CO2 Concentrating Mechanisms (CCM) in Algae By: Reza Taheri Tehrani Supervisor: Dr. Shariati

Headlines: Why do some algae need CO2 concentrating mechanisms (CCM)? The evidences for existence of CCM in algae A general model for CCM in algae The types of CCM in algae Modulation of CCM by environmental factors

Why do some algae need CO2 concentrating? Aquatic photosynthetic microorganisms account for almost 50% of the world’s photosynthesis. These organisms face several challenges in acquiring and fixing CO2 as follows: the Km(CO2) of Rubisco in C3 plants and cyanobacteria is 20 µM and 200 µM respectively. inorganic carbon concentration and diffusion rate in the water is much less than air. In algae, Rubisco is operating at no more than 30% of its capacity.

CO2 concentrating can solve this problem Microalgae (like to C4 and CAM plants) have developed ways to concentrate the CO2 at the location of Rubisco. CO2 concentrating mechanisms (CCM) is a biological adaptation to low CO2 concentration. Some algae can concentrate CO2 within the cell more than 100 fold over ambient CO2 levels

Evidence for CO2 Concentrating Algae have the ability to efficiently fix CO2 even in low external CO2 concentration. Ci uptake has been measured directly in a number of laboratories, and the Ci concentration inside the cell is higher than can be accounted for by diffusion alone. Further evidence comes from mutant studies. Mutant strains that grew well on elevated CO2 but poorly on low CO2 has been yielded mutants in CCMs

General model for CO2 concentrating Algae with CO2 concentrating ability have certain common properties that allow them to use CO2 efficiently. Rubisco packaging in a very specific compartment (carboxysome and pyrenoid) The ability to accumulate Ci (inorganic carbon= CO2 + HCO3- ) . presence of a CA (carbonic anhydrase) near the location of Rubisco.

General model for CO2 concentrating: Above: in cyanobacteria Below: in a eukaryotic algae cCA: chlorpalstinc carbonic anhydrse pCA: periplasmic CA PGA: 3-phophoglyceric acid

A. Rubisco packaging In most microalgae (in contrast to higher plant) Rubisco is concentrated in a specific location, carboxysome and pyrenoid. The carboxysome is the specialized compartment that is surrounded by a protein shell.

Electron micrograph of the carboxysome in a cyanobacteria

In microalgae, Rubisco is also packaged in pyrenoid. The amount of Rubisco in the pyrenoid varied with growth and nutrient conditions Left, Electron micrograph of the pyrenoid in a green alga Right, Immunogold labeling of the pyrenoid.

Pyrenoids (P) from C.reinhardtii in high CO2 (A) and low CO2 concentration (B).

B. The ability to accumulate Ci Algae display a variety of ways to concentrate CO2 as follows: C4 mechanisms Active transport of Ci (inorganic carbon= CO2 + HCO3- ) pH gradient in chloroplast.

1st CCM type: C4 mechanisms in algae C4 photosynthesis and CAM in terrestrial higher plants were the first photosynthetic CCMs to be described in detail. The presence of C4 or CAM-like metabolism has been observed in submerged aquatic plants and algae. A schematic model for C4-like mechanism in eukaryotic algal cells

2nd CCM type: Active transport of Ci Algae have a CCM which involves a variety of active CO2 and HCO3- uptake systems.

Ci transporters in cyanobacteria At least five distinct Ci transport systems are known for cyanobacteria with low and high affinity for Ci. Name Uptake Condition Place affinity comment BCT1 HCO3- Ci deficiency Pm H. A Na+ independent BicA constitutive L. A Na+ dependent SbtA NDH13 CO2 Tyl CA activity NDH14 Constitutive CA like Ci= inorganic carbon H.A= high affinity L.A= low affinity

Changing in transporters under Ci limitation

3rd CCM type: pH gradient in chloroplast. This type of CCM found in eukaryotic algae relies on the pH gradient across the thylakoid membrane in the light.

Leakage and regeneration of CO2 CO2 can easily diffuse through biological membranes. Microalgae are composed of only one or a few cells and ready access to the environment. Microalgae overcome the problem of CO2 diffusion by accumulating a charged species (HCO3-) that diffuses much more slowly than CO2. CA converts HCO3- to CO2 that is the substrate required by Rubisco.

C. Role of CA (Carbonic Anhydrase) in CO2 regeneration The role of CA is the dehydration of accumulated HCO3 in order to elevate concentration of CO2 in the carboxysome. HCO3- + H+ H2CO3 CO2 + H2O Loss of CA leads to a cell that cannot grow well on limiting levels of CO2 while accumulate HCO3- to higher levels than wild type cells

A transformed species with a human CA requires high CO2 for growth. more about CA A transformed species with a human CA requires high CO2 for growth. The human CA is localized to the cytoplasm and converts the accumulated HCO3_ to CO2 that leake from the cell and could not beused efficiently for photosynthesis. The location of the internal CA is as important as the packaging of Rubisco.

CCMs modulation Environmental conditions and nutrient availability play a significant role in modulating CCM activity. Macronutrient deficiency can affect photosynthesis so understanding the interactions between CCM activity, Environmental conditions, and nutrient availability is important. The amount of acquiring carbon through a CCM is coupled to the availability of other nutrients.

Nutrients phosphorus limitation would affect CCMs because of the dependency of Ci on ATP supply. Zinc is used as the metal cofactor of CA and is directly involved in the catalytic mechanism of these enzymes thus, it can be expected that a reduction in Zn availability would affect CCMs Iron can often compromise light-harvesting processes and photosynthesis.

Conclusion Rubisco biochemical properties and aquatic condition are the most important reasons that algae need to CCM CCM is a biological adaptation to low carbon dioxide concentrations in the environment Rubisco packaging, CO2 concentrating, and CA operating are essential for a perfect CCM. CO2 concentrating (at least) has three types; C4 like, active transport and pH gradient in chloroplast. CCM can be modulated by environmental factors and nutritions.

Thank you your attention for The file of presentation may be downloaded from following link: http://sci.ui.ac.ir/~reza_taheri/