Abstract:Green tides, dominated by Ulva prolifera, have occurred each summer in the Yellow Sea of China from 2007 to 2023 and are characterized by a huge biomass, long duration, and extensive influence areas. During the post-bloom period, millions of tons of U. prolifera settle to the sea floor and release carbon, nitrogen, and phosphorus into the surrounding waters, notably impacting coastal environments. Organic matter released from macroalgae are important contributors to biogeochemical cycles in marine ecosystems. Particulate organic carbon (POC) is an important fraction of the marine organic carbon pool and is crucial in the marine carbon cycle by regulating dissolved organic carbon (DOC); sediment organic carbon; and inorganic carbon via deposition, degradation, and mineralization. Additionally, the ratio of POC and particulate organic nitrogen (PON) affects the sea-air CO2 flux and the efficiency of carbon sequestration. Till date, POC and PON released during the degradation of U. prolifera remain poorly quantified and microbial regulations of POC and PON release remain unclear. We investigated the changes in POC and PON concentrations and their molar ratios, and microbial abundance under different degradation densities (1 g/L and 5 g/L) during a 90-d laboratory degradation of U. prolifera. Under dark conditions, 50 g and 250 g (fresh weight) of U. prolifera were added to polyethylene carboys containing 50 L filtered seawater to conduct 1 g/L and 5 g/L degradation experiments. Triplicate replicates were performed for each treatment. Samples for analyzing POC, PON, and microbial abundance were collected on days 0, 4, 6, 8, 14, 21, 28, 60, and 90. The results showed that the degradation period of U. prolifera was divided into the leaching stage (0–14 d), during which soluble materials were lost, and the microbial degradation stage (14–90 d), during which the debris was digested by bacterial or fungal extracellular enzymes. The POC, the maximal values: (90.17±24.77) μmol/L and (219.99±45.11) μmol/L under 1 g/L and 5 g/L, respectively, and PON, the maximal values: (16.15±0.71) μmol/L and (23.20±7.16) μmol/L under 1 g/L and 5 g/L, respectively, concentrations changed significantly during degradation, however, showed different trends. Specifically, the POC and PON concentrations first increased and then decreased during days 0–60; however, POC continued to decrease (approximately 49%) and PON increased (approximately 430%) during days 60–90. The decrease in POC concentrations can be explained by the conversion of POC to DOC by macroalgae-associated microbes and subsequently, DOC was mineralized into dissolved inorganic carbon. The enrichment of nitrogen due to bacterial colonization of particle surfaces may largely explain the increase in PON concentrations. POC:PON first increased and then decreased, indicating that PON showed a lagged release compared to POC when U. prolifera began to degrade, and the subsequent decline of POC:PON can be attributed to nitrogen fixation by microbial and carbon consumption via respiration. Microbial abundance increased during days 0–28, the maximal values: (9.81±3.81)×105 and (26.24±6.98)×105 cells/mL under 1 g/L and 5 g/L, respectively, indicating that the released organic matter was utilized and transformed into microbial biomass. The microbial abundance then decreased during days 28–90. This change may be explained by the decrease in organic matter contents and bioavailability, and the contents of organic matter were deficient for microbial growth, leading to the decrease in microbial abundance. Microbial abundance showed significant correlations with the POC and PON concentrations, indicating the critical roles of microbes in the release of POC and PON during the degradation of U. prolifera. No significant correlations were observed between the microbial abundance and POC:PON. Microbial regulations of POC and PON release during the degradation of U. prolifera are complex and further studies on microbial community structure may help to explore the role of microbes in the release of POC and PON. Degradation density significantly impacted POC and PON concentrations. At the high degradation-density treatment, we observed slow changes in POC and PON concentrations and 2–3 times higher maximal concentrations of the two compared to those at lower degradation-density conditions. We observed that the higher the degradation density, the longer the leaching phase of organic matter. However, POC and PON concentrations did not change proportionally with degradation density. This result may be attributed to the changes in other factors such as pH, dissolved oxygen, and initial nutrient concentrations. The changes in POC and PON concentrations at the end of degradation suggested that more extensive studies are necessary to elucidate the long-term relationship between U. prolifera degradation and microbial communities. Our study provides an important basis for clarifying the changes in POC and PON and their correlations with microbial abundance during the degradation of U. prolifera. This helps to generate a better understanding of the regulation and mechanisms of microbes on U. prolifera degradation.
