The concentration of TPCs was in the μgL −1 range in the co-culture medium and increased with co-culture time. These compounds are degradable in water, implying that they are secreted continuously by submerged macrophytes and result in long-term stress to the phytoplankton. Sustained exposure to TPC leads to formation of excess ROS/NO that triggers PCD in M. aeruginosa. Ross et al. (2006) reported that caspase-3-like activity was elicited in M. aeruginosa in the wild environment treated with H2O2, and Ding et al. (2012) contrib- uted to our understanding of PCD in M. aeruginosa by describ- ing the diverse PCD markers that appeared after exposure to H2O2. It is well known that ROS can directly react with bio- molecules and damage DNA, proteins (enzymes), lipids (mem- branes), etc. (Hemnani and Parihar 1998), as well as can act as signaling radical that activate the intracellular caspases and trig- gerPCD(Simonetal. 2000).Moreover,studiesindicatethatthe level of ROS required for triggering PCD is significantly lower than the level that causes direct irreversible oxidative damage to M. aeruginosa cells (Lu, unpublished paper, 2014).