To date, only solid heavy metals such as gold or tungsten have been used as DNA carriers in biolistic bombardment of algae. In this study, we show that even a metal oxide of lower density can act as a DNA carrier. We investigated the potency of size-controlled mesoporous titanium dioxide (TiO2) particles. Among the six tested gas pressures, TiO2 particles best facilitated transformation of the green alga Chlamydomonas reinhardtii at 1100 psi (approximately 7.6 MPa) and 2000 psi (approximately 14 MPa). Surprisingly, a mesoporous metal oxide with a density of approximately only one-tenth that of gold or tungsten could be effective as a DNA carrier in biolistic bombardment of a rigid cell wall-containing alga. In addition, we found two peaks of gas pressures in the transformation ratio irrespective of whether the particles were made of gold, tungsten, or TiO2.