fatty acid and photochemical data allowed the separation of controls and cells exposed to bezafibrate with high classification efficiency, namely for photochemical traits, suggesting their validity as suitable biomarkers of bezafibrate exposure. Further evaluations of the occurrence of a metabolic shift in diatoms due to exposure to bezafibrate is paramount, as ultimately it may reduce O₂ generation and CO₂ fixation in aquatic ecosystems with ensuing consequences for neighboring heterotrophic organisms.

Microalgae and cyanobacteria modeling in water resource recovery facilities: A critical review

Microalgal and cyanobacterial resource recovery systems could significantly advance nutrient recovery from wastewater by achieving effluent nitrogen (N) and phosphorus (P) levels below the current limit of technology. The successful implementation of phytoplankton, however, requires the formulation of process models that balance fidelity and simplicity to accurately simulate dynamic performance in response to environmental conditions. This work synthesizes the range of model structures that have been leveraged for algae and cyanobacteria modeling and core model features that are required to enable reliable process modeling in the context of water resource recovery facilities. Results from an extensive literature review of over 300 published phytoplankton models are presented, with particular attention to similarities with and differences from existing strategies to model chemotrophic wastewater treatment processes (e.g., via the Activated Sludge Models, ASMs). Building on published process models, the core requirements of a model structure for algal and cyanobacterial processes are presented, including detailed recommendations for the prediction of growth (under phototrophic, heterotrophic, and mixotrophic conditions), nutrient uptake, carbon uptake and storage, and respiration.

Growth and lipid accumulation by different nutrients in the microalga Chlamydomonas reinhardtii

BACKGROUND

Individual nutrient depletion is widely used to induce lipid accumulation in microalgae, which also causes cell growth inhibition and decreases the total biomass. Thus, improving the lipid accumulation without biomass loss in the nutrient deficiency cells becomes a potential cost-effective treatment for cheaper biofuels.

METHODS

In this study, the effects of different nutritional conditions on the growth and contents of lipids in Chlamydomonas reinhardtii were compared, and the metabolic profiles under different nutritional conditions were also investigated.

RESULTS

We showed that similar to other microalgae, nitrogen or phosphorus deficiency inhibited the growth of Chlamydomonas and combined nutrition deficiency reduced biomass by up to 31.7%, though lipid contents in cells (g/g dry weight [DW]) were significantly increased. The addition of sodium acetate countered this growth inhibition that resulted from nitrogen and phosphorus deficiency, with significantly increased biomass. Furthermore, the combination of 4 g/L sodium acetate supplementation with nitrogen and phosphorous deficiency increased total fatty acid yield (mg/L) by 93.0 and 150.1% compared to nutrient-depleted and normal culture conditions, respectively. Metabolite content was affected by the different nutritional conditions, especially metabolites that are involved in lipid metabolism, amino acid metabolism and metabolism of external substances.

CONCLUSION

Further research into these metabolites could shed light onto the relationship between cell growth inhibition and fatty acid accumulation in Chlamydomonas.

Generation of Mutants of Nuclear-Encoded Plastid Proteins Using CRISPR/Cas9 in the Diatom Phaeodactylum tricornutum

Genome modifications in microalgae are becoming a widespread and mandatory tool for research in both fundamental and applied biology. Among genome editing methods in these photosynthetic organisms, CRISPR/Cas9 offers a specific, powerful and efficient tool for genome engineering by inducing mutations in targeted regions of the genome. Here we described a protocol that allows the generation of knockout mutants by CRISPR/Cas9 in the diatom Phaeodactylum tricornutum using biolistic transformation.

Modulation of lipid biosynthesis by stress in diatoms

Diatoms are responsible for up to 40% of the carbon fixation in our oceans. The fixed carbon is moved through carbon metabolism towards the synthesis of organic molecules that are consumed through interlocking foodwebs, and this process is strongly impacted by the abiotic environment. However, it has become evident that diatoms can be used as 'platform' organisms for the production of high valuable bio-products such as lipids, pigments and carbohydrates where stress conditions can be used to direct carbon metabolism towards the commercial production of these compounds. In the first section of this review, some aspects of carbon metabolism in diatoms and how it is impacted by environmental factors are briefly described. The second section is focused on the biosynthesis of lipids and in particular omega-3 long-chain polyunsaturated fatty acids and how low temperature stress impacts on the production of these compounds. In a third section, we review the recent advances in bioengineering for lipid production. Finally, we discuss new perspectives for designing strains for the sustainable production of high-value lipids.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.