Cyanobacteria are recognized as the main N
2-fixing microorganisms in the marine environment that participate in the universal nitrogen cycle
[10][41]. N
2-fixing cyanobacteria are grouped into filamentous non-heterocyst, unicellular cyanobacteria, and filamentous heterocyst-forming symbionts
Figure 2 [10]. Regarding filamentous non-heterocyst, the genus
Trichodesmium represents the most conspicuous marine organisms in terms of N
2 fixing
[42]. They are estimated to be responsible for approximately 50% of the global natural N
2 fixation. Their nitrogen fixation process occurs during the day at temperatures ranging between 24–30°C, but stops at night due to inactivation and degradation of the nitrogenase enzyme
[43][44]. Where N
2 fixation occurs in differentiated cells of
Trichodesmium spp., these cells consistently remain in clusters of about 3 to 20
[45]. The N products, such as ammonia (NH
3), ammonium (NH
4+), or the amino acid glutamine, are directly produced as a result of the reduction of N
2 [46][47]. The following synthesis of glutamine and glutamate by a glutamine synthetase/glutamine oxoglutarate aminotransferase (GS/GOGAT) reaction also requires energy in the form of 1 ATP and 1 NADPH + H
+. Thus,
Trichodesmium’s capacity to fix nitrogen is highly reliant on the bioavailability of energy
[48]. Unicellular N
2-fixing cyanobacteria were first discovered through the amplification of nitrogenase genes and gene transcripts (mRNAs) from oceanic water samples
[49]. According to recent studies, unicellular cyanobacteria (UCYN) have a high ability to fix N
2 within a cell-size fraction below 10 μm. These unicellular cyanobacteria have been classified into three groups based on their
nifH gene phylogeny: UCYN-A, UCYN-B, and UCYN-C
[49][50][51]. In the UCYN-A group, cyanobacterial nitrogenase gene sequences were most tightly linked to sequences from the marine unicellular cyanobacterium
Cyanothece sp. strain ATCC 51142
[52]. Despite numerous attempts, UCYN-A cyanobacteria have not been successfully cultivated
[53]. UCYN-A cyanobacteria have genotypes that have not previously been identified in free-living cyanobacteria, and they lack the genetic ability for oxygenic photosynthesis. Furthermore, nitrogenase genes are most abundant in UCYN-A during the light period, and thus can fix N
2 during daylight
[51]. UCYN-B is a group of free-living unicellular cyanobacteria that fix N
2 [54]. By studying the geographical populations of these free-living creatures, it may be concluded that they significantly contribute to the global N
2 fixation process
[55]. UCYN-B have only been identified in surface water and at 25% light depths (14 m) and is small in size (<10 μm)
[56][57]. They fix N
2 during the night
[10]. In UCYN-B (
Crocosphaera watsonii), nitrogenase enzyme synthesis begins just before nightfall to prepare for the upcoming N
2 fixing activity during the night
[58]. UCYN-C is a group of unicellular free-living cyanobacteria that fix N
2 during the night
[55][59]. It includes several cultivated cyanobacteria, such as
Cyanothece sp. strain ATCC51142 and TW3
[60][61]. By 1993, Reddy and colleagues isolated a unicellular cyanobacterium,
Cyanothece sp. ATCC51142, on the intertidal sands of the Texas Gulf Coast
[62]. Their symbiotic associations play vital roles in chemical defense, as well as supplying partners with energy and organic products of carbon or nitrogen fixation
[63]. In numerous oceanic diatom taxa, heterocyst-forming cyanobacterial symbionts are frequently observed
[64]. In the microenvironment of a photosynthetic symbiotic partner cell, such as diatoms, heterocyst-forming cyanobacteria have a beneficial effect, where the oxygen-sensitive nitrogenase protein is inhibited as a result of O
2 concentrations
[10].
Richelia intracellularis J. Schmidt 1901 is a filamentous cyanobacteria frequently found either free-living or in symbiosis with the diatoms
Rhizosolenia Brightwell 1858 and
Chaetoceros Ehrenberg 1844 in the plankton of the warm oceans
[63]. Cyanobacteria nitrogen fixation has various impacts that can be explained as follows: firstly, cyanobacteria are significant bioavailable nitrogen suppliers to the pelagic and benthic food webs that support fish productivity by fixing dissolved N
2. The food web benefits from bioavailable nitrogen in addition to the fresh or decaying filamentous cyanobacteria to nourish various invertebrates and release more nitrogen by cyanobacterial cells. Secondly, their ability to get beyond summertime nitrogen limitation by the fixation of dissolved nitrogen. Thirdly, they can contribute to enhancing productivity in different agricultural and ecological situations by building up soil fertility and increasing yield
[65][66][67]. The researchers conclude that only filamentous non-heterocyst cases have contributed to the explanation of the mechanism of nitrogen fixation. On the other hand, there are limitations in the data related to unicellular cyanobacteria, and filamentous heterocyst-forming symbionts, as there are no papers explaining the role of the nitrogen fixation mechanism in these cases. Taken together, nitrogen fixation has important impacts in various fields, i.e., agriculture and ecology, reflecting a potential benefit on the economy.
Industrialization and the burning of fossil fuels are to blame for the alarmingly high CO
2 levels in the atmosphere. CO
2 concentrations in surface waters have increased with the rise in atmospheric CO
2 during the past century. Cyanobacteria are one of the most promising organisms for CO
2 capture
[68][69][70]. The most common photoautotrophic lineage on Earth comprises cyanobacteria. Their effectiveness in photoautotrophism relies on a collection of adaptations known as the CO
2-concentrating mechanism (CCM). The CCM aims to increase the efficiency of CO
2 fixation by promoting the carboxylase reaction through improving the chemical conditions around the main CO
2-fixing enzyme, D-ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), and suppressing the oxygenase reaction
[71]. In cyanobacteria, RubisCO is enclosed within a class of protein-rich structures known as carboxysomes. The selectively permeable protein shell of the carboxysomes contains the CO
2-fixation enzyme, as well as the carbonic anhydrase enzyme, which provides CO
2 from a cytoplasmic bicarbonate pool
[71][72]. The carboxysomes have two main types, α-cyanobacteria, which are found in marine water and β-cyanobacteria, present in fresh water
[72]. In α-cyanobacteria, the RubisCO is categorized as (RuBisCO Form IA)
[73]. In α-cyanobacteria, up to two different types of plasma membrane-associated bicarbonate transporters, SbtA2 (high-affinity Na
+/HCO
3−) and BicA2 (medium to low-affinity Na
+/HCO
3−) promote CO
2 fixation
[71]. In the first stage of the (CCM), bicarbonate is concentrated inside the cell via transporters in the cell membrane. In the second stage of (CCM), the carboxysome plays a vital role in the enhancement of CO
2 fixation by co-localizing the two enzymes (CA) and (RuBisCO). Bicarbonate is assumed to enter the carboxysome through proteinaceous shell pores, and once inside, it is converted to CO
2 and used by RuBisCO. The conversion of HCO
3− to CO
2 is catalyzed by the CA enzyme. After CO
2 fixation, some of it exits into the cytosol, while some promptly combines with RuBP to form phosphoglycolate
Figure 3 [72][74]. The components of the cyanobacterial CO
2-concentrating mechanism (CCM) have a vital role in the improvement of the efficiency of photosynthetic CO
2 fixation in the chloroplasts of crop plants via specific cyanobacterial transporters, accordingly leading to better crop yields
[75]. Finally, the researchers conclude that the importance of CO
2 revolves around the conversion of inorganic carbon to organic carbon through vital processes.