Different types of algae are harvest in different ways. Macro algae is easier to harvest than micro algae and can be harvested more energy efficient. One way of harvesting macro algae is to simply have a net installed in the pond and raise it to bring the algae up and out of the water. The algae are then harvested using mechanical cutters similar to lawnmowers. This process was earlier done by hand. You can usually harvest macro algae like this 3-4 times each season by simply lowering the net again after harvesting and allowing the algae to grow back. It should however be said that the first harvest usually is the biggest one while the subsequent harvests tend to be smaller and smaller.
Micro algae are smaller and harder to harvest. It is also a more energy requiring process. Micro algae are the type of algae primarily grown in photobioreactors. There are several ways to harvest them; some are one step processes while others require two steps. Examples of common methods include >settling or flotation, centrifugation and filtration. All processes uses flocculation in one way or another to get algae cells to clump together forming larger clumps that can be harvested. Exactly how hard a micro algae species is to harvest and how energy consuming it is to do so depends on the size of the species in question. The smaller the algae, the harder the harvest.
Micro algae and especially very small micro algae are usually the most fast growing kinds of algae and therefore among the most attractive to farm. They are however also the hardest to harvest so you need a balance between algae size and growth rate to find an optimal algae to farm. A more efficient way to harvest micro algae could enhance the viability of algae bio fuels by miles.