We studied microrheological properties of $F$-actin across the isotropic-nematic phase transition region by video particle tracking (VPT) and by laser deflection particle tracking (LDPT). Both methods track the motion of thermally driven micron-sized beads, and convert the temporal mean square displacement (MSD) to shear moduli. The two methods give consistent results for the elastic modulus $G^{\prime }$ and less so for the loss modulus $G^{″}$. As the nematic order parameter increases with actin concentration, $G_{\parallel }^{\prime }$ (measured parallel to the nematic director) and $G_{\perp }^{\prime }$ (perpendicular to the director) grow apart, with $G_{\perp }^{\prime }$ larger than $G_{\parallel }^{\prime }$. The moduli scale with actin concentration as $G_{\parallel }^{\prime }\sim c^{0.54\pm 0.13}$ and $G_{\perp }^{\prime }\sim c^{1.38\pm 0.15}$. Furthermore, $G^{\prime }$ and $G^{″}$ dependence on $\left[{\mathrm{Mg}}^{2+}\right]$ were measured and compared for $1\mathrm{mg}∕\mathrm{ml}$ isotropic and $4\mathrm{mg}∕\mathrm{ml}$ nematic $F$-actin solutions, respectively. In the isotropic phase, $G^{\prime }$ increases with $\left[{\mathrm{Mg}}^{2+}\right]$ up to $6\mathrm{mM}$ and then plateaus. In the nematic phase, $G_{\perp }^{\prime }$ is larger than $G_{\parallel }^{\prime }$, and both $G_{\perp }^{\prime }$ and $G_{\parallel }^{\prime }$ increase with $\left[{\mathrm{Mg}}^{2+}\right]$ progressively up to $16\mathrm{mM}$, above which $F$-actin form large bundles. In both isotropic and nematic phases, $G^{″}$ only weakly depends on $\left[{\mathrm{Mg}}^{2+}\right]$. In conclusion, particle tracking microrheology reveals rich rheological features of $F$-actin affected by the isotropic-nematic phase transition and by tuning weak electrostatic interactions among the protein filaments.

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