Decrease, Photosynthesis

refers to a reduction in the efficiency and/or capacity of photosynthetic organisms to convert light energy into chemical energy stored in organic carbon compounds. This key event encompasses impairments in the light-dependent reactions, the carbon fixation reactions, or both, resulting in diminished overall photosynthetic performance. At the mechanistic level, this KE can be caused by damage or inhibition of photosystems, particularly photosystem II (PSII), or interference with the photosynthetic electron transport chain (ETC). Such perturbations reduce the generation of ATP and NADPH, which are required to drive carbon fixation in the Calvin–Benson cycle. decrease in photosynthesis leads to reduced primary productivity, lower carbohydrate synthesis, and impaired energy availability for growth, reproduction, and maintenance. In aquatic and terrestrial primary producers, this KE represents a critical point of vulnerability linking molecular or cellular stressors (e.g., chemical inhibitors, oxidative stress, nutrient imbalance, or physical stressors such as light limitation) to higher-level adverse outcomes, including reduced biomass accumulation, altered community structure, and ecosystem-level impacts.

A decrease in photosynthesis can be quantified using a suite of complementary physiological and biochemical measurements that capture both light-driven energy conversion and carbon assimilation processes.

Carbon fixation rates are most directly assessed using ¹⁴C-bicarbonate uptake assays, which quantify the incorporation of inorganic carbon into organic compounds during photosynthesis. This approach provides an integrative measure of photosynthetic carbon assimilation and is widely applied in algal, phytoplankton, and plant systems, with careful interpretation required to distinguish gross versus net fixation under different experimental conditions (Grant and Howard, 1980; Milligan, Halsey and Behrenfeld, 2015).

Oxygen evolution measurements offer a direct proxy for the activity of the photosynthetic light reactions, particularly Photosystem II. Using Clark-type oxygen electrodes or polarographic methods, the rate of O₂ production under illumination can be quantified in intact tissues, leaf discs, or isolated chloroplasts, providing sensitive detection of functional impairment in the photosynthetic electron transport chain (DELIEU and WALKER, 1981; van Gorkom and Gast, 1996).

At the whole-organism or leaf level, infrared gas analysis (IRGA) is commonly employed to measure net CO₂ uptake. This non-invasive technique allows continuous monitoring of photosynthetic performance under controlled environmental conditions and integrates stomatal conductance, biochemical capacity, and photochemical efficiency into a single functional endpoint (Amthor and Baldocchi, 2001; Xie et al., 2019).

To resolve downstream biochemical constraints, Rubisco activity assays are used to quantify the catalytic capacity of ribulose-1,5-bisphosphate carboxylase/oxygenase. Both ¹⁴C-based assays and NADH-linked spectrophotometric or microtiter plate methods enable discrimination between limitations arising from carbon fixation enzymes versus upstream photochemical processes (Lilley and Walker, 1974; Sales, da Silva and Carmo-Silva, 2020).

Together, these measurement approaches provide mechanistically informative and quantitatively robust indicators of decreased photosynthesis, supporting their use as key event measurements in Adverse Outcome Pathway development and ecotoxicological hazard characterization (van Gorkom and Gast, 1996)

This KE applies broadly to organisms that perform oxygenic photosynthesis, including terrestrial higher plants, freshwater and marine macrophytes, macroalgae, and phytoplankton, because all of these taxa utilize PSII–mediated light reactions and downstream carbon fixation to convert light energy into chemical energy. Photosystem II is a multisubunit pigment–protein complex present across cyanobacteria, algae, and plants that catalyzes light-driven water oxidation and initiates electron transport, providing the reducing power required for CO₂ assimilation and organic carbon synthesis. Disruption of PSII electron transport or carbon fixation directly results in decreased photochemical efficiency and reduced primary productivity in these diverse taxa (Sundby et al., 1993; Broser et al., 2011). For example, herbicides and other stressors that target PSII competitively bind to quinone acceptor sites in the D1 protein, blocking electron transport and diminishing carbon fixation efficiency, with effects documented across several photosynthetic groups (Broser et al., 2011; King et al., 2021). Environmental stressors such as high light intensity and other abiotic pressures further exacerbate impairment of PSII function through photoinhibition, in which damage to PSII and imbalances in repair processes reduce photosynthetic rates in both aquatic and terrestrial organisms (Murata et al., 2007).

Because the PSII reaction center and associated processes are highly conserved among oxygenic photosynthetic organisms, decreases in photosynthetic performance induced by chemical or physical stressors are broadly applicable across taxa that contribute to ecosystem primary productivity. This KE is relevant across life stages where photosynthetic activity supports growth and energy capture, from juvenile algal cells to mature plant leaves, and under a range of environmental contexts, including natural light variation and anthropogenic pollutant exposure (Sundby et al., 1993; Murata et al., 2007). The universal importance of PSII integrity for carbon fixation and energy transduction supports the domain of applicability of Decrease, Photosynthesis as a mechanistically grounded indicator of photochemical and autotrophic dysfunction across photosynthetic lineages.

Broser, M., Glöckner, C., Gabdulkhakov, A., Guskov, A., Buchta, J., Kern, J., Müh, F., Dau, H., Saenger, W. and Zouni, A. 2011. Structural basis of cyanobacterial photosystem II inhibition by the herbicide terbutryn. J Biol Chem 286(18), 15964–15972.

DELIEU, T. and WALKER, D.A. 1981. Polarographic measurement of photosynthetic oxygen evolution by leaf discs. New Phytologist 89(2), 165–178.

Grant, B.R. and Howard, R.J. 1980. Kinetics of C distribution during photosynthesis by chloroplast preparations isolated from the siphonous alga Caulerpa simpliciuscula. Plant Physiol 66(1), 29–33.

Lilley, R.M. and Walker, D.A. 1974. An improved spectrophotometric assay for ribulosebisphosphate carboxylase. Biochimica et Biophysica Acta (BBA) – Enzymology 358(1), 226–229.

Milligan, A.J., Halsey, K.H. and Behrenfeld, M.J. 2015. Advancing interpretations of 14C-uptake measurements in the context of phytoplankton physiology and ecology. Journal of Plankton Research 37(4), 692–698.

Murata, N., Takahashi, S., Nishiyama, Y. and Allakhverdiev, S.I. 2007. Photoinhibition of photosystem II under environmental stress. Biochimica et Biophysica Acta (review).

Photosystem II. 2025. Wikipedia, The Free Encyclopedia.

Sales, C.R.G., da Silva, A.B. and Carmo-Silva, E. 2020. Measuring Rubisco activity: challenges and opportunities of NADH-linked microtiter plate-based and 14C-based assays. Journal of Experimental Botany 71(18), 5302–5312.

Sundby, C., Chow, W.S. and Anderson, J.M. 1993. Effects on Photosystem II function, photoinhibition, and plant performance of the spontaneous mutation of serine-264 in the Photosystem II reaction center D1 protein in triazine-resistant Brassica napus L. Plant Physiol 103(1), 105–113.

van Gorkom, H.J. and Gast, P. 1996. Measurement of photosynthetic oxygen evolution. Biophysical Techniques in Photosynthesis 3, 391–405.

Xie, L., Solhaug, K.A., Song, Y., Brede, D.A., Lind, O.C., Salbu, B. and Tollefsen, K.E. 2019. Modes of action and adverse effects of gamma radiation in an aquatic macrophyte Lemna minor. Science of the Total Environment 680, 23–34.