Due to its interesting properties, attention has been oriented to the production of cellulose nanofibrils. However, the cellulose isolation requires the combination of several steps which are generally very energy consuming. This is one of the main limitations for the use and industrialization of such materials. Trying to participate in the resolution of this probleme, lignin-containing cellulose nanofibrils are produced for the first time from Posidonia oceanica waste. Fibers are alkali-extracted by a conventional soda cooking in autoclave or by alkaline steam explosion. As conventional refining of unbleached fibers is difficult, a comparative study is carried out to test alternative processes for fiber fibrillation (twin-screw extrusion (1 pass) or steam explosion (during 3 times)). Actually, these two processes are known to be environmentally friendly and low energy consuming. For microfibrillation, an ultra-fine friction grinder is used to prepare lignin-containing cellulose nanofibrils. The obtained gels are characterized by several properties such as morphological analysis (Morfi, optical microscopy, scanning electron microscopy, transmission electron microscopy) and turbidity measurements. Papers and nanopapers are prepared by filtration to investigate the mechanical properties. All these analysis methods allow discussing the efficiency of the different process combinations. It is concluded that steam explosion could be an alternative method for Posidona fiber extraction, while for refining the process conditions should be optimized. Finally, it is found that LCNF produced from extruded pulps (whatever the fiber extraction method) exhibit promising properties comparing with other unbleached CNF.