Antioxidant activity recorded by endogenous fluorescence monitoring in green microalga Chlorella sp. exposed to chosen antioxidants

Algae represent a biotechnologically important group of organisms whose physiological processes can be studied using endogenous fluorescence. This study tested fluorescence methods for evaluating the effect of antioxidants on the algae Chlorella sp. using endogenous fluorescence, in order to determine the best approach for monitoring antioxidant activity in living cells. The algae were cultured with several antioxidants, including ascorbic acid, gallic acid, catechin, quercetin, and resveratrol for 4–6 days. Antioxidant properties were evaluated using two different approaches. The TPC, DPPH, ABTS, and FRAP methods provide information on reducing capacity and the content of antioxidant-active compounds. TPC values ranged from 1.71 ± 0.02 to 2.21 ± 0.02 mg GAE/g; while DPPH, ABTS, and FRAP values ranged from 25.54 ± 0.23 to 38.16 ± 0.18 TE/g, 27.41 ± 0.16 to 40.71 ± 0.57 and 1.79 ± 0.02 to 3.78 ± 0.08 µmol TE/g. In contrast, NAD(P)H fluorescence reflects the intracellular metabolic and redox state of cells. Results demonstrated that NAD(P)H fluorescence was the best indicator of antioxidant activity, as all antioxidants increased the total photon counts of NAD(P)H fluorescence and prolonged the Tau2 fluorescence lifetime in the algae, in agreement with the expected action on NADPH. Specific responses in a1/a2, or Tau1, of NAD(P)H fluorescence lifetimes, and/or on the red/green endogenous fluorescence were also observed for individual antioxidants. The gathered results demonstrate that recording endogenous fluorescence, predominantly in the blue spectral region, is an effective approach for directly monitoring the antioxidant activity in living algae.