Physiological Responses of Sargassum muticum, a Potential Golden Tide Species, to Different Levels of Light and Nitrogen

Abstract

Sargassum golden tides have bloomed frequently in many sea areas throughout the world, and negatively impacted on the local marine ecology. Sargassum muticum commonly inhabits rocky shores. It is now distributed worldwide due to its invasiveness, and recently drifting individuals have been observed on the coasts of Canary Islands. However, as a potential golden tide alga, physiological, and ecological studies of this species have not been frequently explored. To investigate the responses of S. muticum to light and nitrogen, two key environmental factors in golden tide formation, we established three light levels (LL, low light, 10 μmol photons m–2 s–1; ML, medium light, 60 μmol photons m–2 s–1, and HL, high light, 300 μmol photons m–2 s–1) and two nitrogen levels (LN, low nitrogen, 25.0 μM of natural seawater; HN, high nitrogen, 125.0 μM), and cultivated the thalli under different conditions for 12 days before measuring the physiological properties of alga. The results showed that higher light and/or nitrogen levels enhanced the relative algal growth rate. The maximum net photosynthesis rate of alga increased with the light, while it remained unaffected by the nitrogen. The HN treatment had no effect on the apparent photosynthetic efficiency of algae in the LL culture, while increased it in the ML and HL cultures. The irradiance saturation point of photosynthesis was approximately 300 μmol photons m–2 s–1 with no significant difference among the six treatments, except for a slight increase under HLHN in contrast to the LLHN and MLLN treatments. HL treatment decreased the maximum quantum yield of photosynthesis (Fv/Fm) in both nitrogen levels. In the HN culture, ML and HL led to lower values of photoinhibition, indicating higher survivability in the alga. The HN culture led to higher nitrogen uptake but had no effects on Fv/Fm and the contents of pigments and soluble protein, regardless of culture light level. Based on these results, we speculate that drifting individuals of S. muticum would be possible to form a golden tide owing to its rapid growth rate at light level of 300 μmol photons m–2 s–1, when they encountered the sustained lower light level on the sea surface (≤300 μmol photons m–2 s–1). A high nitrogen supply caused by eutrophication of seawater might facilitate this process. Our results provide an important reference for the prediction of golden tides formed by S. muticum.

Growth and photosynthetic physiology of holopelagic Sargassum (Phaeophyceae) under laboratory conditions

“…Nevertheless, the responses to light changes found in this study were similar to those observed in other algae, including benthic Sargassum species and the few data published on holopelagic species. For example, in S. muticum and S. henslowianum, increases in P max and respiration rates and a decrease in pigment content have been reported at increasing irradiances (Zou et al 2018;Yan et al 2021). P max and respiration rates obtained in S. fluitans and S. natans I at LL had similar ranges to those of tropical shallow macrophytes, but were much lower than those of the highly productive temperate M. pyrifera (Chisholm 2003;Colombo-Pallotta et al 2006;Meyer et al 2016;V asquez-Elizondo & Enríquez 2016).…”

“…Under LL, S. fluitans and S. natans I had high Chl a content, comparable to that of the subtidal Macrocystis pyrifera (Colombo-Pallotta et al 2006), whereas the Chl c content was much higher in S. fluitans than in S. natans I, increasing absorption and allowing higher α. In contrast, HL reduced the pigment content, probably as a strategy to change the size and number of PSUs and/or the ratio of PSII to PSI to avoid excess light (Kirk 2011;Yan et al 2021). According to these observations, P max and respiration were low at LL and increased under HL in both holopelagic species.…”

“…In general, macroalgal responses to increasing irradiance involve reductions in the photosynthetic unit (PSU) (size and number) and changes in the photosynthetic response to light and growth (Falkowski & Raven 2007;Polo et al 2014;Chen & Zou 2014;Yan et al 2021). Thus, the high photosynthetic efficiency and low metabolism normally observed in low-light-adapted species change in opposite directions when gradually exposed to higher irradiances (Ramus et al 1976a(Ramus et al , 1976bCopertino et al 2006).…”

“…For Sargassum polycystum, a tropical benthic species, growing under an irradiance gradient from 20 to 200 μmol quanta m À2 s À1 , a decrease in its carotenoid and chlorophyll a (Chl a) contents was observed above 100 μmol quanta m À2 s À1 , whereas maximal growth rates occurred below this value (Zou et al 2018). Similarly, in Sargassum muticum growing for 10 days at 300 μmol quanta m À2 s À1 , their Chl a and Chl c contents decreased with increases in P max and dark respiration (Yan et al 2021). Laboratory cultures under 30, 80, and 300 μmol quanta m À2 s À1 of intertidal Sargassum thunbergii showed a reduction in photosynthetic efficiency and saturation irradiance (E k ) at the highest intensity, while maximal growth rates occurred at Table 1.…”

“…2014; Chen & Zou 2014; Yan et al . 2021). Thus, the high photosynthetic efficiency and low metabolism normally observed in low‐light‐adapted species change in opposite directions when gradually exposed to higher irradiances (Ramus et al .…”