They really are heart shaped, the cells of of the blue-green algae Gomphosphaeria:
Although it’s not as nicely visible here as in Joostens (2006) book, see page 153.
This is the type species Gomphosphaeria aponina, because it has a thin, yet very clear layer enveloping the individual cells:
There used to be a lot of different Gomphosphaeria-species, but then – in 1980’s – misters Komárek and Hindák took a closer look at these species and divided three genera out of this group: Gomphosphaeria Kützing 1836, Woronichinia Elenkin 1933 and Snowella Elenkin 1938.
They also mention the genus Coelosphaerium Nägeli 1849 as a member of the subfamily Gomphosphaerioideae. Coelosphaerium differs from the rest because there are no stalks inside the colony.
Joosten (2006) on the other hand thinks that “possibly only a single genus is justified” (p. 128) for all these four genera. But, as he also says, for such a drastic revision we shall have to wait for the results of molecular studies.
Which again have a tiny obstacle in the fact that these species are difficult to cultivate (Komárek & Hindák 1988). Maybe a friendly “Good morning” to the cultures every morning would help here too, as Komárek told us during the blue-green course last summer, was (one of) the secret(s) of the Japanese culturing Aphanizomenon so succesfully?
Joosten, A.M.T 2006. Flora of the blue-green algae of the Netherlands. I The non filamentous species of inland waters. KNNV Publishing, Utrecht. 239 s.
Komárek, J., Anagnostidis, K. 1999. Cyanoprokaryota, 1.Teil: Chroococcales. In: Ettl H., Gärtner G., Heynigh H., Mollenhauer D. (ed.). Süsswasserflora von Mitteleuropa Band 19/1.
Komárek J, & Hindák F. 1988. Taxonomic review of natural populations of the cyanophytes from the Gomphosphaeria-complex. Arch. Hydrobiol. Suppl. 80, 1-4. Algolocigal Studies 50-53. 203-225.