Wide variations exist in units of measurement, and standardization is required with regard to the growth conditions of algae to permit comparison of outputs (Coronet, 2010). On a volume basis, biomass in several species of autotrophic algae varied considerably between 0.002 and 4 g L-1d-1 and 1.7 to 7.4 g L-1d-1 in the heterotrophic algae (Table 13.1); on an areal basis, values ranged from 0.57 to 150 g m-2d-1 (Table 13.1). The highest production of algal biomass (120 to 150 g m-2d-1) has been reported in PBRs under artificial light (Tsoglin and Gabel, 2000).

The success of microalgal biotechnology entrepreneurship depends on the opti­mization of biomass and production yields. It is necessary to establish to what extent these variations are intra-specific or inter-specific, whether or not these yields are based on optimal growth conditions, and how to prime the algal production. Between several species of Dunaliella, cell division rates ranged from 0.12 to 3.0 div d-1 (Subba Rao, 2009). Within the one species, Chlorella sorokiniana, biomass produc­tion rates (div d-1) varied between 0.32 and 4.0 div d-1; and in Dunaliella teriolecta, rates varied between 0.15 and 3.0 div d-1 (Subba Rao, 2009). Such variations could be due to differences in strains of isolates and/or culture conditions. Even in the most commonly used strain, Neochloris oleoabundans UTCC 1185, biomass varied between 0.03 and 1.50 g L-1d-1 (Table 13.2).

In Dunaliella tertiolecta, a green alga often used in biotechnology, Duarte and Subba Rao (2009) discussed the relationship between biomass (B determined as Chl-a), photosynthesis (P), and light energy I (pmol m-2s-1):

PB = {PBs[1 — exp(-aB//PBs)]exp(- pB//PBs>) + PBd

where PBs is the maximum potential photosynthesis in the absence of photo­inhibition, and PBd is the intercept of the P-I curve on the y-axis and has the same units as PBm. In D. teriolecta, PBm varied between 3.3 and 7.43 mg C mg Chl-a h-1 (Duarte and Subba Rao, 2009). They showed that the photosynthesis and respira­tion activities were dependent on the light energy and the cell density; that is, over a 21-day period, gross production and respiration decreased by sevenfold and fourfold, respectively, at 42 pmol m-2s-1. The optimal light energy for photosynthesis ranged between 627 and 1,356 pmol m-2s-1. Also, the gross primary production:respiration ratio decreased with higher cell densities. It will be crucial in biotechnology opera­tions to optimize the relationships among high biomass yields, photosynthetic effi­ciencies, and yield of bioactive compounds. These criteria are crucial and could greatly improve commercial algal harvest.

Grobbelaar (2010), while discussing the light energy relationships in algae, sug­gested that by optimizing light, photosynthetic yield could be doubled from 1.79 g (DW) m-2d-1 and pointed out that several factors determine volumetric yields of mass algal cultures. Furthermore, Grobbelaar pointed out that many biotechnology start-up companies make the mistake of simple extrapolation of controlled labora­tory rates to large-scale outdoor production systems.

TABLE 13.1 Summary of Variations in Microalgal Biomass Production Criteria Example Minimum Maximum Biomass Haematococcus pluvialis 0.06 1.2 Haematococcus pluvialis 0.06 0.55 Haematococcus pluvialis 0.28 Haematococcus pluvialis 1.2 Chlorella sorokiniana 0.32 Chlorella sorokiniana 0.5 Chlorella sorokiniana 1.8 Chlorella sorokiniana 4 Six microalgae 0.09 0.21 0.04 0.37 0.03 0.48 Haematococcus oleoabundans 0.63 Haematococcus oleoabundans 0.4 Chlorella protothecoides 0.002-0.02 Chlorella protothecoides 1.7-7.4 Production Dunaliella 2 Nannochloropsis sp. 20 120-150 Haematococcus pluvialis 50-90 Several species 0.91-38 20 species 0.57-130
Remarks Bubble reactor 130 pmol photons mr2s_1 Tubular reactor 2,500 pmol photons mr2s_1 1,200 pmol photons mr2s_1	Ref. Garcia-Malea et al., 2006 Garcia-Malea et al., 2005 Esperanza Del Rio, 2005 Hunt et al., 2010 Janssen et al., 2003 Chang and Yang, 2003 Lee et al., 1996 Gouveia et al., 2009 Chen et al., 2011 Pitman et al., 2011 Li et al., 2008 Chen et al., 2011 Chen et al., 2011 Ben — Amoz, 2009 Ben-Amoz, 2009 Tsoglin and Gabel, 2000 Olaizola, 2000 Chisti, 2007; Khan et al., 2009; Harun et al., 2010; Pitman et al., 2011; Chen et al., 2011 Mata et al., 2010
10 mM Sodium nitrate enrichment 5 mM Sodium nitrate enrichment Phototrophic cultivation Heterotrophic cultivation Rue gas enriched Bioreactors, artificial light
Note: Variations in microalgal biomass (g L 1d *) and production (g m 2d 1). * All values are for autotrophic cultivation unless specified as heterotrophic.