I wonder how closely the authors investigated Type IV secretion systems when concluding that a T-DNA could be transferred by V. chlorellavorus into Chlorella cells.

T-strands transfer by A. tumefaciens into eukaryotic cells is essentially an anomaly of Type IV secretion systems. Either systems are used for conjugation between bacteria, or they deliver effector proteins into host cells. For instance, A. tumefaciens strain C58 carries several other Type IV secretion systems on plasmids that are used simply for conjugative purposes, despite close homology to the VirB system that it uses to transfer T-DNA into plant cells. Likewise, close relatives of the A. tumefaciens system, such as the Type IV secretion system found in Brucella, are only used for secreting proteins as well.

It is far more likely that, if the Type IV secretion system is used at all during this process, V. chlorellavorus transfers many of its proteases and other degradative enzymes (and most likely, DNases!) directly into Chlorella without resorting to genetic transformation of Chlorella cells.