def convert_coord_to_axis(coord):
    """
    Converts coordinate type to its single character axis identifier (tzyx).

    :param coord: (str) The coordinate to convert.
    :return: (str) The single character axis identifier of the coordinate (tzyx).
    """

    axis_dict = {"time": "t", "longitude": "x", "latitude": "y", "level": "z"}
    return axis_dict.get(coord, None)