Only a few species of algae contain extractable cellulose.

The extraction procedure is similar to that used in wood industry, but in milder conditions.

Ulva rigida doesn't contain a lot of pulp but is used in the paper production (see Patent US5472569 .

I advice you to use agricultural wastes for cellulose extraction.

Because you may need a high quantities of algae to produce smAll mount of cellulose and will take long time.

The severe eutrophication of coastal areas is one of the biggest environmental threats of our time, as excessive blooming by green algae, triggered by industrial pollutants in water, jeopardizes the fragile balance of ecosystems.1–3 The increased sliming of shore areas, piers, and fishing equipment is an obvious sign of worsening seawater quality. This sliming is caused by the dense growth of microalgae and filamentous green algae. Such growth also reduces the transparency of the water; this means that the species that grow deeper in the water suffer from a lack of sunlight. Annual filamentous green Cladophora algae, which grow on submerged rocks and stones, can then proliferate. Figure 1 shows Cladophora algae growing on a rocky substrate. As these algae age and die, they drift into the water, forming dense mats and harming other forms of aquatic life. These mats of filamentous algae eventually fall to the seabed, where they are decomposed by bacteria. This process can consume a considerable amount of oxygen, and the supply of oxygen on the seabed may even run out altogether. The excessive supply of nutrients and consequent eutrophication and oxygen depletion on the seabed all weaken the functioning of marine ecosystems.

Thank you everyone, for your replies.They really helped me and made me got an insight of using algae for cellulose extraction :)