Algal biofuels represent one of the most promising means of sustainably replacing liquid fuels. However, significant challenges
remain before alga-based fuels become competitive with fossil fuels. One of the largest challenges is the ability to harvest the algae
in an economical and low-energy manner. In this article, we describe the isolation of a bacterial strain, Bacillus sp. strain
RP1137, which can rapidly aggregate several algae that are candidates for biofuel production, including a Nannochloropsis sp.
This bacterium aggregates algae in a pH-dependent and reversible manner and retains its aggregation ability after paraformaldehyde
fixation, opening the possibility for reuse of the cells. The optimal ratio of bacteria to algae is described, as is the robustness
of aggregation at different salinities and temperatures. Aggregation is dependent on the presence of calcium or magnesium ions.
The efficiency of aggregation of Nannochloropsis oceanica IMET1 is between 70 and 95% and is comparable to that obtained by
other means of harvest; however, the rate of harvest is fast, with aggregates forming in 30 s.