What is the maximum possible yield of second-generation biofuels? The most efficient photosynthesizers known are among the blue-green algae. For instance...

Chlorella - Wikipedia, the free encyclopedia

Chlorella is a genus of single-celled green algae, belonging to the phylum Chlorophyta. It is spherical in shape, about 2 to 10 μm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast. Through photosynthesis it multiplies rapidly requiring only carbon dioxide, water, sunlight, and a small amount of minerals to reproduce.

...

Many people believed Chlorella could serve as a potential source of food and energy because its photosynthetic efficiency can theoretically reach 8%,^[1] comparable with other highly efficient crops such as sugar cane. It is also an attractive food source because it is high in protein and other essential nutrients; when dried, it is about 45% protein, 20% fat, 20% carbohydrate, 5% fiber, and 10% minerals and vitamins. However, because it is a single-celled algae, harvest posed practical difficulties for its large-scale use as a food source. Mass production methods are now being used to cultivate it in large artificial circular ponds.

What does "8% efficiency" mean?

Photosynthetic efficiency - Wikipedia, the free encyclopedia

The photosynthetic efficiency is the fraction of light energy converted into other forms of energy for use. Trees convert light in to chemical energy through the process of photosynthesis with a photosynthetic efficiency of approximately 0.2-0.5%. Other numbers reported range up to 6%, a more detailed analysis is required. By comparison solar panels convert light into electric energy at a photosynthetic efficiency of approximately 10-20%. The photosynthetic efficiency varies with the frequency of the light being converted.

So, assume you cover a hectare of land with a pond full of chlorella and manage to capture 100% of the light and convert it at 8% efficiency into second-generation biofuel feedstock. How much area does Europe need to cover with chlorella in order to satisfy 10% of its liquid fuel demand? Note that this land use is compatible with, for instance, puttin it on the roofs of buildings though in urban environments it's probably more sensible to use solar panels to power the buildings themselves.

We have met the enemy, and it is us — Pogo