Biochemical responses of red alga Gracilaria corticata (Gracilariales, Rhodophyta) to salinity induced oxidative stress

The biochemical responses of Gracilaria corticata (J. Agardh) J. Agardh to salinity induced oxidative stress were studied following the exposure to different salinities ranging from 15, 25, 35 (control), 45 to 55 in laboratory conditions. The growth was highest under 25 (3.14±0.69% DGR) and 35 (3.58±0.32% DGR) and decreased significantly in both extreme lower (15) and hyper (55) salinities. Both phycoerythrin (PE) and allophycocyanin (APC) were significantly higher in hyper-salinity (45) with an increase of almost 70% and 52% from their initial contents. Conversely, the level of increase of the same in hypo-salinities was considerably lower as compared with that of hyper-salinity. Both hypo- and hyper-salinity treatments induced almost two fold increase in the contents of polyphenols, proline and the activities of antioxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) especially for 6d exposure. The Na+ ions readily displaced the K+ and Ca2+ from their uptake sites at extreme hyper-salinity (55) and accounted for substantial increase in the ratio of Na+/K+ and Na+/Ca2+ that impeded the growth under long term exposure (>6d). The survivability at salinity 45 even with considerably higher ratio of Na+/K+ and Na+/Ca2+ suggests the compartmentalization of Na+ into the vacuoles. Further, the micro nutrients such as Zn, Fe and Mn were decreased at both high and low end salinities with highest at extreme hyper-salinity. The C18:1(n-9) cis, C18:2(n-6), C18:3(n-3) and C20:4(n-6) were found in significant amounts in hyper-salinities. The C18:1(n-9) cis in particular increased by 60.25% and 70.51% for salinities 45 and 55, respectively from their initial amounts. The ratio of total unsaturated to saturated fatty acids (UFA/SFA) also increased linearly with increasing salinity. These results collectively suggest the potential role of antioxidative enzymes, phycobiliproteins, PUFAs and mineral nutrients to combat the salinity induced oxidative stress in G. corticata.