Are diatoms a food source for Antarctic sponges?

Living diatoms are commonly found within Antarctic Porifera, and generally interpreted as additional food source, or as mutualists or parasites of sponge tissues. However, no data are available about temporal variations of the abundance of diatoms inside sponges especially during the winter period. In this paper we analysed the amount of diatom frustules and chlorophaeopigment concentration in six species of Antarctic sponges (Dendrilla antarctica, Homaxinella flagelliformis, Kirkpatrickia variolosa, Suberites montiniger, Haliclona dancoi, Haliclona penicillata) sampled weekly from November 2001 - before the ice melting - to February 2002. Frustule concentration in the sponge tissues was very low at the beginning of November in all the sponge species, and increased between 9 and 22 January, to reach maximum values between 29 January and 7 February. Diatom abundances were significantly higher in H. dancoi tissues, comparing to the other sponge species, reaching values up to 1217′178 cells *10 g - 1 dw at the end of January. Chlorophaeopigments, very low at the beginning of November, increased between 5 and 19 December, before the peak of diatoms. Maximum chlphaeo values (650.5 ′ 5.9 μg g - 1 dw) were observed in D. antarctica. The planktonic Fragilariopsis curta was the most common diatom species recorded inside sponges. These data clearly indicate that diatom concentration inside the sponge tissues is related to the summer phytoplankton bloom. The shift between the pigment and frustule peaks strongly suggests that diatoms are used as a food source by sponges and that their frustules are accumulated inside the sponge body. The lack of frustules at the beginning of summer indicates that diatom frustules are expelled or dissolved during the cold season.