Optimal lower bounds for first eigenvalues of Riemann surfaces for large genus

In this article we study the first eigenvalues of closed Riemann surfaces for large genus. We show that for every closed Riemann surface $X_g$ of genus $g$ $(g\geq 2)$, the first eigenvalue of $X_g$ is greater than ${\cal L}_1(X_g)\over g^2$ up to a uniform positive constant multiplication. Where ${\cal L}_1(X_g)$ is the shortest length of multi closed curves separating $X_g$. Moreover,we also show that this new lower bound is optimal as $g\to\infty$.