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The protein content of seaweeds: a universal nitrogen-to-protein conversion factor of five

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A global drive to source additional and sustainable biomass for the production of protein has resulted in a renewed interest in the protein content of seaweeds. However, to determine accurately the potential of seaweeds as a source of protein requires reliable quantitative methods. This article systematically analysed the literature to assess the approaches and methods of protein determination and to provide an evidence-based conversion factor for nitrogen to protein that is specific to seaweeds. Almost 95 % of studies on seaweeds determined protein either by direct extraction procedures (42 % of all studies) or by applying an indirect nitrogen-to-protein conversion factor of 6.25 (52 % of all studies), with the latter as the most widely used method in the last 6 years. Meta-analysis of the true protein content, defined as the sum of the proteomic amino acids, demonstrated that direct extraction procedures underestimated protein content by 33 %, while the most commonly used indirect nitrogen-to-protein conversion factor of 6.25 over-estimated protein content by 43 %. We therefore determined whether a single nitrogen-to-protein conversion factor could be used for seaweeds and evaluated how robust this would be by analysing the variation in this factor for 103 species across 44 studies that span three phyla, multiple geographic regions and a range of nitrogen contents. An overall median nitrogen-to-protein conversion factor of 4.97 was established and an overall mean nitrogen-to-protein conversion factor of 4.76. We propose that the overall median value of 5 be used as the most accurate universal seaweed nitrogen-to-protein (SNP) conversion factor.

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This research is part of the MBD Energy Research and Development programme for Biological Carbon Capture and Storage. The project is supported by the Advanced Manufacturing Cooperative Research Centre (AMCRC), funded through the Australian Government’s Cooperative Research Centre Scheme. We thank Simon Angell for developing an Excel macro program that enabled efficient extraction of data from the literature.

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Correspondence to Alex R. Angell.

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Figure S1

Number of papers identified through the review per year of publication as the sum of the three main extraction methods—Extraction, N × 6.25 and total amino acids (TAA). (TIFF 33974 kb)

High Resolution (GIF 34 kb)

Figure S2

Nitrogen-to-protein conversion factors calculated for extraction and total amino acid (TAA) determination methods. Dashes represent medians, crosses represent means, boxes represent 25th percentiles and whiskers represent 5th/95th percentiles. The horizontal dashed line represents a ratio of 6.25. (TIFF 7622 kb)

High Resolution (GIF 9 kb)

Figure S3

Quantitative tissue nitrogen measurements (% DW) of papers examined in this review analysed using the two main methods; analysis by combustion and by variants of the Kjeldahl method. Dashes represent medians, crosses represent means, boxes represent 25th percentiles and whiskers represent 5th/95th percentiles. (TIFF 6995 kb)

High Resolution (GIF 7 kb)

Figure S4

Concentration of nitrogen in the total amino acid (TAA) and non-TAA fractions of seaweeds analysed for N-prot factors in this review. Dashes represent medians, crosses represent means, boxes represent 25th percentiles and whiskers represent 5th/95th percentiles. (TIFF 8079 kb)

High Resolution (GIF 9 kb)

Figure S5

The within-species variation in (a) nitrogen-to-protein conversion factors and (b) the concentration of nitrogen in the total amino acid (TAA) and non-TAA fractions of the green seaweed Ulva ohnoi. All data is from Angell et al. (2014). Dashes represent medians, crosses represent means, boxes represent 25th percentiles and whiskers represent 5th/95th percentiles. (TIFF 17174 kb)

High Resolution (GIF 15 kb)

Table S1

Descriptive statistics for protein content data (% DW) overall and for each categorisation. Data shown for all and each method of protein determination. (DOCX 14 kb)

Table S2

Mean N-protein factors for all species examined (raw data) and those included in the 5th/95th percentile range which were used for analysis in this meta-analysis. Numbers in parenthesis indicate medians for N-protein factors. (DOCX 28 kb)

Table S3

Correlations between N content and N-protein factor for all seaweeds overall and all possible sub-groups. r 2 values, direction and p value given. Bold text indicates a significant correlation (p < 0.05). Sub-groups not shown had no data available. (DOCX 16 kb)

Appendix 1

Additional articles included in meta-analysis that were not retrieved by search string as well as the list of all articles examined in qualitative and quantitative meta-analysis. (DOCX 42 kb)

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Angell, A.R., Mata, L., de Nys, R. et al. The protein content of seaweeds: a universal nitrogen-to-protein conversion factor of five. J Appl Phycol 28, 511–524 (2016).

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