| Nitrogen is an essential element of metabolism in living organisms.In most of the global oceans,nitrogen is the limiting factor controlling the primary productivity and carbon exportation,and it regulates the atmosphere CO2 concentrations through the biological pumps.Under the intermediated of nitrifying microorganisms,ammonium is oxidized to nitrite,which is the substrate for denitrification and anammox.Therefore,ammonia oxidation process plays a central role in the nitrogen cycle,regulating the source and sink of nitrogen flow.In addition,the ammonia oxidation process produces greenhouse gas,nitrous oxide,as a byproduct.Therefore,studying the response of ammonia oxidation to temperature changes in the context of global warming is critical for modelling the future climate change.In this study,we used the 15N-labeling technique to investigate the temperature sensitivity of ammonia oxidation rate under different nutrient levels in the Jiulong River(estuary),Wuyuan Bay(coastal)and South China Sea(sea basin).The main conclusions are as follows:In the eutrophic Jiulong River estuary,we investigated the temperature effect of ammonia oxidation rate in summer and spring seasons.The results showed that the response of ammonia oxidation rate to temperature in different seasons followed the Arrhenius curve.That is,the rate increased synchronously with the temperature below the optimum temperature,and decreased with increasing temperature when the incubation temperature was exceeded optimum.At the same time,it was also found that the ammonia oxidation rate and the optimum temperature at the high salinity station(low ammonium concentration)were both lower than that in the low salinity stations(high ammonium concentration).Meanwhile,we synchronous determined the bulk ammonia oxidation rate(AORB)at different particle concenltrations and the free-living ammonia oxidation rate(AORF)after removal of particles>3 μm,and obtained the particle-associate rate(AORp)by differential subtraction of AORF from AORB.Results revealed that the optimum temperature of AORp is higher～5°C)than that of AORf.Which together implies that the substrate concentration or turbidity influences the temperature sensitivity of the ammonia oxidation rate.In addition,the addition of ammonium oxidizing bacteria(AOB)inhibitors was used to investigate the response of AOB and ammonium oxidizing archaea(AOA)to temperature.For the first time in field experiments,we found the thermal response of AOA and AOB were similar.The AORB and AORF was measured at different particle concentrations in autumn,winter,and summer in a eutrophic Wuyuan Bay(coastal).The highest AORB occurred in autumn,when particle concentrations were highest.The results show that AORp is systematically greater than AORF,and AORp increases with the particle concentration.Regardless of seasonally distinctive temperature and particle concentrations,ammonia oxidation exhibited consistent temperature dependence in all cases(including bulk,particle-associated,and free-living)with a Q10 value of～2.2.However,the community structure and abundance were distinctive among seasons,thus,our findings illustrated that AOA and AOB may have similar responses to temperature change.This was consistent with the finding in Jiulong River.Meanwhile,the optimum temperature for AORp was also～5 °C higher than that for AORF in Wuyuan Bay.By correlating the in-situ ammonia oxidation rates of multiple sites with environmental factors(pH,salinity,turbidity,temperature),we found that particle concentration and temperature were key factors that controlling the ammonia oxidation rate.Furrthermore,based on the robust relation between turbidity,temperature and ammonia oxidation rate,we established a simple model to predict the ammonia oxidation rate in Wuyuan Bay by monitoring the suspended particulate concentration and temperature.A series temperature manipulation experiments was conducted in the South China Sea.We found the ammonia oxidation rate was positively correlated with the temperature when substrate(ammonium)was high,while the rate was inversely related to the temperature when the substrate was low.After adding substrate,the rate in low substrate station showed positive temperature dependency.A batch of Michaelis-Menten kinetics experiments were conducted for whole water community.We found both maximum rate(Vmax)and half-saturation constant(Km)were temperature dependent;however,the temperature sensitivity(Q10)of Vmax(Q10-Vmax)was higher than that of Km(Q10-km).Through Michaelis-Menten kinetics experiments and model calculation,we found the optimum temperature increased with substrate concentration when Q10-Vmax<Q10-Km.Therefore,the reversed temperature effect observed at low substrate concentration stations is probably due to the optimum temperature being lower than the minimum temperature in temperature manipulation experiment.Based on the experimental data of the estuaries and offshore,we also found that the optimum temperature was a saturating function of substrate concentration.That is,at the low substrate concentration range,the optimum temperature was increased with substrate concentration,while when the substrate threshold is reached,the optimum temperature no longer increases with the substrate.In summary,ammonia oxidation rates showed different temperature sensentivity under changed trophic level.From the estuary to the sea basin,the optimum temperature of ammonia oxidation rate decreases as the ammonia oxidation substrate concentration decreased.Our results implied that global warming may have dissimilar effects on ammonia oxidation in different seas with diverse trophic level.In the eutrophic zone,warming may promote the ammonia oxidation due to the higher optimum temperature.However,in oligotrophic oceans with low nutrient,warming may inhibit the ammonia oxidation.Future climate models should be able to distinguish the effects between oligotrophic and eutrophic zones when simulating the nitrogen cycle response to warming. |
