Differential Physiological Responses of Small Thalassiosira pseudonana and Large Thalassiosira punctigera to the Shifted-High Light and Nitrogen

Abstract

With global warming, the intensity and frequency of extreme episodic weather events such as typhoons are rising in tropical and subtropical regions, disturbing the water column and shifting phytoplankton therein from deep to surface layers, and exposing them to high light as well as nutrients. To explore how phytoplankton respond to such environmental changes, we tracked the growth, cell compositions and physiology of small Thalassiosira pseudonana and large Thalassiosira punctigera from simulated ambient to upward-shifted light and nitrogen (N) conditions. Shifting to high levels of light caused a limited effect on the growth of small T. pseudonana, but reduced that of large T. punctigera by 36%, with supplemental N alleviating the light-caused growth reduction. The upward-shifted light reduced the cellular pigments contents in small T. pseudonana, but not in large T. punctigera. The upward-shifted light reduced the photosynthetic capability (FV/FM) of both species, as well as the light utilization efficiency (α) and maximal relative electron transport rate (rETRmax), but it enhanced their dark reparations. Moreover, the upward-shifted light did not affect the superoxide dismutase (SOD) activity of small T. pseudonana, but it did enhance that of large T. punctigera. In addition, the supplemental N showed a limited effect on cellular pigments and the dark respiration of T. pseudonana, but it reduced that of T. punctigera. Our results showed that the growth responses of Thalassiosira to upward-shifted light and nitrogen vary with species and possibly with cell size, indicating that the field species composition might change after the occurrence of extreme weather events.

Photosynthetic Characteristics of Smaller and Larger Cell Size-Fractioned Phytoplankton Assemblies in the Daya Bay, Northern South China Sea

“…For example, the photosynthetic performance of phytoplankton assemblages exhibited a clearly light-dependent diel variation in coastal waters of, e.g., the Daya Bay, northern South China Sea [ 17 ] and Ariake Bay, Japan [ 18 ], as well as in pelagic waters of, e.g., the South China Sea [ 19 ], Southern Ocean [ 20 ] and Northeast Pacific Ocean [ 21 ]. Cell compositions basically mediate such the integrated physiological responses of phytoplankton assemblages to the L:D cycle; while this has been convincingly shown in our laboratorial studies [ 8 , 13 , 14 , 15 , 22 ] and in the work of others [ 16 ], there is still a lack of evidence under field conditions.…”

“…The photosynthetic parameter of F V /F M usually decreases under stressful high light, caused by the photoinactivation of PS II [ 13 , 51 , 52 ]; therefore, it is often used as an indicator of photosynthetic capacity of phytoplankton cells [ 8 , 17 ]. Consistently, the F V /F M was lower at noontime than the other times ( Figure 4 A); while this decline was less in piconano- than in micro-cell assemblies, which, together with a similar decline of the FLC-derived light utilization efficiency (α, Figure 5 A), implies that smaller cells are more resistant to high light.…”

“…Meanwhile, smaller cells have lower pigment “packaging effect” which enables them to possess a greater light-harvesting ability, and thus grow better under dim-light conditions [ 7 ]. On the other hand, these biological traits also make smaller cells more vulnerable to stressful high light or harmful UV radiation if considering the photoinactivation [ 8 ] or DNA damage [ 9 ]; because, the higher pigment- or volume-specific light absorption usually leads to the higher light-harvesting capacity, as well as the UV exposure per unit pigment or per cell volume [ 10 , 11 ]. The small cell volume also limits the storage of many indispensable resources, e.g., lipids and proteins [ 12 ], which may be predicted to count them against surviving in rapidly changing coastal environments.…”

Lowering pO2 Interacts with Photoperiod to Alter Physiological Performance of the Coastal Diatom Thalassiosira pseudonana

“…Similar to previous studies [30,31], the light intensity of 150 µmol photons m −2 s −1 was expected to saturate the cell growth under a shorter photoperiod and may have oversaturated under the longer photoperiod, as indicated by an insignificant difference of the µ between the 8:16 and 16:8 L:D cycles (Figure 1). It is easily understood that phytoplankton cells usually lower the synthesis and accumulation of Chl a in oversaturated light, to lessen the excessive energy harvesting [43] and thus to alleviate the excessive light energy-caused photodamage or photoinhibition [30,50]. Moreover, the decreased pO 2 interacted with the prolonged photoperiod to aggravate the decrease in Chl a (F (2,12) = 7.53, p < 0.01), as also supported by the transcriptome results (Figure 6).…”

“…After centrifugation (10,000× g, 10 min, 4 • C), the supernatant was used to quantify the protein using a protein assay kit (A045-2, Nanjing Jiancheng Biological Engineering Co., Nanjing, China) following the manufacturer's protocol with the bicinchoninic acid (BCA) method [31,32]. The malondialdehyde (MDA) in the protein solution, a product of membrane lipid peroxidation, was determined using an assay kit (A003-1, Nanjing Jiancheng Biological Engineering Co., Nanjing, China) [32], as well as the superoxide dismutase (SOD) activity with an assay kit (A001-3, Nanjing Jiancheng Biological Engineering Co., Nanjing, China) [43] following the protocol of the manufacturer.…”