Fuji Tamaki's Algae is packed full of so much symbolism and feeling in so few pages. A magnificent read.
The pool symbolizes the girls' past together and the graduation ceremony symbolizes their separation. While the dark-haired girl's tears indicate pain, they ultimately foreshadow healing. The dark-haired girl is saddened that she and the light-haired girl are parting ways.
Water generally symbolizes change and strong emotions. The presence of algae in the (stagnant) water indicates a lack of change and indifference. In other words, the two girls have experienced a consistent platonic relationship, but have not directly allowed their emotions to interfere.
Again, the algae symbolizes a lack of emotion. When the light-haired girl says that the algae is coiled around her heart, she means that the two girls' emotional distance has been eating away at her heart. Alternatively, algae can symbolize growth; in this case, the light-haired girl's sentimental growth with respect to the dark-haired girl.
Later, "tangled flagella" are pictured as tangled legs, presumably those of our two main characters. Since flagella are active, moving parts from creatures in the Kingdom Animalia, it directly contrasts with the stagnant, (e)motionless algae from the Kingdom Plantae.
In conclusion, "our algae" does not reveal an emotionless, decaying relationship, but rather reveals an active, loving, growing, and affection-filled relationship.
