Effects of elevated pCO2 and nutrient enrichment on the growth, photosynthesis, and biochemical compositions of the brown alga Saccharina japonica (Laminariaceae, Phaeophyta)

Response of Juvenile Saccharina japonica to the Combined Stressors of Elevated pCO2 and Excess Copper

Coastal macroalgae may be subjected to global and local environmental stressors, such as ocean acidification and heavy-metal pollution. We investigated the growth, photosynthetic characteristics, and biochemical compositions of juvenile sporophytes of Saccharina japonica cultivated at two pCO₂ levels (400 and 1000 ppmv) and four copper concentrations (natural seawater, control; 0.2 μM, low level; 0.5 μM, medium level; and 1 μM, high level) to better understand how macroalgae respond to ongoing environmental changes. The results showed that the responses of juvenile S. japonica to copper concentrations depended on the pCO₂ level. Under the 400 ppmv condition, medium and high copper concentrations significantly decreased the relative growth rate (RGR) and non-photochemical quenching (NPQ) but increased the relative electron transfer rate (rETR) and chlorophyll a (Chl a), chlorophyll c (Chl c), carotenoid (Car), and soluble carbohydrate contents. At 1000 ppmv, however, none of the parameters had significant differences between the different copper concentrations. Our data suggest that excess copper may inhibit the growth of juvenile sporophytes of S. japonica, but this negative effect could be alleviated by CO₂-induced ocean acidification.

Decreased Salinity Offsets the Stimulation of Elevated pCO2 on Photosynthesis and Synergistically Inhibits the Growth of Juvenile Sporophyte of Saccharina japonica (Laminariaceae, Phaeophyta)

The combined effect of elevated pCO₂ (Partial Pressure of Carbon Dioxide) and decreased salinity, which is mainly caused by freshwater input, on the growth and physiological traits of algae has been poorly assessed. In order to investigate their individual and interactive effects on the development of commercially farmed algae, the juvenile sporophytes of Saccharina japonica were cultivated under different levels of salinity (30, 25 and 20 psu) and pCO₂ (400 and 1000 µatm). Individually, decreased salinity significantly reduced the growth rate and pigments of S. japonica, indicating that the alga was low-salinity stressed. The maximum quantum yield, F_v/F_m, declined at low salinities independent of pCO₂, suggesting that the hyposalinity showed the main effect. Unexpectedly, the higher pCO₂ enhanced the maximum relative electron transport rate (rETR_max) but decreased the growth rate, pigments and soluble carbohydrates contents. This implies a decoupling between the photosynthesis and growth of this alga, which may be linked to an energetic reallocation among the different metabolic processes. Interactively and previously untested, the decreased salinity offset the improvement of rETR_max and aggravated the declines of growth rate and pigment content caused by the elevated pCO₂. These behaviors could be associated with the additionally decreased pH that was induced by the low salinity. Our data, therefore, unveils that the decreased salinity may increase the risks of future CO₂-induced ocean acidification on the production of S. japonica.

The CsGPA1-CsAQPs module is essential for salt tolerance of cucumber seedlings

CsGPA1 interacts with CsTIP1.1 (a member of CsAQPs) and suppression of CsGPA1 results the reverse expression of CsAQPs in leaves and roots, resulting in declining water content of cucumber seedlings under salt stress. Salt stress seriously affects cucumber growth and development. Whether the G-protein alpha subunit functions in cucumber during salt stress and its regulation mechanism remains unknown. We interrogated CsGPA1-RNAi lines to identify the role of CsGPA1 during salt stress. Phenotypically, compared with wild type, leaves were severely withered, and root cells showed signs of senescence under salt stress for RNAi lines. Compared with WT, SOD and CAT activity, soluble protein and proline contents all decreased in RNAi lines, while malondialdehyde and relative electrical conductivity increased. Through screening the yeast two-hybrid library and combined with yeast two-hybrid and GST pull-down, the interaction of CsGPA1 with CsTIP1.1 was found the first time in a plant. Then, the expression of aquaporin (AQP) family genes was detected. The expression of CsAQP genes in leaves and roots was primarily up-regulated in WT under salt stress. However, interference by CsGPA1 resulted in enhanced expression of CsAQPs except for CsTIP3.2 in leaves, but reduced expression of some CsAQPs in roots under salt stress. Furthermore, principal component analysis of CsAQP expression profiles and linear regression analysis between CsGPA1 and CsAQPs revealed that CsGPA1 reversely regulated the expression of CsAQPs in leaves and roots under salt stress. Moreover, the water content in leaves and roots of RNAi seedlings significantly decreased compared with WT under salt stress. Overall, CsGPA1 interacts with CsTIP1.1 and suppression of CsGPA1 results in opposite patterns of expression of CsAQPs in leaves and roots, resulting in declining water content of cucumber under salt stress.