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Nitric oxide (NO) has been identified as a key physiological modulator and signaling molecule in animals and plants. However, due to its high reactivity, our knowledge of its production and consumption pathways in the ocean remain limited. Laboratory experiments showed that Ulva prolifera can produce NO, producing as much as 0.44 ± 0.04 nmol h−1 g−1. During the growth period, U. prolifera released NO, but during the decay period NO was absorbed by U. prolifera and bacteria. Furthermore, field investigations examined NO concentrations in the coastal waters of the Yellow Sea off Qingdao, where the U. prolifera green tide occurred in summer 2018. The average concentrations of NO in the surface seawater were 70.2 ± 38.2 pmol L−1 and 18.9 ± 10.3 pmol L−1 in the late- and after-bloom periods, respectively. NO release by U. prolifera was the primary contributor to the high NO concentrations during the late-bloom period. The study area was a net source of NO to the atmosphere during the study period, with average NO sea-air fluxes from the Qingdao coastal waters being 1.5 × 10−12 mol m−2 s−1 and 0.4 × 10−12 mol m−2 s−1 in the late- and after-bloom periods, respectively. This study concluded that the coastal waters of the Yellow Sea off Qingdao contributed more NO to the atmosphere during the bloom of U. prolifera than afterward. Image 1 • The growing U. prolifera could produce NO with the maximum rate of 0.44 nmol h−1 g−1. • The rate of NO release by U. prolifera was lower than the photoproduction rate. • NO concentration during the late-bloom was 2.7 times higher than after the bloom. • The study area acted as a source of NO to the air with a higher flux during a bloom. [ABSTRACT FROM AUTHOR]

• The carbonate system was affected intensely during the Ulva prolifera bloom. • The calcification in the seawater affected by the Ulva prolifera bloom was enhanced. • The release and distribution of organic acids were controlled by the bloom. • Qingdao coastal waters changed from a sink of atmospheric CO 2 to a strong source. As an environmental nuisance, Ulva prolifera green tides have occurred annually in the southern Yellow Sea since 2007. While it is expected that high levels of biological activity during these blooms can alter seawater carbonate chemistry, there has been little research on the responses of marine carbonate system to green tides. Here, the effects of the bloom on the carbonate system were examined on three cruises in June, July, and September, corresponding to the early-, late-, and after-bloom periods of the U. prolifera bloom in Qingdao coastal waters in 2018. Among these three stages, the pH (National Bureau of Standards scale), dissolved inorganic carbon (DIC), total alkalinity (TA), and partial pressure of CO 2 (p CO 2) were all affected by bloom, with the highest pH and lowest DIC and TA concentrations of the surface seawater occurring at the late-bloom stage. While p CO 2 continuously increased from the beginning to the end of the bloom. TA increased by ∼40 μmol kg−1 between the early- and after-bloom periods likely due to the shifts in the carbonate system equilibrium caused by increased CO 3 2− concentrations and the organic matter released by U. prolifera during decomposition. Compared to nearby areas with no U. prolifera bloom, the green tide, along with increasing temperature, reduced the pH and DIC but increased the TA and p CO 2. This large-scale bloom also turned the coastal waters from being an atmospheric CO 2 sink to a strong source, with the estimation of air-sea CO 2 fluxes about 1.69 ± 1.70, 2.28 ± 1.16, and 7.44 ± 5.84 mmol m−2 d−1 during the early-, late-, and after-bloom periods, respectively. This bloom event also promoted the formation of CaCO 3 and was an important source of low molecular weight organic acids. These new findings provide nuances for the current conversations on the role of biological processes in modulating marine carbonate system and the contribution of organic matter to alkalinity. [Display omitted] [ABSTRACT FROM AUTHOR]