[2] Koster began her botanical career in 1930 as an unpaid assistant at Rijksherbarium, Leiden, the national Herbarium of The Netherlands.
[3] She held this position until 1933 when she was promoted to curator of the Weber-Van Bosse algae collection, which was recently acquired by the herbarium.
In June 1935, she defended her thesis titled The Compositae of the Malayan Archipelago I. Vernonieae and Eupatorieae.
[3] Dr. Koster retired in 1967 and was honored by a special issue of Blumea.
Throughout her career, she published 88 texts on Cyanophyceae to the Index nominum genericorum, a compilation of generic names published for all organisms covered by the International Code of Botanical Nomenclature.