• Abacella cornuta Shuysky, 1973 †: Algae
• Abacella delicatula J.H.Johnson, 1964 †: Algae
• Abacella dentata Shuysky, 1973 †: Algae
• Abacella pertusa Maslov, 1956 †: Algae
• Abacella pustulata Shuysky, 1973 †: Algae
• Abacella stellata Shuysky, 1991 †: Algae
• Abbottella concinna Hollenberg, 1967: Algae
• Abedinium dasypus (Cachon & Cachon-Enjumet) Loeblich Jr & Loeblich III, 1966: Algae
• Abratopdinium cardioforme S.Mao & B.A.R.Mohr †: Algae
• Abratopdinium kerguelense S.Mao & B.A.R.Mohr †: Algae
• Abroteia suborbicularis (Harvey) J.Agardh, 1878: Algae
• Abruptophycus compositus Vologdin, 1962 †: Algae
• Acaciella angepena W.V.Preiss, 1972 †: Algae
• Acaciella augusta W.V.Preiss, 1972 †: Algae
• Acaciella australica (Howchin) M.R.Walter, 1972 †: Algae
• Acaciella savoryensis K.Grey & M.R.Walter, 1994 †: Algae
• Acanthina bullosa Korde, 1973 †: Algae
• Acanthina multiformis Korde, 1973 †: Algae
• Acanthochloris bacillifera Pascher, 1938: Algae
• Acanthochloris brevispinosa Pascher, 1930: Algae
• Acanthochloris ingens Margalef, 1956: Algae
• Acanthochloris scherffelii Pascher, 1938: Algae
• Acanthochloris verrucosa Ettl, 1977: Algae
• Acanthococcus antarcticus Hooker f. & Harvey, 1845: Algae
• Acanthodinium caryophyllum Kofoid, 1907: Algae
• Acanthogonyaulax acanthosphaera (Sargeant) Sargeant †: Algae
• Acanthogonyaulax paliuros (Sargeant) Sargeant †: Algae
• Acanthogonyaulax spinifera (Murray & Whitting) H.W.Graham, 1942: Algae
• Acanthoica acanthifera Lohmann ex Lohmann, 1913: Algae
• Acanthoica acanthos J.Schiller, 1925: Algae
• Acanthoica bidentula Lecal-Schlauder, 1951: Algae
• Acanthoica biscayensis A.Kleijne, 1992: Algae
• Acanthoica coronata Lohmann, 1903: Algae
• Acanthoica janchenii J.Schiller, 1925: Algae
• Acanthoica lithostratos J.Schiller, 1926: Algae
• Acanthoica maxima Heimdal, 1981: Algae
• Acanthoica monospina J.Schiller, 1925: Algae
• Acanthoica ornata Conrad, 1928: Algae
• Acanthoica quattrospina Lohmann, 1903: Algae
• Acanthoica rubens Kamptner, 1941: Algae
• Acanthoica schilleri Conrad, 1928: Algae
• Acanthoica trispinosa Ostenfeld: Algae
• Acanthophora aokii Okamura, 1934: Macroalgae
• Acanthophora dendroides Harvey, 1855: Macroalgae
• Acanthophora muscoides (Linnaeus) Bory de Saint-Vincent, 1828: Macroalgae
• Acanthophora nayadiformis (Delile) Papenfuss, 1968: Macroalgae
• Acanthophora pacifica (Setchell) Kraft, 1979: Macroalgae
• Acanthophora ramulosa Lindenberg ex Kützing, 1843: Macroalgae
• Acanthophora spicifera (M.Vahl) Børgesen, 1910: Macroalgae
• Acanthosiphonia echinata (Harvey) Savoie & G.W.Saunders, 2018: Algae
• Acanthosolenia mediterranea F.C.Bernard, 1939: Algae
• Acanthosphaera zachariasii Lemmermann, 1899: Macroalgae
• Acanthosphaeridium reticulatum M.Hajós & H.Stradner, 1975 †: Algae
• Acarposporophycos brasiliensis (Necchi) Necchi & A.S.García, 2019: Algae
• Acaryochloris thomasi Partensky & al., 2018: Algae
• Acavomonas peruviana Tikhonenkov, Janouškovec, Mylnikov, Mikhailov, Simdyanov, Aleoshin & P.J.Keeling, 2014: Algae
• Acetabularia acetabulum (Linnaeus) P.C. Silva, 1952: Macroalgae
• Acetabularia caliculus J.V.Lamouroux, 1824: Macroalgae
• Acetabularia chiavonica Squinabol, 1902 †: Macroalgae
• Acetabularia crenulata J.V. Lamouroux, 1816: Macroalgae
• Acetabularia crenulata J.V.Lamouroux, 1816: Macroalgae
• Acetabularia dentata Solms-Laubach, 1895: Macroalgae
• Acetabularia dentata Solms-Laubach, 1895: Macroalgae
• Acetabularia farlowii Solms-Laubach, 1895: Macroalgae
• Acetabularia farlowii Solms-Laubach, 1895: Macroalgae
• Acetabularia haemmerlingii H.G.Schweiger & Berger, 1975: Macroalgae
• Acetabularia jalakanyakae Sani, Madhu, R.K.Kohli, K.Gupta & Bast, 2021: Macroalgae
• Acetabularia kilneri J.Agardh, 1887: Macroalgae
• Acetabularia major G.Martens, 1866: Macroalgae
• Acetabularia moldavica Barattolo, Ionesi & Ţibuleac, 2019 †: Macroalgae
• Acetabularia peniculus (R.Brown ex Turner) Solms-Laubach, 1895: Macroalgae
• Acetabularia ryukyuensis Okamura & Yamada, 1932: Macroalgae
• Acetabularia schenckii K. Möbius, 1889: Macroalgae
• Acetabularia schenckii Möbius, 1889: Macroalgae
• Achoma brachiatum Novis & Visnovsky, 2012: Algae
• Achomosphaera alcicornu (Eisenack) R.J.Davey & G.L.Williams, 1966 †: Algae
• Achomosphaera antleriformis P.Schiøler †: Algae
• Achomosphaera argesensis E.Demetrescu †: Algae
• Achomosphaera gralliforme (Brosius) R.J.Davey & G.L.Williams, 1969 †: Algae
• Achomosphaera hirundo (Eisenack) R.J.Davey & G.L.Williams, 1969 †: Algae
• Achomosphaera improcera M.A.Islam, 1983 †: Algae
• Achomosphaera linifera (Cookson & Eisenack) Stover & Evitt, 1978 †: Algae
• Achomosphaera neptuni (Eisenack) R.J.Davey & G.L.Williams, 1966 †: Algae
• Achomosphaera recurvata K.P.Jain, S.C.D.Sah & R.Y.Singh †: Algae
• Achomosphaera sagena R.J.Davey & G.L.Williams, 1966 †: Algae
• Achomosphaera triangulata (Gerlach) R.J.Davey & G.L.Williams, 1969 †: Algae
• Achroonema articulatum Skuja, 1956: Algae
• Achroonema gotlandicum Skuja, 1956: Algae
• Achroonema inaequale Skuja, 1956: Algae
• Achroonema lentum Skuja, 1956: Algae
• Achroonema macromeres Skuja, 1956: Algae
• Achroonema proteiforme Skuja, 1956: Algae
• Achroonema simplex Skuja, 1956: Algae
• Achroonema spiroideum Skuja, 1948: Algae
• Achroonema splendens Skuja, 1956: Algae
• Achroonema sporogenum Skuja, 1956: Algae
• Acicularia alaica Maslov, 1956 †: Algae
• Acicularia albrightii Johnson & Kaska: Algae
• Acicularia americana Konishi & Epis: Algae
• Acicularia andrusovii Livental, 1946 †: Algae