• Abruptophycus colonialis Vologdin, 1962 †
• Actinophycus obrutschevii Korde, 1954 †
• Aegeococcus thuretii Konstantinou & Gkelis, 2020
• Agmenellum quadruplicatum (Meneghini) Brébisson, 1839 accepted as Merismopedia quadruplicata (Meneghini) Trevisan, 1845
• Albrightia tortuosa J.Copeland, 1936
• Aliinostoc vietnamicum S.Maltseva, E.Kezlya & Y.Maltsev, 2022
• Alkalinema pantanalense Vieira Vaz et al., 2015
• Alternantia ramifera J.Schiller, 1954
• Amazonocrinus nigriterrae Alvarenga & al., 2021
• Ammatoidea murmanica Petrov, 1961
• Amorphonostoc entophytum (Bornet & Flahault) Elenkin, 1936 accepted as Nostoc paludosum Kützing ex Bornet & Flahault, 1886
• Amphirytos necridica Panou & Gkelis, 2022
• Amphithrix janthina var. torulosa Bornet & Flahault, 1886
• Acanthina multiformis Korde, 1973 †
• Actinothrix stokesiana J.E.Gray, 1864 (uncertain)
• Affixifilum floridanum Lefler, D.E.Berthold & Laughinghouse, 2021
• Albertania alaskensis Strunecky, Raabova, Bernardova, Ivanova, Semanova, Crossley & Kaftan, 2019
• Aliinostoc magnakinetifex Kabirnataj, Nematzadeh, Talebi, Saraf, Suradkar, Tabatabaei & Singh, 2020
• Aliterella compacta P.Jung, Büdel & Lakatos, 2021
• Altericista variichlora Averina, E. Polyakova, Senatskaya & Pinevich, 2021
• Amalia elegans (Wołoszyńska) De Toni, 1934 accepted as Romeria elegans (Woloszynska) Geitler, 1932
• Ammassolinea attenuata P.Hasler, P.Dvorák, A.Poulícková & D.A.Casamatta, 2014
• Ammatoidea sinensis S.-H.Ley, 1944
• Amorphonostoc punctiforme f. polymorphum Kukk accepted as Nostoc commune Vaucher ex Bornet & Flahault, 1888
• Amphithrix hospita Kützing, 1849 accepted as Dasyactis hospita (Kützing) P.L.Crouan & H.M.Crouan, 1867
• Acaryochloris thomasi Partensky & al., 2018
• Adrianema adriaticum (Ercegovic) G.De Toni, 1936
• Agmenellum prasiola (Meneghini) Brébisson, 1839 accepted as Trochiscia prasiola Meneghini, 1837
• Alborzia kermanshahica Nowruzi & F.Soares, 2021
• Aliinostoc soli A.Saraf & al., 2018 † accepted as Pseudoaliinostoc soli (A.Saraf & al.) Lee, Bang, Kim, Ki & Lee, 2021 †
• Aliterella shaanxiensis unknon authority (uncertain)
• Alternantia geitleri J.Schiller, 1954
• Amalia leopoliensis (Raciborski) De Toni, 1934 accepted as Romeria leopoliensis (Raciborski) Koczwara, 1932
• Ammatoidea breviarticulata Hirose & Chung, 1970
• Ammatoidea xizangensis Zhu & Y.-Y.Li, 1984
• Amorphonostoc symbioticum (F.Wettstein) Elenkin, 1936 accepted as Nostoc symbioticum F.V.Wettstein, 1915
• Amphithrix janthina Bornet & Flahault, 1886 accepted as Tapinothrix janthina (Bornet & Flahault) Bohunická & J.R.Johansen, 2011
• Acaciella angepena Preiss (uncertain)
• Actinophycus ramosus A.G.Vologdin, 1962 †
• Aerosakkonema funiforme Nanda Kyaw Thu & Watanabe, 2012
• Agmenellum tranquillum (Ehrenberg) Trevisan, 1842 accepted as Merismopedia tranquilla (Ehrenberg) Trevisan, 1845
• Aliinostoc catenatum Kabirnataj, Nematzadeh, Talebi, Saraf, Suradkar, Tabatabaei & Singh, 2020
• Aliterella atlantica J.Rigonato et al., 2016 (uncertain)
• Alpinella graeca Dragastan, Gieslich, D.K.Richter, Grewer, Kaziur, Kube & Radusch, 1994 †
• Alternantia tabularis J.Schiller, 1954
• Ambigolamellatus ovatus Maithy, 1979 †
• Ammatoidea olivacea Yoneda, 1952
• Amorphonostoc punctiforme (Hariot) Elenkin, 1936 accepted as Nostoc paludosum Kützing ex Bornet & Flahault, 1886
• Amphithrix amoena Kützing, 1843 accepted as Phormidium ambiguum Gomont, 1892
• Amphithrix leprieurei Montagne, 1861 (uncertain)
• Acanthina bullosa Korde, 1973 †
• Actinothorax stokesiana (J.E.Gray) J.E.Gray accepted as Actinothrix stokesiana J.E.Gray, 1864
• Aetokthonos hydrillicola S.B.Wilde & J.R.Johansen, 2014
• Ainactis alpina Kützing (uncertain)
• Aliinostoc constrictum Kabirnataj et al., 2020 accepted as Pseudoaliinostoc constrictum (Kabirnataj & al.) Lee, N.-J., Bang, S.-D., Kim, T., Ki., J.-S. & Lee, O.-M., 2021
• Aliterella chasmolithica P.Jung, Schermer, Mikhailyuk & Büdel, 2020
• Altericista lacusladogae Averina, E.Polyakova, Senetskaya & Pinevich, 2021
• Alternia silva Vologdin, 1962 †
• Amganella glabra (Vologdin) Reitlinger, 1959 † accepted as Proaulopora glabra Vologdin, 1938 †
• Ammatoidea simplex Woronichin, 1923
• Amorphonostoc punctiforme (Kützing) Elenkin (uncertain)
• Amphithrix barbata Kützing, 1847 (uncertain)
• Amphithrix meneghiniana Kützing, 1843 (uncertain)
• Acaryochloris marina H.Miyashita & M.Chihara, 2003
• Acuasiphonoria ordovica Lijing Liu, Yasheng Wu, H.J.Yang & Riding, 2016 †
• Affixifilum granulosum (Caires, Sant'Anna & Nunes) Lefler, D.E.Bertholdt & Laughinghouse, 2021
• Albertania skiophila Zammit, 2018
• Aliinostoc morphoplasticum Bagchi, Dubey & P.Singh, 2017
• Aliterella gigantea P.Jung, Büdel & Lakatos, 2021
• Altericista violacea Averina, E.Polyakova, Senatskaya & Pinevich, 2021
• Amalia gracilis (Koczwara) G.B.De Toni, 1934 accepted as Romeria gracilis (Koczwara) Koczwara, 1932
• Ammatoidea aegaea Anagnostidis & Pantazidou, 1991
• Ammatoidea singularis Hirose & Chung, 1970
• Amorphonostoc punctiforme f. populorum (Geitler) Hollerbach, 1936 accepted as Nostoc punctiforme f. populorum (Geitler) Hollerbach, 1936
• Amphithrix incrustata Kützing, 1847 (uncertain)
• Abruptophycus compositus Vologdin, 1962 †
• Actinophycus parvus A.G.Vologdin, 1962 †
• Aerofilum fasciculatum Chakraborty & J.Mukherjee, 2021
• Agmenellum thermale (Kützing) Drouet & Daily, 1956 accepted as Merismopedia thermalis Kützing, 1843
• Aliinostoc alkaliphilum Christodoulou, Economou-Amilli, M.Fiore & Sivonen, 2022
• Aliterella antarctica J.Rigonato et al., 2016
• Allocoleopsis franciscana Moreira-Fernandes, Giraldo-Silva, D.Roush & Garcia-Pichel, 2021
• Alternantia serra J.Schiller, 1954
• Amazonocrinus thailandica Tawong, Pongcharoen, Pongpadung, Ponza & Saijuntha, 2022
• Ammatoidea normanii West & G.S.West, 1897
• Amorphonostoc paludosum (Bornet & Flahault) Elenkin, 1936 accepted as Nostoc paludosum Kützing ex Bornet & Flahault, 1886
• Amphithrix amethystea (Kützing) Kützing, 1849 (uncertain)
• Amphithrix laminariae Kuckuck, 1894 accepted as Tapinothrix violacea Bohunická & J.R.Johansen, 2011
• Actinophycus kordeae A.G.Vologdin, 1962 †
• Aegeococcus anagnostidisii Konstantinou & Gkelis, 2020
• Agmenellum quadrijugum (Turpin) Trevisan, 1842 (uncertain)
• Albrightia roraimae Kastovský, Fucíková, Hauer & Bohunická, 2011
• Aliinostoc tiwarii A.Saraf, P.Singh & al., 2018 accepted as Pseudoaliinostoc tiwarii (A.Saraf, P.Singh & al.) Lee, Bang, Kim, Ki & Lee, 2021
• Aliterella vladivostokensis Abdullin, A.Y.Nikulin, Bagmet & V.Y.Nikulin, 2021
• Alternantia quadriga J.Schiller, 1954
• Amazoninema brasiliense Genuário, W.R.de Souza, R.T.R.Monteiro, Sant'Anna & I.T.S.Melo, 2020
• Ammatoidea multispora Yoneda, 1952
• Ammatoidea yellowstonensis J.J.Copeland, 1936
• Amphiheterocytum lacustre Sant'Anna et al., 2018
• Amphithrix janthina f. torulosa (Bornet & Flahault) Elenkin, 1949 accepted as Amphithrix janthina var. torulosa Bornet & Flahault, 1886