Spacetime Superalgebra in $AdS_4 \times S^7$ via Supermembrane Probe

The spacetime superalgebra via the supermembrane probe in the background of $AdS_4 \times S^7$ is discussed to the lowest order in the spinor coordinate $\t$. To obtain the correct spacetime superalgebras, all $\t^2$ order corrections for supervielbein and super 3-form gauge potential have to be included. The central extension of the superalgebra OSp(8|4) of the super isometries for $AdS_4 \times S^7$ is found.