## 2 Results and Discussion  
### Algae Are Harvested and Concentrated onto Amberlite Anion Exchange Resin  
Amberlite CG-400, a divinylbenzene-based resin containing quaternary ammonium groups (3.8 mmol/g [21]), has been previously shown to bind and concentrate two different species of green algae for removal from the water supply [22]. Here, we used Amberlite to concentrate and dewater biomass from two potentially high oil-producing green algal species, _Neochloris oleoabundans_[23] and KAS 603 [24]. For routine comparisons, algal suspension was passed through the column until saturation was achieved. Algae appeared to bind on contact, initially accumulating as a band at the top of the resin bed. As more algae were added, the upper green layer progressively expanded down to the bottom of the resin bed. Prior to the resin bed becoming solid green, the flow-through was clear and colorless. Once the resin bed became solid green, color quickly began to appear in the effluent and this was taken as the saturation point. To determine the binding capacity of the resin for _Neochloris_ and KAS 603, the OD\({}_{680}\) of the algal suspension was measured before and after passing the algae through an Amberlite column. The concentration of algae in the initial suspension and the in flow-through were determined from algal OD\({}_{680}\) absorbance versus dry cell weight (DCW) calibration curves generated for both types of algae (Supplementary Figure S1). The flow-through included the solution from the algae algal suspension that went through the resin and the subsequent wash to remove unbound algae. The total amount of algae bound to the resin was then determined by subtracting the amount of algae in the flow-through from the total that was added to the column. For all the experiments algal binding to the resin was tested at pH 7, although there was no significant difference in binding over the pH range of 6-9 (Supplementary Figure S2). Since for Amberlite, algal binding decreases with increasing ionic strength, binding studies for both algae were tested under freshwater conditions (salinity 5 psu). The algal suspensions were used at 0.4 g/L, the value that we typically obtain in our simple airlift photobioreactors. When more dilute algal suspensions were tested, saturation of the resin was still achieved but, as would be expected, the volume of algal suspension and time required to achieve saturation increased. Since algae can be entrained in the resin beads, our protocol included a washing step between binding and elution to ensure only algae that were actually bound to the resin were counted. Figure 1(a) gives a schematic for resin-binding and elution experiments shown in Figures 1-3. Algal binding capacity was determined for each cycle of algal binding. Reusability of the resin was assessed in terms of loss of binding capacity over multiple cycles of algal binding and elution. These experiments were carried out either using only fresh sulfuric acid/methanol reagent to elute the algae (Series A, \"fresh\") or using sulfuric acid/methanol that was recycled after the first use (Series B, \"used\"). Although the flow through approach in Figure 1(a) gave repeatable values for saturation and was used for all experiments except that shown in Figure 4, we later found that longer contact times, which involved stirring excess algae with resin for up to 15 min, gave somewhat larger binding capacities than those obtained using the flow through method. However, while the binding capacity numbers would be improved with longer contact times, the patterns remain the same. Figure 1: Algal binding capacity of Amberlite and resin reuse. (**a**) Experiment series for determining binding capacity and reusability of resin and transesterification reagent. (**b**) Binding capacities of Amberlite (mg algal dry weight / gram resin) were determined for _Neochloris_ and KAS 603. (**c**) _Neochloris_ or (**d**) KAS 603 were loaded onto resin columns and eluted with 100 mL of 5% sulfuric acid/methanol reagent.