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Candidatus Atelocyanobacterium thalassa, also referred to as UCYN-A, is a diazotrophic species of cyanobacteria commonly found in measurable quantities throughout all the world's oceans and some seas.[1][2] A. thalassa are spheroid in shape and are 1-2µm in diameter.[3] A. thalassa provide nitrogen to ocean regions by fixing non biologically available atmospheric nitrogen into biologically available ammonium that other marine microorganisms can use.[1] Unlike many other cyanobacteria, the A. thalassa genome does not contain genes for RuBisCO, photosystem II, or the TCA cycle.[4] Consequently, A. thalassa lacks the ability to fix carbon via photosynthesis. Some genes specific to the cyanobacteria group are also absent from the A. thalassa genome despite being an evolutionary descendent of this group.[4] With the inability to fix their own carbon, A. thalassa are obligate symbionts that have been found within photosynthetic pikeoeukaryotic algae.[4] Most notably, the UCYN-A2 sublineage has been observed as an endosymbiont in the Braarudosphaera bigelowii algae with a minimum of 1-2 endosymbionts per host.[1][5] A. thalassa fixes nitrogen for the algae, while the algae provides carbon for A. thalassa through photosynthesis.[6] There are many sublineages of A. thalassa that are distributed across a wide range of marine environments and host organisms.[2] It appears that some sublineages of A. thalassa have a preference for oligotrophic ocean waters while other sublineages prefer coastal waters.[7] Much is still unknown about all of A. thalassa's hosts and host preferences.[1]
Ecology
Nitrogen fixation
Nitrogen fixation, which is the reduction of N2 to biologically available nitrogen, is an important source of N for aquatic ecosystems. For many decades, N2 fixation was vastly underestimated. The assumption that N2 fixation only occurred via Trichodesmium (Links to an external site.) and Richelia led to the conclusion that in the oceans, nitrogen output exceeded the input. However, researchers found that the nitrogenase fixation complex has variable evolutionary histories. With the utilization of polymerase chain reaction (PCR), the requirements of cultivation or microscopy to identify N2 fixing microorganism is no longer needed. As a result, marine N2-fixing microorganisms other than Trichodesimum were found by sequencing PCR-amplified nitrogenase gene (nifH) fragments.
In 1989, a short nifH gene sequence was discovered. Then, in the following 15 years, it was revealed to be an unusual cyanobacterium that is widely distributed.[8] The microbe was originally given the name UCYN-A for "unicellular cyanobacteria group A". In the research published in 1998, nifH was amplified directly from Pacific and Atlantic as sequences from bacterial, unicellular cyanobacterial nifH, and Trichodesmium and diatom symbionts.[9] With the use of cultivation-independent PCR and quantitative PCR (qPCR) targeting the nifH gene, studies found that A. thalassa is distributed in many ocean regions, showing that the oceanic plankton contains a broader range of nitrogen-fixing microorganisms than was previously believed.
Habitat
In addition to being found in all of the oceans, A. thalassa have also been found in the North Sea, Mediterranean Sea, Adriatic Sea, Red Sea, Arabian Sea, South China Sea, and the Coral Sea.[10] A. thalassa possess a nitrogen-fixation characteristic, so it is found in areas of the ocean where atmospheric nitrogen is present.[10] It contains a nifH gene, which is the gene that is transcribed and translated to produce NifH protein (a nitrogenase). This nitrogenase is the enzyme that catalyzes nitrogen fixation, and studies have shown that this gene is widely distributed throughout the different parts of the ocean.[11] This indicates that A. thalassa is found in a wide variety of ocean environments and continuously expands in range, further reinforcing its significant role in nitrogen fixation.[10] Although A. thalassa is ubiquitous, its abundance is highly regulated by various abiotic factors such as temperature and nutrients.[12] Studies have shown that it occupies cooler waters compared to other diazotrophs.[13]
Furthermore, analysis of the A. thalassa genome shows a wide variety of nifH gene sequences. This means that this cyanobacterium group is further divided into genetically distinct sublineages, four of which have been identified and defined. These four sublineages are: UCYN-A1, UCYN-A2, UCYN-A3, and UCYN-A4, and studies have shown that these groups are adapted to different marine environments.[2] UCYN-A1 and UCYN-A3 co-exist in open ocean oligotrophic waters. while UCYN-A2 and UCYN-A4 co-exist in coastal waters.[7]
Metabolism
Obligate photoheterotroph
A. thalassa is categorized as a photoheterotroph. Complete genome assembly reveals a reduced size genome (only 1.44 megabases) and the lack of pathways needed for metabolic self-sufficiency common to cyanobacteria.[14] A. thalassa lacks all genes for photosystem II of the photosynthetic apparatus, RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), and enzymes of the Calvin and tricarboxylic acid (TCA) cycle.[15][16] Due to the lack of metabolically essential genes, A. thalassa requires external sources of carbon and other biosynthetic compounds.[14] A. thalassa lacks the tricarboxylic acid cycle, however expresses a putative dicarboxylic-acid transporter.[14] This suggests that A. thalassa fills its requirement for dicarboxylic acids from an external source.[14] The complete or partial lack of biosynthetic enzymes required for valine, leucine, isoleucine, phenylalanine, tyrosine and tryptophan biosynthesis further suggests the need for external sources of amino acids.[14] A. thalassa still possesses the Fe-III transport genes (afuABC) which allow Fe-III transport into the cell.[4]
Obligate symbiosis
A. thalassa has been discovered to be in an obligate symbiotic relationship with an algae, the calcifying haptophyte Braarudosphaera bigelowii.[1] Stable isotope experiments revealed that A. thalassa fixes 15N2 and exchanges fixed nitrogen with the partner, while H13CO3- was fixed by B. bigelowii and exchanged to A. thalassa. A. thalassa receives ~16% of the total carbon of the symbiotic partner, and exchanges ~85 -95% of total fixed nitrogen in return.[1][17]
A. thalassa must live in close physical association with its metabolically dependent symbiosis partner; however, the details of the physical interaction are still unclear due to a lack of clear microscopy images.[4] A. thalassa may be a true endosymbiont and fully enclosed within the host’s cell membrane or has molecular mechanisms to allow for secure attachment and transfer of metabolites.[17] This symbiotic connection must not allow the passage of oxygen while maintaining an exchange of fixed nitrogen and carbon.[17] Such close symbiosis also requires signalling pathways between the partners and synchronized growth.[17]
Daytime N-fixation
A. thalassa is unicellular, hence it cannot have specialized cellular compartments (heterocysts) to protect the nitrogenase (nifH) from oxygen exposure. Other nitrogen-fixing organisms employ temporal separation by fixing nitrogen only at night-time, however, A. thalassa has been found to express the nifH gene during the day-time.[18][15] This is possible due to the absence of photosystem II and, therefore, oxygen and transcriptional control.[15][19] It is hypothesized that the day-time nitrogen-fixation is more energy-efficient than night-time fixation common in other diazotrophs because light energy can be used directly for the energy-intensive nitrogen fixation.[19]
Life cycle
The lifecycle of A. thalassa is not well understood. As obligate endosymbionts, A. thalassa is thought to be unable to survive outside of the host, suggesting its entire life cycle takes place inside of the host.[4] The division and replication of A. thalassa are at least partially under the control of the host cell.[20] It is thought that a signal transduction pathway exists to regulate the amount of A. thalassa cells within the host to ensure a sufficient amount of A. thalassa cells are supplied to the host's daughter cell during cell division.[4]
Diversity
A. thalassa has been found in measurable amounts in nearly all oceanic bodies.[10] The lineages of A. thalassa are split by their determining oligotypes. There is a very high level of similarity between all sublineages in their amino acid sequencing, but some variance was found in their nifH sequences. The analyzed oligotype of A. thalassa is its nitrogenase (nifH) sequencing. After an oligotyping analysis of the nifH sequence, it was determined that there were thirteen positions of variance (entropy).[2] The variances would cause different oligotypes/sublineages of A. thalassa to be found in different relative abundances and have different impacts on the ecosystems they would be found in.
Oligotyping
Four main sublineages have been identified from oligotype analysis, and their respective oligotypes are: UCYN-A1/ Oligo1, UCYN-A2/Oligo2, UCYN-A3/Oligo3, UCYN-A4/Oligo4. UCYN-A1 was the most abundant oligotype found across the oceans.[2] The UCYN-A1 sublineage has an abundance of nitrogenase in a range of 104 - 107 copies of nifH per litre.[21] UCYN-A1 and UCYN-A2 also have a significantly reduced genome size. UCYN-A2 differs from UCYN-A1 in that its oligo2 oligotyping has 10/13 differing positions of entropy from oligo1 (UCYN-A1). UCYN-A3 differs from UCYN-A1 with its oligo3 differing from oligo1 with an entropy position difference of 8/13. UCYN-A4 also differs from UCYN-A1 by 8/13 entropy positions in a different set.
Range
UCYN-A1 and UCYN-A2 can be found in coastal regions and are co-occurring.[2] UCYN-A2 is typically found in high latitude temperate coastal waters. In addition, it can also be found co-occurring with UCYN-A4 in the coastal bodies of water. UCYN-A3 was found to be in greater abundance in the surface of the open ocean in the subtropics. In addition, UCYN-A3 has only been found to co-occur with UCYN-A1 thus far.
References
External links
- HAMAP: cyanobacterium UCYN-A complete proteome
- Life Stripped Down
- UCYN-A, la cyanobactérie qui fixe l'azote mais ignore la photosynthèse French journal article about UCYN-A
- Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II
- ↑ a b c d e f Thompson AW, Foster RA, Krupke A, Carter BJ, Musat N, Vaulot D, Kuypers MM, Zehr JP: Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga. In: Science. 337. Jahrgang, Nr. 6101, September 2012, S. 1546–1550, doi:10.1126/science.1222700, PMID 22997339, bibcode:2012Sci...337.1546T.
- ↑ a b c d e f Turk-Kubo KA, Farnelid HM, Shilova IN, Henke B, Zehr JP: Distinct ecological niches of marine symbiotic N2 -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa sublineages. In: Journal of Phycology. 53. Jahrgang, Nr. 2, April 2017, S. 451–461, doi:10.1111/jpy.12505, PMID 27992651.
- ↑ Kyoko Hagino, Ryo Onuma, Masanobu Kawachi, Takeo Horiguchi: Discovery of an Endosymbiotic Nitrogen-Fixing Cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae). In: PLoS ONE. 8. Jahrgang, Nr. 12, 4. Dezember 2013, ISSN 1932-6203, S. e81749, doi:10.1371/journal.pone.0081749, PMID 24324722, PMC 3852252 (freier Volltext) – (nih.gov).
- ↑ a b c d e f g Zehr JP, Shilova IN, Farnelid HM, Muñoz-Marín MD, Turk-Kubo KA: Unusual marine unicellular symbiosis with the nitrogen-fixing cyanobacterium UCYN-A. In: Nature Microbiology. 2. Jahrgang, Nr. 1, Dezember 2016, S. 16214, doi:10.1038/nmicrobiol.2016.214, PMID 27996008 (escholarship.org).
- ↑ Anne Thompson, Brandon J. Carter, Kendra Turk-Kubo, Francesca Malfatti, Farooq Azam, Jonathan P. Zehr: Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host: UCYN-A genetic diversity. In: Environmental Microbiology. 16. Jahrgang, Nr. 10, Oktober 2014, S. 3238–3249, doi:10.1111/1462-2920.12490 (englisch, wiley.com).
- ↑ Stephens T: Unusual symbiosis discovered in marine microorganisms. University of California Santa Cruz Newscenter, 20. September 2012, abgerufen am 13. Januar 2017.
- ↑ a b Cabello AM, Turk-Kubo KA, Hayashi K, Jacobs L, Kudela RM, Zehr JP: Unexpected presence of the nitrogen-fixing symbiotic cyanobacterium UCYN-A in Monterey Bay, California. In: Journal of Phycology. 56. Jahrgang, Nr. 6, Dezember 2020, S. 1521–1533, doi:10.1111/jpy.13045, PMID 32609873, PMC 7754506 (freier Volltext).
- ↑ Zehr JP, McReynolds LA: Use of degenerate oligonucleotides for amplification of the nifH gene from the marine cyanobacterium Trichodesmium thiebautii. In: Applied and Environmental Microbiology. 55. Jahrgang, Nr. 10, Oktober 1989, S. 2522–2526, doi:10.1128/aem.55.10.2522-2526.1989, PMID 2513774, PMC 203115 (freier Volltext), bibcode:1989ApEnM..55.2522Z.
- ↑ Zehr JP, Mellon MT, Zani S: New Nitrogen-Fixing Microorganisms Detected in Oligotrophic Oceans by Amplification of Nitrogenase (nifH) Genes. In: Applied and Environmental Microbiology. 64. Jahrgang, Nr. 12, Dezember 1998, S. 5067, doi:10.1128/aem.64.12.5067-5067.1998, PMID 16349571, PMC 90973 (freier Volltext), bibcode:1998ApEnM..64.5067Z.
- ↑ a b c d e Farnelid H, Turk-Kubo K, del Carmen Muñoz-Marín M, Zehr JP: New insights into the ecology of the globally significant uncultured nitrogen-fixing symbiont UCYN-A. In: Aquatic Microbial Ecology. 77. Jahrgang, Nr. 3, 16. September 2016, ISSN 0948-3055, S. 125–138, doi:10.3354/ame01794 (englisch, int-res.com).
- ↑ Zehr JP, Turner PJ: Nitrogen fixation: Nitrogenase genes and gene expression. In: Methods in Microbiology (= Marine Microbiology). 30. Jahrgang. Academic Press, 1. Januar 2001, S. 271–286, doi:10.1016/s0580-9517(01)30049-1 (englisch).
- ↑ Goebel NL, Turk KA, Achilles KM, Paerl R, Hewson I, Morrison AE, Montoya JP, Edwards CA, Zehr JP: Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N₂ fixation in the tropical Atlantic Ocean. In: Environmental Microbiology. 12. Jahrgang, Nr. 12, Dezember 2010, S. 3272–3289, doi:10.1111/j.1462-2920.2010.02303.x, PMID 20678117.
- ↑ Moisander PH, Beinart RA, Hewson I, White AE, Johnson KS, Carlson CA, Montoya JP, Zehr JP: Unicellular cyanobacterial distributions broaden the oceanic N2 fixation domain. In: Science. 327. Jahrgang, Nr. 5972, März 2010, S. 1512–1514, doi:10.1126/science.1185468, PMID 20185682.
- ↑ a b c d e Tripp HJ, Bench SR, Turk KA, Foster RA, Desany BA, Niazi F, Affourtit JP, Zehr JP: Metabolic streamlining in an open-ocean nitrogen-fixing cyanobacterium. In: Nature. 464. Jahrgang, Nr. 7285, März 2010, S. 90–94, doi:10.1038/nature08786, PMID 20173737, bibcode:2010Natur.464...90T.
- ↑ a b c Zehr JP, Bench SR, Carter BJ, Hewson I, Niazi F, Shi T, Tripp HJ, Affourtit JP: Globally distributed uncultivated oceanic N2-fixing cyanobacteria lack oxygenic photosystem II. In: Science. 322. Jahrgang, Nr. 5904, November 2008, S. 1110–1112, doi:10.1126/science.1165340, PMID 19008448, bibcode:2008Sci...322.1110Z.
- ↑ Bothe H, Tripp HJ, Zehr JP: Unicellular cyanobacteria with a new mode of life: the lack of photosynthetic oxygen evolution allows nitrogen fixation to proceed. In: Archives of Microbiology. 192. Jahrgang, Nr. 10, Oktober 2010, S. 783–790, doi:10.1007/s00203-010-0621-5, PMID 20803290.
- ↑ a b c d Krupke A, Mohr W, LaRoche J, Fuchs BM, Amann RI, Kuypers MM: The effect of nutrients on carbon and nitrogen fixation by the UCYN-A-haptophyte symbiosis. In: The ISME Journal. 9. Jahrgang, Nr. 7, Juli 2015, S. 1635–1647, doi:10.1038/ismej.2014.253, PMID 25535939, PMC 4478704 (freier Volltext).
- ↑ Church MJ, Short CM, Jenkins BD, Karl DM, Zehr JP: Temporal patterns of nitrogenase gene (nifH) expression in the oligotrophic North Pacific Ocean. In: Applied and Environmental Microbiology. 71. Jahrgang, Nr. 9, September 2005, S. 5362–5370, doi:10.1128/aem.71.9.5362-5370.2005, PMID 16151126, PMC 1214674 (freier Volltext), bibcode:2005ApEnM..71.5362C.
- ↑ a b del Carmen Muñoz-Marin M, Shilova IN, Shi T, Farnelid H, Cabello AM, Zehr JP: A transcriptional cycle suited to daytime N2 fixation in the unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A). In: bioRxiv. 14. November 2018, doi:10.1101/469395.
- ↑ Marine Landa, Kendra A. Turk-Kubo, Francisco M. Cornejo-Castillo, Britt A. Henke, Jonathan P. Zehr: Critical Role of Light in the Growth and Activity of the Marine N2-Fixing UCYN-A Symbiosis. In: Frontiers in Microbiology. 12. Jahrgang, 5. Mai 2021, ISSN 1664-302X, doi:10.3389/fmicb.2021.666739 (doi.org).
- ↑ Mulholland MR, Bernhardt PW, Blanco-Garcia JL, Mannino A, Hyde K, Mondragon E, Turk K, Moisander PH, Zehr JP: Rates of dinitrogen fixation and the abundance of diazotrophs in North American coastal waters between Cape Hatteras and Georges Bank. In: Limnology and Oceanography. 57. Jahrgang, Nr. 4, 24. Juni 2012, ISSN 0024-3590, S. 1067–1083, doi:10.4319/lo.2012.57.4.1067, bibcode:2012LimOc..57.1067M.