Abstract
Exacerbating deoxygenation is extensively affecting marine organisms, with no exception for phytoplankton. To probe these effects, we comparably explored the growth, cell compositions, photosynthesis, and transcriptome of a diatom Thalassiosira pseudonana under a matrix of pO2 levels and Light:Dark cycles at an optimal growth light. The growth rate (μ) of T. pseudonana under a 8:16 L:D cycle was enhanced by 34% by low pO2 but reduced by 22% by hypoxia. Under a 16:8 L:D cycle, however, the μ decreased with decreasing pO2 level. The cellular Chl a content decreased with decreasing pO2 under a 8:16 L:D cycle, whereas the protein content decreased under a 16:8 L:D cycle. The prolonged photoperiod reduced the Chl a but enhanced the protein contents. The lowered pO2 reduced the maximal PSII photochemical quantum yield (FV/FM), photosynthetic oxygen evolution rate (Pn), and respiration rate (Rd) under the 8:16 or 16:8 L:D cycles. Cellular malondialdehyde (MDA) content and superoxide dismutase (SOD) activity were higher under low pO2 than ambient pO2 or hypoxia. Moreover, the prolonged photoperiod reduced the FV/FM and Pn among all three pO2 levels but enhanced the Rd, MDA, and SOD activity. Transcriptome data showed that most of 26 differentially expressed genes (DEGs) that mainly relate to photosynthesis, respiration, and metabolism were down-regulated by hypoxia, with varying expression degrees between the 8:16 and 16:8 L:D cycles. In addition, our results demonstrated that the positive or negative effect of lowering pO2 upon the growth of diatoms depends on the pO2 level and is mediated by the photoperiod.
Highlights
Anthropogenic marine eutrophication and warming are exacerbating the deoxygenation in the water column through, for example, unbalancing O2 production and consumption, lowering O2 solubility, and hindering the exchange with atmospheric O2 [1,2]
Most previous studies reported the individual effect upon phytoplankton physiology
At lowered pO2, the reduction in respiration and photosynthesis occurred under both the 8:16 and 16:8 L:D cycles, as compared to ambient pO2 ; while the enhancement of the growth rate was present under the former but reduced under the latter L:D cycle, which indicated the alteration of light duration on the balance of the lowered pO2 -induced savings of consumption and the reduction in photosynthetic production
Summary
Anthropogenic marine eutrophication and warming are exacerbating the deoxygenation in the water column through, for example, unbalancing O2 production and consumption, lowering O2 solubility, and hindering the exchange with atmospheric O2 [1,2]. Over the past five decades, the global oceans have had an estimated loss of ~2% of their oxygen [3]; the degree and area of O2 -deficiency or O2 -limitation have extended quickly [1,4]. The number of hypoxia zones (dissolved oxygen, DO < 2.5 mg L−1 , ~25% saturation) have exceeded 500 worldwide, with the hypoxic area reaching over 1,000,000 km2 [4]. Dissolved O2 in seawater is important in marine ecosystems as the water must contain sufficient O2 to maintain aquatic biota. Apart from affecting these aerobic organisms, the lowered available O2 influences the photosynthetically O2 producing organisms [6–8], as these O2 producers, including phytoplankton, need surrounding O2 to maintain mitochondrial respiration under the limited-light or dark conditions, which can supply energy
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