Effects of ocean acidification and eutrophication on the growth and photosynthetic performances of a green tide alga Ulva prolifera

Abstract

With the impact of fossil fuel burning and industrialization, atmospheric CO2 concentration will reach about 1000 ppmv in 2100, and more and more CO2 will be absorbed by ocean, resulting in ocean acidification. The Chinese coastal waters are showing unexpectedly high levels of acidification due to a combination of global ocean acidification and severe regional eutrophication, which is caused by natural accumulation or human activities such as aquacultural tail water input, potentially affecting macroalgal blooms. However, little is known about the combined effects of ocean acidification and entrophication on the eco-physiology of bloom-forming macroalgae. This study investigated Ulva prolifera, a dominant species causing green tide in the South Yellow Sea, and explored its growth and physiological responses under the combination conditions of ocean acidification and enriched nutrients. In this study, U. prolifera thalli were cultured under two CO2 conditions (air and 1000 μatm) and two nutrient conditions (High Nutrient, HN, 135 μmol L-1 N and 8.5 μmol L-1 P; Normal Nutrient, NN, 27 μmol L-1 N and 1.7 μmol L-1 P). The results showed that eutrophication conditions obviously enhanced the relative growth rate and photosynthetic performance of U. prolifera. Elevated pCO2 had no significant effect on U. prolifera growth and photosynthetic performance under normal nutrient conditions. However, under eutrophication conditions elevated pCO2 inhibited U. prolifera growth. Moreover, eutrophication conditions markedly improved the contents of chlorophyll a, chlorophyll b and nitrate reductase activity and inhibited the soluble carbohydrate content, but elevated pCO2 had no significant effect on them under nutrient-replete conditions. In addition, elevated pCO2 significantly reduced the carotenoid content under eutrophication conditions and had no effect on it under normal nutrient conditions. These findings indicate that seawater eutrophication would greatly accelerate U. prolifera bloom, which may also be suppressed to a certain extent by ocean acidification in the future. The study can provide valuable information for predicting the future outbreaks of U. prolifera green tide in nearshore regions.

Review of Allelopathy in Green Tides: The Case of Ulva prolifera in the South Yellow Sea

“…These show that fatty acids significantly influence stress and the control of biological responses and that these pathways are essential to the environmental adaptability and growth of algae [ 59 ]. Particularly acidifying circumstances significantly affect the levels of polyunsaturated fatty acids and palmitic acid [ 38 ].…”

“…On the other hand, eutrophication and acidification of the ocean have proven green macroalgae to be remarkably adaptive [ 81 ]. In these situations, algal physiological regulating ability is increased in part by phenolic substances [ 38 ]. Furthermore, some research has demonstrated that green macroalgae can successfully regulate the amount of cyanobacteria by producing phenolic chemicals [ 82 ].…”

Multi-Factors Synthetically Contribute to Ulva prolifera Outbreaks in the South Yellow Sea of China

“…In addition to the aforementioned anthropogenic factors, it is also important to consider that U. prolifera absorbs a significant amount of nutrients such as nitrogen and phosphorus from seawater to support its growth and reproduction during the growing phase, and releases substantial amounts of carbon, nitrogen, and phosphorus back into the seawater during the decaying phase [71,72]. We posit that the timing and magnitude of U. prolifera harvesting may influence the degree of seawater eutrophication, thus affecting the scale of U. prolifera blooms in the subsequent year.…”

Two new coastal time-series of seawater carbonate system variables in the NW Mediterranean Sea: rates and mechanisms controlling pH changes

“…The ocean carbon cycle community is increasingly recognizing coastal and nearshore areas as hotspots for carbon and biogeochemical variability (Cai et al, 1998;Cai et al, 2020;Cai et al, 2021;Dai et al, 2022), especially vulnerable to global change, likely sites for marine CO 2 removal interventions, and regions of great importance, as coastal ecosystems provide invaluable resources and services, including various for climate change mitigation (Duarte et al, 2013b). The increased CO 2 uptake of coastal waters is not only contributing to their acidification but also exacerbating existing global challenges such as eutrophication, pollution, and habitat degradation (Andersson and Gledhill, 2013;Cai et al, 2023). Therefore, a robust understanding of carbonate chemistry is key to properly assessing habitat vulnerability to ocean acidification.…”