Roles of Algae in Biodiversity

 Cyanobacteria and the Origin of an Oxygen-Rich Atmosphere

 By the process of the oxygenic photosynthesis the cyanobacteria influence Earth’s atmospheric chemistry by releasing gaseous oxygen. It is estimated that the oxygenic cyanobacteria was first appeared more than 2.7 billion years ago and by about 2.4 billion years ago, atmospheric oxygen had become abundant enough that organisms could use it as an electron acceptor in more efficient aerobic respiration.  This event fostered the origin and early diversification of eukaryotes. So, oxygenic cyanobacteria were also essential to the origin of the first eukaryotic algae. Together with the cyanobacteria, early eukaryotic algae continued to produce oxygen, with the result that atmospheric levels had nearly reached modern level i.e. 21% by 550 million years ago.

The another major impact of oxygenic photosynthesis was that by 1 billion years ago, the interaction of atmospheric oxygen with solar ultraviolet radiation had generated a stratospheric ozone shield sufficient to protect surface life from UV damage.

So, oxygen-rich atmosphere as an essential precursor to the formation of Earth’s earliest ozone shield. The modern production of atmospheric oxygen by cyanobacteria, eukaryotic algae, and plants continued to build Earth’s ozone shield, protecting all the living beings from harmful radiations. 

Algae and the Carbon Cycle

However the cyanobacteria, algae, and plants are generating oxygen for billions of years, it does not explain how oxygen accumulated in Earth’s atmosphere or today’s relatively high and stable levels of atmospheric oxygen. The algae affect the Earth’s carbon cycle in two ways. First way is that, the capability of algae and plants to produce organic compounds that are resistant to microbial breakdown and thus is readily buried in anoxic ocean or lake sediments. So, those sediments are not oxidized back to carbon dioxide gas. Another effect is that it generates the large deposits of sedimentary carbonates which decrease the CO₂ content of atmosphere.

Algae and Organic Carbon Sequestration

Organic carbon compounds produced by algae can sink though the water column to the bottom without being completely decomposed due to which the organic carbon is degraded to CO₂ more slowly than it is produced. Thus algal organic carbon can be sequestered in deep anoxic sediments where it is sheltered from microbial oxidation.  Algae produce many kinds of organic compounds that are resistant to chemical breakdown and decay and are thus described as refractory carbon. The refractory carbon, together with more degradable organic compounds that may be associated with it, can build up as hydrocarbon-rich sedimentary deposits called kerogens. Similarly, green algae that are closely related to land plant produce zygotes that have degradation-resistance cell wall layers. These are similar to the sporopollenin. Dinoflagellates often produce resistant-walled cyst stages, which enable survival through stressful conditions, and, in some cases, these cyst walls contain a resistant substance that is distinct from sporopollenin.

 Aquatic bacteria can convert degradable organic carbon exuded from living algae or released from decomposing algae into fairly decay-resistant colloids. These colloidal materials forms the acylpolysaccharides (APS) which aggregate with bacteria, zooplankton remains, and fecal pellets to form larger particles known as marine snow, that readily sink. Marine snow is very important mechanism by which organic carbon reaches sediments.

The Role of Algae in Carbonate Formation

Various groups of algae with cyanobacteria have transformed very large amounts of carbon from the atmosphere into carbonate sediments and rocks such as limestone. Carbonates are important because they contain 40% of the world’s hydrocarbon reserves.  Carbonic acids formed by reacting H₂O and increased CO₂ caused by human activities have lead the acidification of ocean as carbonic acid dissolves carbonate minerals. For this reason, the production of carbonates by algae is an important topic.

The process by which the algae (cyanobacteria, some fresh water green algae, and green, red and brown seaweeds) produce calcium carbonate is called calcification. Although the calcification and exact mechanism by which it occurs are not completely understood. In some algae, calcification may help prevent photosynthesis from becoming limited by the availability of carbon dioxide.

Impact of Modern Carbon Dioxide Levels on Algal Photosynthesis

Geochemical evidence indicates that carbon dioxide levels in the atmosphere and water were much higher, and oxygen levels much lower, 2.2 billion years ago than at present. Oxygenic photosynthesis and the sedimentation of organic carbon and carbonates caused carbon dioxide levels to decrease to today’s relatively low levels. The cellular adaptations involve rubisco, the enzyme that converts inorganic carbon dioxide to reduced organic compounds. Another reason for reduced CO₂ and increased O₂ was the evolution of carbon concentrating mechanisms (CCMs). Algal CCMs may involve cell membrane inorganic carbon transporters, enzymes that interconvert CO₂ and bicarbonate, calcification-linked processes, and specialized cellular structures.

Carbon Concentration Mechanisms of Cyanobacteria

Many bacteria rely on CCMs because their rubisco has relatively low specificity for CO₂. If CO₂ is available, the gas diffuses into cells and is likely converted into bicarbonate ion at the thylakoid membranes. Before bicarbonate ion can be used in photosynthesis, it must be converted into CO₂ because only CO₂ can be used directly in carbon fixation.

HCO₃^- + H⁺                                         CO₂ + H₂O

Carbon Concentration Mechanisms of Eukaryotic Algae

Eukaryotic algae possess a wide range of mechanisms for acquiring inorganic carbon for photosynthesis.  Some algae possess plastid structures known as pyrenoids, which are through to a play in CCMs. Some eukaryotic algae may have a C₄-like photosynthesis, in which the enzyme PEP carboxylase traps inorganic carbon into a 4-carbon organic compound. CO₂ later released from this compound for fixation by rubisco. Algae can generate CO₂ from environmental bicarbonate is to excrete protons (H⁺) across cell membranes. The protons react with bicarbonate to yield carbon dioxide and water, whereupon the CO₂ can be taken into cells and plastids.  Such processes are often linked to calcification.

Algal Use of Organic Carbon

Non-photosynthetic algal species rely on the uptake of organic carbon from their environment. Those that consume dissolved organic carbon are described as osmotrophs, and those taking up particles of organic material are phagotrophs. Many algae possess transport system that allows the active transport of sugars such as glucose. Some algae utilize dissolved organic carbon only in the light. For example, diatom.

Studies of model species have revealed the molecular and biochemical basis for uptake of organic compounds by algae and its interaction with CCMs. The presence of diverse CCMs and widespread ability to use organic carbon mean that growth of most modern algae is not generally limited by inorganic carbon availability, despite today’s relatively low atmospheric CO₂ levels. If atmospheric CO₂ levels continue to increase in the future, the utility of algal CCMs might decline. Thus nitrogen, phosphorus, iron, silica minerals, and light probably limit algal growth more often than does inorganic carbon.