Responses of marine primary producers to interactions between ocean acidification, solar radiation, and warming

Abstract

Anthropogenic CO2 is accumulating in the atmosphere and trapping backward infrared radiation, resulting in warming of both terrestrial and ocean ecosystems. At the same time, dissolution of CO2 into seawater is increasing surface ocean acidity, a process known as ocean acidification. Phytoplankton cells in natural environments experience diurnal changes of solar radiation, from light-limiting to light-saturating and then, most often in upper layers, to stressful light levels in the presence of UV radiation. Subsequently, ocean acidification can interact with solar radiation to bring about synergistic, antagonistic or balanced effects on marine primary producers at different depths or under changing weather conditions. In fact, both solar radiation and pCO2 can fluctuate over different time scales to range from limiting to saturating or even stressful levels. On the other hand, shoaling of the upper mixed layer (enhanced stratification) due to ocean warming and freshening (rain, ice melting) can lead to additional photosynthetically active radiation (PAR) and ultraviolet (UV) exposure, which can have both benefits and costs to photosynthetic organisms. Within limits, elevated CO2 concentrations under low or moderate levels of PAR have been shown to act synergistically benefiting photosynthesis or growth in both marine phytoplankton and macroscopic algae; excessive levels of PAR, however, can lead to additional inhibition of photosynthesis or growth under elevated CO2, and addition of UV radiation (280-400 nm) can increase such inhibition. While solar UV-B (280-315 nm) radiation often harms algal cells, UV-A (315-400 nm) at moderate levels stimulates photosynthetic carbon fixation in both phytoplankton and macroalgae. Both the inhibitory impacts of UV-B and stimulatory effects of UV-A vary in amplitude with changes in seawater chemistry associated with ocean acidification. In view of warming effects, increased temperatures have been shown to enhance photorepair of UV-damaged molecules, though it simultaneously enhances respiratory carbon loss. The net effects of ocean acidification on marine primary producers are therefore largely dependent on the photobiological conditions (light limitation, light or UV stress), as well as interactions with rising temperature and other variables such as altered nutrient availability. Hence, feedbacks between changing carbonate chemistry and solar radiation across the entire spectrum present complications to interpret and understand ocean acidification effects based on single-factor experiments.