Fig. 5

Automatic freezing unit (AFU) for controlled freezing and preparation of biological samples at freezing temperatures. a Top view. b Front view. The original lid of the laboratory freezer (1) was replaced by a top unit made out of thermally insulating material (2) which was covered by a detachable transparent Plexiglas^® pane (3). The top unit had two holes (4) through which thermally insulated gloves (5) for manipulation purposes were inserted. Temperature control was realized by two software controlled heating elements (6) which were equipped with miniature fans. Additionally, at the bottom of the freezer an array of four powerful fans (7) was placed to ensure good ventilation and temperature homogeneity within the freezer. A wire mesh (8) prevented the unintentional drop down of tools from the working (9) and the storage shelf (10). For freezing algal suspensions the working shelf was equipped with a fan (11) which blew air directly to the underside of the object carrier (12) to which three glass dishes with each three indentations (13) were placed. The central indentation, into which the control temperature sensor was plunged, was filled with NaCl-solution (black). For freezing of leaves a different working shelf without object carrier was used (not shown). Arrows indicate the main directions of the internal airflows