Algae are a diverse range of aquatic “plants”, comprising both unicellular and multi-cellular forms, which generally possess chlorophyll, but are without true stems and roots. The algae can be divided by size into two groups: macroalgae commonly known as “seaweed” and microalgae, microscopic single cell organisms ranging in size from a few micrometres (µm) to a few hundred micrometres [
1]. Unlike terrestrial crops cultivated for biofuel, algae do not require agricultural land for cultivation and many species grow in brackish or salt water avoiding competition for land and fresh water required for food production [
2,
3,
4]. The potential biomass yield of algae per unit area is also often higher than that of terrestrial plants [
5,
6,
7] with, for example, brown seaweeds grown “under cultured conditions” having yields of ~13.1 kg dry weight m
−2·yr
−1 compared to ~10 kg dry weight m
−2·yr
−1 from sugarcane [
8,
9]. Algae, therefore are considered as among the most potentially significant future sources of sustainable biofuels [
10], and have been described as potential sunlight-driven cell factories for the conversion of carbon dioxide to biofuels and chemical feedstocks [
3]. Nevertheless despite their obvious potential, there are no economically-viable commercial-scale quantities of fuel from either micro- or macroalgae [
11,
12,
13].
In the case of microalgae, part of the problem lies in the low values calculated for energy return on investment (EROI) (1–3) compared to that for petroleum and diesel (4–5) [
14]. This is now being addressed by a raft of research initiatives aimed at considering the full spectrum of products that might be obtained from microalgae in addition to biofuel, in so-called “biorefineries” [
2]. By contrast in the case of macroalgae there has been a paucity of research directed towards producing fuels or developing feedstocks for fuels [
12,
15]. However, the use of macroalgal feedstocks for non-fuel uses is currently 100 times bigger in wet tonnage terms than that for microalgae [
16]. Today the global utilisation of (non-fuel) products obtained from macroalgae is a multi-billion dollar industry [
17], with Asia being the main market [
18,
19]. Current uses of seaweeds include human foods, fertilisers, cosmetics ingredients and phycocolloids [
20]. Alginates, agars, and carrageenans (E400, E406 and E407, respectively), three major phycocolloids, are widely used in the food industry and have a combined annual production of 86,000 tonnes [
21,
22]. Worldwide 221 species of macroalgae are known to be exploited by humankind with 66% of the species used as food [
23]. However, the majority of algal biomass comes from a relatively small number of species [
24] with five genera,
Laminaria (reclassified as
Saccharina for some species),
Undaria,
Porphyra,
Euchema, and
Gracilaria, representing 76% of the total tonnage for cultured macroalgae [
25].
This review draws on available literature resources to unravel the opportunities and limitations for exploiting macroalgae as a source of biofuel, particularly from the perspective of EROI, and highlights areas where further research effort and investment needs to be directed in order to realise their potential as sustainable biofuel resources.