Cultivation of the wild seaweed Gracilaria gracilis under laboratory scale: The Effect of Light Intensity and Nutrients on Growth, Pigment and Total Soluble Protein Content

Considered primary sources of bioactive natural compounds with enormous applicability in various scientific-technological sectors, macroalgae have gained some prominence in recent years. The red macroalgae Gracilaria gracilis, one of the most dominant in worldwide cultivation systems, is fundamentally characterized not only by its high agar content, but also for its extraordinarily high phycobiliproteins (PBPs) levels endowed with a remarkable importance in the food industry, cosmetics, pharmaceuticals and in the field of bioactivities. The main objectives of this study consisted in manipulate laboratorily certain cultivation conditions of G. gracilis from the Lagoa de Óbidos (namely, light intensity and concentration of the macronutrients N and P (in the form of ammonia and phosphate, respectively)) and test their effects on growth, on the content of the Rphycoerythrin (R-PE) and R-phycocyanin (R-PC) PBPs, total soluble protein (TSP) content of the algal tissue and evaluate different methods of PBPs extraction applied to different states and pretreatments of algal biomass to assess the most effective method. As to the different extractions techniques used, for oven-dried wild samples, a maximum of 1.186 ± 0.008 mg of R-PE was obtained per g of dry weight with the bead beater, whereas enzymatic hydrolysis with cellulase only provided a maximum pigment content of 0.220 ± 0.001 mg R-PE g-1 of dry weight, clearly indicating that the first technique is much more efficient than the last one. For the wet samples, the methodology employing the tissue homogenizer yielded maximum values of 4.568 ± 1.773 mg R-PE g-1 WW and 0.099 ± 0.076 mg R-PC g -1 WW, proving that this extractingwith-water mechanical methodology is the most effective of the three in the extraction of phycobiliproteins from G. gracilis. One of the first things to note in culture assays is that light intensity and quality play a major role on the quality of the biomass and, thus, depending on the parameters desirable to increase in cultivation - DGR, PBPs or TSPs -, it may be preferable to use one or the other culture lamps tested - Cool White Fluorescent (CWF) or Plant Growth Fluorescent (Gro-Lux). The maximum DGR (16.219 ± 3.84 % day-1 ) was obtained for Gro-lux (with nutrient increment on the growth medium) while the obtained for CWF did not exceed the 14.936 ± 1.243 %, revealing a greater importance of nutrients (in particular, ammonium), comparatively to light, for this parameter. The TSPs concentration ranged from 18.06 ± 2.027 (for CWF) to 9.71 ± 0.364 % of dry weight (for Gro-lux without nutrient or light increment). As for R-PE and R-PC pigments, extracted with the tissue homogenizer, the highest peaks were obtained for CWF after two weeks of cultivation (9.599 ± 1.722 and 0.156 ± 0.043 mg g-1 of wet weight, respectively) - originated by a higher light intensity of the CWF lamp in comparison to the Gro-lux one. In sum, the similarity between the high levels of PBPs already studied in Gracilaria and the considerable levels of mechanically extracted R-PE in this study for Gracilaria gracilis, allied with an optimization of cultivation conditions (light and nutrients) to increase DGR, PBPs or TSP content in specific situations, may certainly justify the large-scale cultivation of this macroalgae species