Oxidative pathways in response to polyunsaturated aldehydes in the marine diatom Skeletonema marinoi (Bacillariophyceae)

Polyunsaturated aldehydes (PUA) have recently been shown to induce reactive oxygen species (ROS) and possibly reactive nitrogen species (RNS, e.g., peroxynitrite) in the diatom Skeletonema marinoi (S.marinoi), which produces high amounts of PUA. We now are attempting to acquire better understanding of which reactive molecular species are involved in the oxidative response of S.marinoi to PUA. We used flow cytometry, the dye dihydrorhodamine 123 (DHR) as the main indicator of ROS (but which is also known to partially detect RNS), and different scavengers and inhibitors of both nitric oxide (NO) synthesis and superoxide dismutase activity (SOD). Both the scavengers Tempol (for ROS) and uric acid (UA, for peroxynitrite) induced a lower DHR-derived green fluorescence in S.marinoi cells exposed to the PUA, suggesting that both reactive species were produced. When PUA-exposed S.marinoi cells were treated with the NO scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), an opposite response was observed, with an increase in DHR-derived green fluorescence. A higher DHR-derived green fluorescence was also observed in the presence of sodium tungstate (ST), an inhibitor of NO production via nitrate reductase. In addition, two different SOD inhibitors, 2-methoxyestradiol (2ME) and sodium diethyldithiocarbamate trihydrate (DETC), had an effect, with DETC inducing the strongest inhibition after 20min. These results indicate the involvement of O-2(center dot) generation and SOD activity in H2O2 formation (with downstream ROS generation dependent from H2O2) in response to PUA exposure. This is relevant as it refines the biological impact of PUA and identifies the specific molecules involved in the response. It is speculated that in PUA-exposed S.marinoi cells, beyond a certain threshold of PUA, the intracellular antioxidant system is no longer able to cope with the excess of ROS, thus resulting in the observed accumulation of both O-2(center dot-) and H2O2. This might be particularly relevant for population dynamics at sea, during blooms, when cell lysis increases and PUA are released. It can be envisioned that in the final stages of blooms, higher local PUA concentrations accumulate, which in turn induces intracellular ROS generation that ultimately leads to cell death and bloom decay.