Antifreeze proteins (AFPs), characterized by their ability to separate the melting and growth temperatures of ice and to inhibit ice recrystallization, play an important role in cold adaptation of several polar and cold-tolerant organisms. Recently, a multigene family of AFP genes was found in the diatom Fragilariopsis cylindrus, a dominant species within polar sea ice assemblages. This study presents the AFP from F. cylindrus set in a molecular and crystallographic frame. Differential protein expression after exposure of the diatoms to environmentally relevant conditions underlined the importance of certain AFP isoforms in response to cold. Analyses of the recombinant AFP showed freezing point depression comparable to the activity of other moderate AFPs and further enhanced by salt (up to 0.9 °C in low salinity buffer, 2.5 °C at high salinity). However, unlike other moderate AFPs, its fastest growth direction is perpendicular to the c-axis. The protein also caused strong inhibition of recrystallization at concentrations of 1.2 and 0.12 μM at low and high salinity, respectively. Observations of crystal habit modifications and pitting activity suggested binding of AFPs to multiple faces of the ice crystals. Further analyses showed striations caused by AFPs, interpreted as inclusion in the ice. We suggest that the influence on ice microstructure is the main characteristic of these AFPs in sea ice. © 2011 Elsevier Inc.