Glutamic acid and glutamine specifically
are the substrate for all organic nitrogen based compounds and therefore represent the selleckchem most energy efficient way to store excess nitrogen (Garrett and Grisham 2013). These free amino acids equated to almost 40% of the FAAP and 20% of the total amino acid content for seaweed in the luxury state, almost twice that of the metabolic state. Arginine represented over 25% of the FAAP and almost 13% of total amino acids in the luxury uptake state. In plants, arginine synthesis eliminates excess nitrogen (Nasholm 1994) as well as storage to support future growth (Lipson et al. 1996). High concentrations of arginine have also been reported for long-term studies in Gracilaria secundata (Lignell and Pedersen 1987). Notably, the synthesis of arginine uses glutamine and CFTR activator asparagine for the
amide group (Lobban and Harrison 1997, Garrett and Grisham 2013), and the synthesis of high levels of arginine is proposed as the rationale for the minimal free asparagine quantified in this study. Although the internal N content and the total amino acid content was highest for seaweed in the luxury state, there was a clear trade-off with growth rates, and therefore with potential biomass production. Internal N content did not increase beyond the luxury point (2.6% internal N) unless growth rate was limited by a resource other than nitrogen. The main non-N limiting resources in intensive seaweed cultivation includes light, which was limiting for the majority of the low N treatment cultures, but in contrast it was the dissolved resources until (other macro-nutrients (P), trace elements or carbon; Lobban and Harrison 1997) that are delivered by increasing water renewals which limited growth in the luxury N state. Although it is difficult to identify exactly what the limiting resource was for these luxury state seaweeds, there is opportunity to enhance luxury N production by removing the next limiting resource and potentially maintaining the luxury N state at higher total amino
acid productivities (>1.6 g · m−2 · d−1) with lower water renewal rates. In conclusion, the current study quantified the variation in internal N content and amino acid quantity and quality in the green seaweed U. ohnoi using an innovative provision of nitrogen flux by simultaneously manipulating nitrogen concentration and water renewals. These experiments demonstrated that amino acid quantity and quality varied substantially based on the nitrogen state of the seaweed, which was determined by N flux and growth rate. Amino acid synthesis above the luxury point had limited (lysine) or no (methionine) further gains in amino acids essential to nutrition and any targeted production of these compounds should focus solely on maximizing biomass productivity through high growth rates to ensure that the biomass is maintained in the metabolic state.