The Effects of Temperature, Light Intensity, and Photoperiod on the Organic Carbon Release Rate of Sargassum horneri Seaweed

Abstract

The ocean is the largest carbon pool on earth and plays an important role in the global carbon cycle. Marine organisms contribute over half of the global carbon fixed by photosynthesis every year. Therefore, much attention has been paid to the role of the ocean in the carbon cycle and carbon sequestration. Macroalgae are an important component of coastal ecosystems, with strong and highly efficient carbon fixation capacity. The net primary productivity (NPP) of macroalgae was higher than that of phytoplankton and other primary producers in coastal waters. Parts of the photosynthates are released into the sea in the form of dissolved organic carbon (DOC) and particulate organic carbon (POC) during algae growth. Dissolved organic carbon and POC have attracted widespread attention because they can play important roles in carbon cycling and carbon sequestration in coastal ecosystems. However, the organic carbon release rate of macroalgae, its relationship with primary productivity, and the regulatory factors require further study. The growth of algae and the organic carbon release are influenced by temperature, light intensity, photoperiod, and other factors. However, organic carbon release rates among algae species might be quite different. Therefore, studying the regulatory effects of different factors on organic carbon release by algae and organic carbon release rate distribution has important implications for quantitatively evaluating the organic carbon release ability of algae and its contribution to the coastal ecosystem carbon cycle.In recent years, Sargassum horneri is the most common wild macroalgae in the Yellow Sea. Its ecological role in the coastal ecosystem has attracted widespread attention. Few studies have focused on organic carbon release by S. horneri. This study conducted an orthogonal experiment to measure DOC and POC release rates of S. horneri at different temperatures (5, 15, and 25 ℃), light intensity [86, 172, and 258 μmol/(m2·s)] and photoperiod (L: D=6 h: 18 h, L: D=12 h: 12 h, L: D=24 h: 0 h, L means light time, D means dark time) to study the organic carbon release rate of S. horneri, its relationship with NPP, and the main regulatory factors. The release rates of DOC were 0.653–4.785 mg/(g·h) and the release rates of POC were 0.066–0.322 mg/(g·h). There is an order of magnitude difference between DOC and POC. This indicated that DOC was the main form of organic carbon released by S. horneri. The highest DOC release rate [4.785 mg/(g·h)] occurred at 25 ℃, 172 μmol/(m2·s) light intensity, and L: D = 6 h: 18 h. The highest POC release rate [0.322 mg/(g·h)] occurred at 25 ℃, 258 μmol/(m2·s) light intensity, and L: D = 24 h: 0 h. The organic carbon release ability of S. horneri is consistent with previous studies of other macroalgae.This study found that changes in temperature and light conditions significantly affect the ability of S. horneri to release organic carbon. Temperature was the main factor affecting the release rate of DOC; the DOC release rate increases with increasing temperature. The DOC release rate initially increases, then decreases with increasing light intensity; it increases with a decrease in the photoperiod. Light intensity was the main factor influencing the POC release rate; increasing light intensity and photoperiod accelerated the POC release rate. However, the POC release rate slightly decreased when the light intensity increased from 172 to 258 μmol/(m2·s). There was a greater POC release rate under extreme temperature stress (5 and 25 ℃).This study showed that a proportion of DOC and POC in NPP were unstable in macroalgae. The range of DOC/NPP was 4%–130%. POC/NPP was relatively concentrated to approximately 0.4%–5.9%. The NPP was significantly lower in the experimental group with a high ratio of organic carbon compared with other experimental groups. The DOC release rate negatively correlated with NPP, but the negative correlation was poor in the low temperature groups. There was no significant correlation between the POC release rate and NPP. POC/NPP was relatively stable [approximately (1±0.6)% when NPP>9 mg C/(g·h)]. The growth pressure of S. horneri was high when NPP<4 mg C/(g·h), and the regulatory effect of organic carbon was manifested as the release under environmental stress, with POC/NPP increasing to 4%–6%. This section was modified for clarity. Please check that your meaning was retained. This study provides scientific and technological support for an in-depth understanding of the contribution by macroalgae to the coastal ecosystem carbon cycle.

Keywords

• sargassum horneri
• net primary productivity
• dissolved organic carbon
• particulate organic carbon