# Microalgal Proteins and Bioactives for Food, Feed, and Other Applications
## 1 Introduction
### Microalgae for Food and Functional Food Applications
#### 1.1.1 Proteins and Peptides  
Microalgae are a rich source of proteins, which can make up to 70% of the biomass dry weight for some species. Well-known, protein-rich microalgae species include _Arthrospria_, _Chorella_, _Aphanizomenon_ and _Nostoc_[22]. Generally, microalgae proteins have a balanced total amino acid (TAA) profile and contain all of the essential amino acids (EAA). According to the FAO and WHO, amino-acid profiles of proteins extracted from _Arthrospria_ correspond to those recommended for human consumption [23]. The factors which have to be considered in order to evaluate the suitability of microalgae proteins for human consumption include TAA and EAA content and protein digestibility, bioaccessibility and bioavailability. The cellulosic wall of most microalgae species may interfere with nutrient utilization if consumed. To assess protein quality, several methods are recommended, including the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS) methods. Table 2 lists the PDCAAS values, protein content and cell wall composition for several microalgae consumed as foods, feeds or functional foods today. Quality protein sources such as egg, whey and soy have reported PDCAAS values in the range of 0.9-1.0 [24]. Unfortunately, no information about the DIAAS values of microalgae biomass or microalgae protein products for human foods is currently available [25]. However, high in vivo DIAAS values ranging from 1.0 to 3.6 were recently reported for dry intact-cell meal produced from _Pavlova_ sp. biomass used to feed juvenile Atlantic salmon [26]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Species** & **Protein** & **Lipids** & **Carbohydrates** \\  & **[\% DW]** & **[\% DW]** & **[\% DW]** \\ \hline _Arthrospria platensis_ & 53–70 & 6–20 & 12–24 \\ _Chlorella vulgaris_ & 49–55 & 3–36 & 7–42 \\ _Dunaliella salina_ & 57 & 32 & 6 \\ _Haematococcus pluvialis_ & 48 & 15 & 27 \\ _Nannochloropsis oscanica_ & 29 & 19–24 & 32–39 \\ _Nannochloropsis_ sp. & 29–32 & 15–18 & 9–36 \\ _Schizochtrium_ sp. & 12 & 32 & 38–71 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Compositional analysis of commercially available microalgae. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Species** & **Protein** & **PDCAAS** & **Cell Wall Composition** \\ \hline _Arthrospria platensis_ & 53–70 [21] & 0.84 [27] & Peptidoglycan + outer membrane [28] \\ \hline _Chlorella sorokinian_ & 50 [29] & 0.81 [29] & Glucosamine, rhamne [30] \\ \hline _Chlorella vulgaris_ & 54 [29] & 0.77 [29] & Cellulose [31] \\ \hline _Dunaliella salina_ & 57 [21] & n/d & No cell wall, glycocalyx-type cell covering [32] \\ \hline _Haematococcus pluvialis_ & 48 [21] & n/d & Cellulose, mannan [33] \\ \hline _Isochrysis galbana_ & 29 [34] & n/d & No cell wall [35] \\ \hline _Nannochloropsis gaditana_ & 20–45 [36] & n/d &
\begin{tabular}{c} Cellulose (inner wall) + outer \\ hydrophobic algaean layer [37] \\ \end{tabular} \\ \hline _Nannochloropsis osculata_ & 35 [34] & n/d & Cellulose [38] \\ \hline _Pavlova lutheri_ & 29 [34] & n/d & Cellulose, hemicellulose [38] \\ \hline _Scenedesmus obliquus_ & 50–56 [21] & n/d & - \\ \hline _Schizochtrium_ sp. & 12 [39] & n/d & Galactose [40] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reported protein content, PDCAAS values and cell wall composition for well-known microalgae species. Meat and whey proteins are known to have PDCAAS values closer to 1 and are considered complete protein sources because of their amino acid content and digestibility (as measured using PDCAAS) values. The PDCAAS values found to date for selected microalgae (Table 2) are lower than 1, and this may result from the anti-nutritional factors present in microalgae, including the constituents of the algal cell walls, which may bind to available protein in microalgae when algae are consumed, preventing their complete digestion. Proteins also contribute to the rheological and stability properties of microalgae during manufacture and storage.