The Synergistic Impact of Multiple Stressors on the Photophysiology of the Cyanobacteria Synechococcus Elongatus

The predicted increase of pCO₂ levels from current levels of 410 ppm to 750 ppm or even 1000 ppm by the year 2100 will also produce warmer temperatures, and a shallower (and therefore brighter) mixed layer. While studies have mainly focused on independent stressors affecting phytoplankton, this study will investigate how multiple stressors affect phytoplankton simultaneously. Our lab has previously investigated growth and photophysiology of diatom Thalassiosira pseudonana under different pCO₂ levels, temperature and light regimes. This study uses a similar approach, but with a focus on a different group of phytoplankton – the cyanobacteria, Synechococcus elongatus. We assessed the growth and photophysiology of S. elongatus under three pCO₂ levels (410 ppm, 750 ppm, and 1000 ppm), four temperatures (20, 28, 36, and 44ºC), and three light regimes (sup-optimal, optimum, and inhibitory) in a multi-factorial design. Here, I describe temporal changes in the photophysiology of S. elongatus in response to multiple stressors by comparing (a) quantum yields, and (b) the relative changes in photosynthesis rates. Measurements were made using a Pulse Amplitude Modulated (PAM) type fluorometer, the Aquapen AP110-C PAM fluorometer. Our results revealed some unusual trends. For example, higher, stable quantum yields were observed at present-day pCO₂ levels, across all temperatures and light intensities. At elevated pCO₂ levels of 750 ppm, photosystems appear stressed, however, as pCO₂ levels increase to extreme concentrations of 1000 ppm, the photosystems appear to regain stability. The results highlight the need to factor in phytoplankton species/taxon specificity, as well as the non-linear nature of the response to multiple simultaneous environmental stressors.