Completely or partially eliminating the harvesting step is a direct way to reduce the cost of harvesting on the process, but is only valuable if it does not impact the downstream processing of products and co-products. There are several ways that are being explored to truncate or eliminate harvesting in algal biofuel production.

Mass culture in a biofilm. Supplying a suitable surface on which to mass culture algae without suspension in a dilute culture medium would allow the direct harvest of concentrated algae rather than trying to separate a dilute suspension of algal cells from their culture medium. In theory, this method would also allow better access to nutrients and sunlight.

A biofilm-based system is currently being commercialized by BioProcessAlgae LLC. Their Grower Harvester™ system provides a substrate on which algae are inoculated and allowed to grow under suitable conditions and then removed in a more concentrated form using a stream of water to remove the cells from the substrate (http://www. bioprocessalgae. com/technology/). Little data are publically available at this time on the economics of this process.

Directly process high moisture content biomass. Elimination or partial elimi­nation of secondary harvesting would mean that the algal pastes do not have to be highly concentrated, leaving a large amount of water with the algal cells (>20 % moisture). A number of different technologies are applicable to process high moisture algal pastes, including hydrothermal liquefaction (Biddy et al. 2013), catalytic hydrothermal gasification (Biller et al. 2011), supercritical methanol conversion to biodiesel (Patil et al. 2011), and hydrothermal carbonization (Heil — mann et al. 2011). All of these processes either require the presence of water for conversion to biofuels or can tolerate high levels of moisture in the biomass without excessive parasitic energy loss. While it is beyond this review to discuss all of these methods, one example is the use of supercritical methanol conversion to biodiesel on algal paste. Open pond-grown Nannochloropsis sp. biomass at <10 % solids was directly converted to fatty acid methyl esters suitable for use in biodiesel (Patil et al. 2011).

Direct harvesting of biofuel or bioproducts. An indirect approach has also found favor in the microalgal industry where the biofuels are not contained within the algal cells but secreted into the medium so that the products are harvested without having to harvest the algal biomass itself (opening the prospect of reuse of the algal biomass for additional product generation).

One unique alga, B. braunii, accumulates lipid on the exterior of the cell as it becomes senescent or stressed. This alga produces some very long chain and unique lipids but grows very slowly. A suggestion that continuous harvest using a bio­compatible solvent in an aqueous/solvent bioreactor has recently been proposed that would allow direct harvesting on a continuous basis to lower costs and offset the slow growth of the biomass (Moheimani et al. 2014; Zhang et al. 2011).

An example of this approach is that currently applied by Algenol for the pro­duction of ethanol from cyanobacteria (bluegreen algae). Secretion of algal lipid into the medium could allow continuous culturing of the algal biomass while removing product from the culture medium through phase separation. This has been demonstrated in bacteria (Sauer and Galinski 1998) and has been proposed for diatoms and other algae (Ramachandra et al. 2009). The company Synthetic Genomics reports that they have developed algal cells that secrete oil in a contin­uous manner to produce an algal biocrude (www. syntheticgenomics. com/what/ renewablefuels. html) and have filed patents protecting the idea (Roessler WO2009076559). Other companies have similar ideas being applied to cyano­bacteria, such as Joule Unlimited with enclosed PBRs and excreted volatile organics, although they are not publishing their methods, so it is hard to calculate the economics.