Evidence for the occurrence of photorespiration in synurophyte algae

Photoautotrophic cultivation of Chlamydomonas reinhardtii in open ponds of greenhouse

Chlamydomonas reinhardtii is one of the most characterized green algae. The open-pond cultivation can be challenging due to sensitivity of strain to fluctuating environmental conditions and unavailability of low-cost photoautotrophic media. In this study, the photoautotrophic growth of C. reinhardtii was evaluated in 1-m² open ponds placed in greenhouse. Sodium bicarbonate (NaHCO₃) was evaluated as an alternative buffering agent to tris. The effect of buffer and pH was tested. The growth was studied in the presence of various nitrogen [urea and ammonium bicarbonate (NH₄HCO₃)] sources. In the study, it was found that 125-ppm NaHCO₃ as an optimum concentration. The buffering agent in the media was found to have major impact on growth. Without buffering agent, culture did not grow, and pH drop was observed. The sodium bicarbonate-buffered media reported to have the lowest bacterial contamination (18.3%), highest AFDW per OD (0.39 ± 0.027 g/L) and higher Fv/Fm (0.714 ± 0.016), whereas these values were found to be 62%, 0.19 ± 0.02 g/L and 0.537 ± 0.053 for tris-grown culture, respectively. The pH 7.0-7.5 was determined as an optimum, whereas pH 6.5-7.0 and 8.0-8.5 were found to affect the growth and induce palmelloidy. The OD and AFDW of culture grown in NH₄HCO₃ were found equivalent to a standard nitrogen source (NH₄Cl), whereas culture shown poor growth in urea. Based on these data, NH₄HCO₃ media recipe and the optimized cultivation parameters were selected for photoautotrophic cultivation of Chlamydomonas in greenhouse open ponds.

Acquisition and metabolism of carbon in the Ochrophyta other than diatoms

The acquisition and assimilation of inorganic C have been investigated in several of the 15 clades of the Ochrophyta other than diatoms, with biochemical, physiological and genomic data indicating significant mechanistic variation. Form ID Rubiscos in the Ochrophyta are characterized by a broad range of kinetics values. In spite of relatively high K0.5CO2 and low CO2 : O2 selectivity, diffusive entry of CO2 occurs in the Chrysophyceae and Synurophyceae. Eustigmatophyceae and Phaeophyceae, on the contrary, have CO2 concentrating mechanisms, usually involving the direct or indirect use of [Formula: see text] This variability is possibly due to the ecological contexts of the organism. In brown algae, C fixation generally takes place through a classical C3 metabolism, but there are some hints of the occurrence of C4 metabolism and low amplitude CAM in a few members of the Fucales. Genomic data show the presence of a number of potential C4 and CAM genes in Ochrophyta other than diatoms, but the other core functions of many of these genes give a very limited diagnostic value to their presence and are insufficient to conclude that C4 photosynthesis is present in these algae.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

The possible evolution and future of CO2-concentrating mechanisms

CO2-concentrating mechanisms (CCMs), based either on active transport of inorganic carbon (biophysical CCMs) or on biochemistry involving supplementary carbon fixation into C4 acids (C4 and CAM), play a major role in global primary productivity. However, the ubiquitous CO2-fixing enzyme in autotrophs, Rubisco, evolved at a time when atmospheric CO2 levels were very much higher than today and O2 was very low and, as CO2 and O2 approached (by no means monotonically), today's levels, at some time subsequently many organisms evolved a CCM that increased the supply of CO2 and decreased Rubisco oxygenase activity. Given that CO2 levels and other environmental factors have altered considerably between when autotrophs evolved and the present day, and are predicted to continue to change into the future, we here examine the drivers for, and possible timing of, evolution of CCMs. CCMs probably evolved when CO2 fell to 2-16 times the present atmospheric level, depending on Rubisco kinetics. We also assess the effects of other key environmental factors such as temperature and nutrient levels on CCM activity and examine the evidence for evolutionary changes in CCM activity and related cellular processes as well as limitations on continuity of CCMs through environmental variations.

Ecological imperatives for aquatic CO2-concentrating mechanisms

In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially oversaturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments, and the frequency of species with a CO2-concentrating mechanism (CCM), and their contribution to productivity, is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity, or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH, and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species, but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton, and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context.

Transcriptome and key genes expression related to carbon fixation pathways in Chlorella PY-ZU1 cells and their growth under high concentrations of CO2

BACKGROUND

The biomass yield of Chlorella PY-ZU1 drastically increased when cultivated under high CO₂ condition compared with that cultivated under air condition. However, less attention has been given to the microalgae photosynthetic mechanisms response to different CO₂ concentrations. The genetic reasons for the higher growth rate, CO₂ fixation rate, and photosynthetic efficiency of microalgal cells under higher CO₂ concentration have not been clearly defined yet.

RESULTS

In this study, the Illumina sequencing and de novo transcriptome assembly of Chlorella PY-ZU1 cells cultivated under 15% CO₂ were performed and compared with those of cells grown under air. It was found that carbonic anhydrase (CAs, enzyme for interconversion of bicarbonate to CO₂) dramatically decreased to near 0 in 15% CO₂-grown cells, which indicated that CO₂ molecules directly permeated into cells under high CO₂ stress without CO₂-concentrating mechanism. Extrapolating from the growth conditions and quantitative Real-Time PCR of CCM-related genes, the K_m (CO₂) (the minimum intracellular CO₂ concentration that rubisco required) of Chlorella PY-ZU1 might be in the range of 80-192 μM. More adenosine triphosphates was saved for carbon fixation-related pathways. The transcript abundance of rubisco (the most important enzyme of CO₂ fixation reaction) was 16.3 times higher in 15% CO₂-grown cells than that under air. Besides, the transcript abundances of most key genes involved in carbon fixation pathways were also enhanced in 15% CO₂-grown cells.

CONCLUSIONS

Carbon fixation and nitrogen metabolism are the two most important metabolisms in the photosynthetic cells. These genes related to the two most metabolisms with significantly differential expressions were beneficial for microalgal growth (2.85 g L^-1) under 15% CO₂ concentration. Considering the micro and macro growth phenomena of Chlorella PY-ZU1 under different concentrations of CO₂ (0.04-60%), CO₂ transport pathways responses to different CO₂ (0.04-60%) concentrations was reconstructed.

Inorganic carbon acquisition in the acid-tolerant alga Chlorella kessleri

The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0-9.0 and was found to take up bicarbonate and CO2 actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO2 uptake. The acidotolerant species maintained an internal pH of 6.1-7.5 over the external pH range 4.5-7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid external pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.0-17.7 µM reached by photosynthetic cells were above the CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. Chlorella kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7-fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO2 at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH.