## 3 Regulation of MAA Biosynthesis  
It is well known that the diversity and concentration of MAAs can vary, depending on environmental factors or culture conditions, such as radiation (PAR and UV), temperature, salinity, nutrients, pH, or desiccation. The main factors involved in MAA biosynthesis are radiation (PAR and UV) and inorganic N availability [51; 61; 62; 63; 64; 65; 66; 67]. PAR and mainly UVR are key factors in the induction of MAA production. Karsten et al. [68] observed that algae exposed to sunlight had 2.5-fold more MAA content than shaded species. Different authors studied the effect of quality and intensity of PAR and UVR in the synthesis of the different MAAs [67; 69]. In C. _crispus_, the MAA palythine was synthesized under blue and white light. In the same study, pre-exposure to blue light followed by growth in UV-A increased the synthesis of shinorine seven-fold compared to treatments without previous exposure to blue light [70]. MAA accumulation (porphyrin-334, palythine and asterina-330) under blue light also occurred in _Neopyropia leucostica_ (previously _Porphyna leucostica_), whereas white, yellow, green, and red lights favored an increase in shinorine content [57]. In C. _crispus_, UV-B radiation induced the synthesis of asterina-330, palythinol, and palythine, whereas UV-A radiation induced the synthesis of shinorine and palythine [71]. MAA palythinol content increased under a high level of PAR in _Gracialriopsis tenufrons_ [72]. MAAs were stimulated in _Agarophytton tenuitipitatum_ (ex-_Gracilaria tenuitipitata_) by UV radiation, including UV-A and UV-B [73]. The gametophytes of _M. laminarioides_ showed a reduction in asterina-330 and palythine in different radiation treatments, including PAR, PAR + UV-A, and PAR + UV-A + UV-B, whereas tetrasporophytes showed an increase in these MAAs, mainly under PAR + UV-A [59]. Availability of nutrients, particularly inorganic N, is another important factor that positively affects MAA biosynthesis. The N can be quickly mobilized to MAAs, which act as N reservoirs [74]. Navarro et al. [75] verified the effects of nitrate (NO\({}_{3}\)\({}^{-}\)) on _M. laminarioides_ grown under solar radiation. The authors observed an increase in myogenic-glycine that occurred according to the increase in nitrate concentration, reaching saturation at 0.18 mM. The interaction between UVR and nutrients was investigated in _Gracialriopsis longissima_. The experiment showed a 65% increase in the MAA content under PAR + UV-A + UV-B and high N availability (300 uM NH\({}_{4}\)Cl and 30 uM HNa\({}_{2}\)PO\({}_{4}\)) compared to the other treatments with low nutrients (10 uM NH\({}_{4}\)Cl and 1 uM HNa\({}_{2}\)PO\({}_{4}\)) and without UVR radiation [76]. The MAAs amount also increased 50% in _Gracialria cornea_ (previously, _Hydropuntia cornea_), 40% in _G. longissima_, and 30% in _Halopithys incurva_ when grown under high nutrient concentration (150 uM of ammonium NH\({}_{4}\)\({}^{+}\) and 15 uM of phosphate PO\({}_{4}\)\({}^{3}\)-) and PAR + UV-A + UV-B radiation, compared with PAR only [54]. _A. tenuitipitatum_ showed an increase in  
Figure 3: Distribution of MAAs in different orders of red macroalgae, according to the values obtained in previous studies [2; 42; 43; 45; 47]. MAAs, reaching constant maximum values at concentrations greater than 0.5 mM nitrate (NO\({}_{3}\)-) [64]. The same species increased MAAs up to eight-fold in the presence of UVR and 0.5 mM NO\({}_{3}\)- [63]. _N. leucosticta_ and _P. umbilicalis_ increased MAA content when grown under 300 uM ammonium and PAR + UV-A + UV-B radiation [58]. _P. columbina_ also increased the concentration of MAAs under 300 uM ammonium, although the increase was greater in PAR + UV-A than in PAR + UV-A + UV-B [77]. _P. columbina_ was also grown under different radiation treatments (PAR and UVR) and different concentrations of nitrate. Samples of this algae were collected during the light and dark (recovery) periods.