Nitrogen availability modulates CO2 tolerance in a symbiotic chlorophyte

Exploring microalgal nutrient-light synergy to enhance CO2 utilization and lipid productivity in sustainable long-term water recycling cultivation

In this work the effects of nutrient availability and light conditions on CO2 utilization and lipid production in Micractinium pusillum KMC8 is reported. The study investigated the ideal nitrogen concentrations for growth and nitrogen utilization in a 15% CO2 environment. Logistic and Gompertz models were employed to analyze the kinetics of KMC8 cell growth. Compared to 17.6 mmol L-1 control nitrogen, which generated 1.6 g L-1 growth, doubling and quadrupling nitrogen concentrations boosted biomass growth by 12.5% and 28.78%. At 8.6 mmol L-1 nitrogen, the growth decreased but lipid productivity increased to 18.62 mg L-1 day-1. At 70.6 mmol L-1 nitrogen, elevated nitrogen levels maintained an alkaline pH above 7 and enhanced CO2 mitigation, achieving 2.27% CO2 utilization efficiency. Nitrogen shows a positive correlation with higher rates of carbon and nitrogen fixation. The investigation extends to find out the influence of phosphorus and light conditions on microalgae. Increasing light intensity incrementally from 150 to 1200 μmol m-2 s-1 with more phosphorus increased biomass productivity by 85% (255 mg L-1 day-1) and lipid productivity by 2.5-fold (84.76 mg L-1 day-1), with 3.3% CO2 utilization efficiency compared to directly using 1200 μmol m-2 s-1. This study suggests a water recycling-fed batch cycle with gradual light feeding, which results in high CO2 fixation (1.1 g L-1 day-1), 7% CO2 utilization, and significant biomass and lipid productivity (577.23 and 150 mg L-1 day-1). This approach promotes lipid synthesis, maintains carbon fixation, and minimizes biomass loss, thus supporting sustainable bioenergy development in a circular bio-economy framework.

Harnessing C/N balance of Chromochloris zofingiensis to overcome the potential conflict in microalgal production

Accumulation of high-value products in microalgae is not conducive with rapid cell growth, which is the potential conflict in microalgal production. Overcoming such conflict faces numerous challenges in comprehensively understanding cell behavior and metabolism. Here, we show a fully integrated interaction between cell behavior, carbon partitioning, carbon availability and path rate of central carbon metabolism, and have practically overcome the production conflict of Chromochloris zofingiensis. We demonstrate that elevated carbon availability and active path rate of precursors are determinants for product biosynthesis, and the former exhibits a superior potential. As protein content reaches a threshold value to confer survival advantages, carbon availability becomes the major limiting factor for product biosynthesis and cell reproduction. Based on integrated interaction, regulating the C/N balance by feeding carbon source under excess light increases content of high-value products without inhibiting cell growth. Our findings provide a new orientation to achieve great productivity improvements in microalgal production.

Ultrastructural patterns of photoacclimation and photodamage to photosynthetic algae cell under environmental stress

In oxygenic phototrophs including unicellular algae, acclimation to and damage by diverse environmental stresses induce profound changes in the ultrastructural organization of the cell. These alterations reflect acclimation of the photosynthetic apparatus to unfavorable conditions (mainly reduction of the chloroplast and its membranal system) and rewiring of the photo-fixed carbon fluxes in the cell. These changes, eventually pursuing mitigation of the photooxidative damage risk, are manifested by the formation of diverse carbon-rich inclusions. Although the physiological and molecular basis of these processes are well understood, the ultrastructural manifestations of the stress responses are often fragmented and frequently controversial. This minireview attempts to generalize on the ultrastructural patterns accompanying stresses in the photosynthetic cell, involving the concerted rearrangements of its assimilatory and storage compartments. The changes characteristic of normal functioning and emergency reduction of the chloroplast thylakoids under harsh stress are also addressed. Special attention is paid to the manifestations of the engagement of photoprotection via active (energy-dependent non-photochemical quenching) and passive mechanisms (e.g. optical shielding by secondary carotenoids). We also underline the potentially important role of autophagy-like processes and provide a more integral view of ultrastructural rearrangements under stress.

Stress-induced changes in the ultrastructure of the photosynthetic apparatus of green microalgae

In photosynthetic organisms including unicellular algae, acclimation to and damage by environmental stresses are readily apparent at the level of the photosynthetic apparatus. Phenotypic manifestations of the stress responses include rapid and dramatic reduction of photosynthetic activity and pigment content aimed at mitigating the risk of photooxidative damage. Although the physiological and molecular mechanisms of these events are well known, the ultrastructural picture of the stress responses is often elusive and frequently controversial. We analyzed an extensive set of transmission electron microscopy images of the microalgal cells obtained across species of Chlorophyta and in a wide range of growth conditions. The results of the analysis allowed us to pinpoint distinct ultrastructural changes typical of normal functioning and emergency reduction of the chloroplast membrane system under high light exposure and/or mineral nutrient starvation. We demonstrate the patterns of the stress-related ultrastructural changes including peculiar thylakoid rearrangements and autophagy-like processes and provide an outlook on their significance for implementation of the stress responses.

Versatility of the green microalga cell vacuole function as revealed by analytical transmission electron microscopy

Vacuole is a multifunctional compartment central to a large number of functions (storage, catabolism, maintenance of the cell homeostasis) in oxygenic phototrophs including microalgae. Still, microalgal cell vacuole is much less studied than that of higher plants although knowledge of the vacuolar structure and function is essential for understanding physiology of nutrition and stress tolerance of microalgae. Here, we combined the advanced analytical and conventional transmission electron microscopy methods to obtain semi-quantitative, spatially resolved at the subcellular level information on elemental composition of the cell vacuoles in several free-living and symbiotic chlorophytes. We obtained a detailed record of the changes in cell and vacuolar ultrastructure in response to environmental stimuli under diverse conditions. We suggested that the vacuolar inclusions could be divided into responsible for storage of phosphorus (mainly in form of polyphosphate) and those accommodating non-protein nitrogen (presumably polyamine) reserves, respectively.The ultrastructural findings, together with the data on elemental composition of different cell compartments, allowed us to speculate on the role of the vacuolar membrane in the biosynthesis and sequestration of polyphosphate. We also describe the ultrastructural evidence of possible involvement of the tonoplast in the membrane lipid turnover and exchange of energy and metabolites between chloroplasts and mitochondria. These processes might play a significant role in acclimation in different stresses including nitrogen starvation and extremely high level of CO₂ and might also be of importance for microalgal biotechnology. Advantages and limitations of application of analytical electron microscopy to biosamples such as microalgal cells are discussed.