Ecophysiology matters: linking inorganic carbon acquisition to ecological preference in four species of microalgae (Chlorophyceae)

Genome-wide identification of genes involved in carbon fixation in Saccharina japonica and responses of putative C4-related genes to bicarbonate concentration and light intensity

Brown algae play a dominant role in the primary productivity of coastal ecosystems and may have an efficient carbon fixation. In this work, 56 genes involved in inorganic carbon fixation were identified from the Saccharina japonica genome. Sequence structure analysis of these genes showed the existence of corresponding function domains and active amino acid sites highly conserved with other stramenopile species. The predicted subcellular localizations showed that Calvin cycle-related enzymes predominantly reside in the plastid and that putative C4-related enzymes are mainly distributed in the mitochondrion. We determined the transcriptional profiles and enzymatic activities of these C4-related enzymes in response to the KHCO3 concentrations and light intensities. Pyruvate orthophosphate dikinase (PPDK) presented the greatest response to low HCO3- concentrations and high light intensity. Phosphoenolpyruvate carboxykinase (PEPCK) was up-regulated at low HCO3- concentrations to compensate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and might be the crucial decarboxylase in this kelp. We propose that S. japonica might possess a PPDK- and PEPCK-dependent C4-like pathway that enables its rapid growth in natural coastal environments.

Nitrate or ammonium: Influences of nitrogen source on the physiology of a green alga

In freshwaters, algal species are exposed to different inorganic nitrogen (Ni) sources whose incorporation varies in biochemical energy demand. We hypothesized that due to the lesser energy requirement of ammonium ( NH 4 + )-use, in contrast to nitrate ( NO 3 - )-use, more energy remains for other metabolic processes, especially under CO2- and phosphorus (Pi) limiting conditions. Therefore, we tested differences in cell characteristics of the green alga Chlamydomonas acidophila grown on NH 4 + or NO 3 - under covariation of CO2 and Pi-supply in order to determine limitations, in a full-factorial design. As expected, results revealed higher carbon fixation rates for NH 4 + -grown cells compared to growth with NO 3 - under low CO2 conditions. NO 3 - -grown cells accumulated more of the nine analyzed amino acids, especially under Pi-limited conditions, compared to cells provided with NH 4 + . This is probably due to a slower protein synthesis in cells provided with NO 3 - . In contrast to our expectations, compared to NH 4 + -grown cells NO 3 - -grown cells had higher photosynthetic efficiency under Pi-limitation. In conclusion, growth on the Ni-source NH 4 + did not result in a clearly enhanced Ci-assimilation, as it was highly dependent on Pi and CO2 conditions (replete or limited). Results are potentially connected to the fact that C. acidophila is able to use only CO2 as its inorganic carbon (Ci) source.

Terrestrial adaptation of green algae Klebsormidium and Zygnema (Charophyta) involves diversity in photosynthetic traits but not in CO2 acquisition

The basal streptophyte Klebsormidium and the advanced Zygnema show adaptation to terrestrialization. Differences are found in photoprotection and resistance to short-term light changes, but not in CO ₂ acquisition. Streptophyte green algae colonized land about 450-500 million years ago giving origin to terrestrial plants. We aim to understand how their physiological adaptations are linked to the ecological conditions (light, water and CO₂) characterizing modern terrestrial habitats. A new Klebsormidium isolate from a strongly acidic environment of a former copper mine (Schwarzwand, Austria) is investigated, in comparison to Klebsormidium cf. flaccidum and Zygnema sp. We show that these genera possess different photosynthetic traits and water requirements. Particularly, the Klebsormidium species displayed a higher photoprotection capacity, concluded from non-photochemical quenching (NPQ) and higher tolerance to high light intensity than Zygnema. However, Klebsormidium suffered from photoinhibition when the light intensity in the environment increased rapidly, indicating that NPQ is involved in photoprotection against strong and stable irradiance. Klebsormidium was also highly resistant to cellular water loss (dehydration) under low light. On the other hand, exposure to relatively high light intensity during dehydration caused a harmful over-reduction of the electron transport chain, leading to PSII damages and impairing the ability to recover after rehydration. Thus, we suggest that dehydration is a selective force shaping the adaptation of this species towards low light. Contrary to the photosynthetic characteristics, the inorganic carbon (C _i ) acquisition was equivalent between Klebsormidium and Zygnema. Despite their different habitats and restriction to hydro-terrestrial environment, the three organisms showed similar use of CO₂ and HCO₃^- as source of C_i for photosynthesis, pointing out a similar adaptation of their CO₂-concentrating mechanisms to terrestrial life.

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.