Haslea karadagensis is a recently-described diatom, the second species of blue diatom to be identified, after Haslea ostrearia, the type species of the genus Haslea, which produces marennine, the water soluble pigment involved in the greening of oysters. Haslea karadagensis also produces a blue grey pigment, with different spectral characteristics from marennine itself. This study demonstrates that the pigment from H. karadagensis can colour the gills of two oysters of economic importance, Crassostrea gigas and Ostrea edulis, when they are fed with a suspension of algae, in the same way as the marennine produced by H. ostrearia. Like marennine, the purified pigment produced by H. karadagensis displays several different biological activities. Both the intracellular and extracellular forms of the pigment have been shown to inhibit the growth of marine bacteria (Polaribacter irgensii, Vibrio aestuarianus, Pseudoalteromonas elyakowii) and fungi (Corollospora maritima, Lulworthia sp., and Dendryphiella salina). The pigment also displays antiviral activity against Herpes simplex virus type 1 (HSV-1). In light of these preliminary results, the use of H. karadagensis and putative applications of its pigment in aquaculture, food chemistry and ecophysiological research, are discussed.