Interactive Effects of Temperature and UV Radiation on Photosynthesis of Chlorella Strains from Polar, Temperate and Tropical Environments: Differential Impacts on Damage and Repair

Abstract

Global warming and ozone depletion, and the resulting increase of ultraviolet radiation (UVR), have far-reaching impacts on biota, especially affecting the algae that form the basis of the food webs in aquatic ecosystems. The aim of the present study was to investigate the interactive effects of temperature and UVR by comparing the photosynthetic responses of similar taxa of Chlorella from Antarctic (Chlorella UMACC 237), temperate (Chlorella vulgaris UMACC 248) and tropical (Chlorella vulgaris UMACC 001) environments. The cultures were exposed to three different treatments: photosynthetically active radiation (PAR; 400–700 nm), PAR plus ultraviolet-A (320–400 nm) radiation (PAR + UV-A) and PAR plus UV-A and ultraviolet-B (280–320 nm) radiation (PAR + UV-A + UV-B) for one hour in incubators set at different temperatures. The Antarctic Chlorella was exposed to 4, 14 and 20°C. The temperate Chlorella was exposed to 11, 18 and 25°C while the tropical Chlorella was exposed to 24, 28 and 30°C. A pulse-amplitude modulated (PAM) fluorometer was used to assess the photosynthetic response of microalgae. Parameters such as the photoadaptive index (Ek) and light harvesting efficiency (α) were determined from rapid light curves. The damage (k) and repair (r) rates were calculated from the decrease in ΦPSIIeff over time during exposure response curves where cells were exposed to the various combinations of PAR and UVR, and fitting the data to the Kok model. The results showed that UV-A caused much lower inhibition than UV-B in photosynthesis in all Chlorella isolates. The three isolates of Chlorella from different regions showed different trends in their photosynthesis responses under the combined effects of UVR (PAR + UV-A + UV-B) and temperature. In accordance with the noted strain-specific characteristics, we can conclude that the repair (r) mechanisms at higher temperatures were not sufficient to overcome damage caused by UVR in the Antarctic Chlorella strain, suggesting negative effects of global climate change on microalgae inhabiting (circum-) polar regions. For temperate and tropical strains of Chlorella, damage from UVR was independent of temperature but the repair constant increased with increasing temperature, implying an improved ability of these strains to recover from UVR stress under global warming.

Photosynthetic UV stress tolerance of the Antarctic snow alga Chlorella sp. modified by enhanced temperature?

“…in Chlorella zofingiensis) or mycosporine-like amino acids (MAAs) [14]. However, when algae were exposed to higher temperatures, repair processes in Antarctic strains increase significantly [27]. This could be a confirmation that elevated temperatures can ameliorate the detrimental impact of UV radiation as has been reported in Antarctic macroalgae [42].…”

“…Up to now, most of the studies on snow algal photobiology have been conducted in ubiquitous flagellate green algal genera such as Chlamydomonas, or Chloromonas, which show an ability to actively migrate along the snow pack column and thus "regulate" their light environment [3,[24][25][26]. In a recent study was demonstrated that Antarctic strains of Chlorella are more sensitive to UV radiation under elevated temperature than their counterparts from temperate or tropical regions [27]. Thus, in the present study the question whether photosynthetic characteristics of non-motile Chlorella sp.…”

Response of Microalgae in a Changing Climate and Environment

“…In another study, Chlorella strains from Antarctic, temperate and tropical regions showed different photosynthetic patterns in response to integrative effects of PAR, UV-A, UV-B with a range of temperatures. The Antarctic Chlorella strain notably showed lower photosynthetic recovery compared to the temperate and tropical strains (Wong et al, 2015). Cell productivity of Scenedesmus acuminatus, Cyclotella meneghiniana, and Microcystis aruginosa increased under combined effects of elevated CO 2 level and temperature.…”

Differential photosynthetic responses of marine planktonic and benthic diatoms to ultraviolet radiation under various temperature regimes

“…With the continuing emission of greenhouse gases, the surface seawater temperature is predicted to increase by up to 4 • C by the end of this century (New et al, 2011), and this could potentially re-shape the phytoplankton assemblages (Thomas et al, 2012). While the situation might be more complex in the natural environment with the consideration of interaction of UVR with other factors (Beardall et al, 2009), for unicellular green algae, an increase of temperature could mitigate UVR harm for temperate species, while exacerbating UV inhibition for polar species (Wong et al, 2015). Moreover, the tolerance of phytoplankton to extreme temperature would be latitudedependent; for tropical areas where the temperature is already high, an increase of temperature reduced the richness of phytoplankton (Thomas et al, 2012).…”

“…The photochemical damage is mainly determined by the intensity and spectrum of light (Heraud and Beardall, 2000) and is temperature insensitive, while the repair process is driven by a series of enzyme-catalyzed reactions and is thus potentially sensitive to temperature changes (Melis, 1999). Previous studies revealed that high temperature alleviated UV inhibition of PSII in green algae (Wong et al, 2015), while it interactively decreased photosynthetic activity in microphytobenthos under excessive PAR conditions (Laviale et al, 2015).…”