Figure 2. Proposed mechanisms involved in the dissolved organic matter (DOM) release of turf algae under nutrient limited and nutrient replete conditions.

Under nutrient limitation carbon fixation is proposed to outpace the N and P supply, leading to the formation of C-rich and high molecular weight (HMW) storage products which are actively released as DOM in an overflow mechanism. An increase in light availability further increases the intracellular C:N:P ratio and thus stimulates DOM release. When nutrients are replete N- and P-rich cell components and low molecular weight (LMW) precursors of cell components are synthesized. These LMW molecules passively diffuse through the cell membrane as long as a concentration gradient prevails. This mechanism is not affected by an increase in light availability. A combination of increased respiration due to an enhanced synthesis and the formation of oxygen radicals and thereafter water to compensate for the imbalance of the intracellular C:N ratio are further suggested to reduce the oxygen release under such conditions. Green and red arrows indicate positive effects (stimulation) and negative effects (reduction), respectively.