Material Transfer And Nutrient Regeneration Of The Marine Ciliate Euplotes Vannus Under Imbalanced Nitrogen And Phosphorus Condition

Bacteria and protozoa are the most important microbes in marine microbial food web.Protozoa play important roles in transfering energy and resources in the marine food web by incorporating biomass of bacteria into their own biomass through feeding and constituting the food basis of higher trophic levels.Besides,protozoa act as nutrient regenerators.They produce various types of organic and inorganic nutrients,which in turn act as important resources for phytoplankton and heterotrophic organisms.The fluctuation of nutrients composition?nitrogen?N?and phosphorus?P??in ambient sea water consequently changes C:N:P stoichiometry of primary produces and bacteria and consequently their quality as food.Ultimately,this effect will propagate along the associated food chain.Facing the imbalance in the nutrient composition,stoichiometric regulation and the alteration of nutrient budgets in protozoa,which is the connection link in the food chain,will affect functions in marine ecosystem.In this study,a common marine ciliate Euplotes vannus and bacteria Pseudomonas putida were chosen to set up experiments to understand the link of prey-consumer.Bacteria with different elemental compositions were obtained by growing bacteria in different N and P conditions:sufficient nutrients?+NP?,low phosphorus?-P?and low nitrogen?-N?.The aim of this study is to expolore the response of physiological characteristics and C,N,P budget of E.vannus on different diets.Grown under contrasting P conditions,P.putida increased its C:P ratio with the increase of C:P ratio of the culture media,indicating a weak stoichiometric homeostasis.The volume-corrected growth and ingestion rate(28.01±1.14 ng C ciliate^-1h^-1,1.47±0.06 d^-1)of E.vannus feeding+NP bacteria were significantly higher than those feeding-P?1/20?bacteria(1.12±0.22 d^-1,8.00±4.20 ng C ciliate^-11 h^-1).Euplotes in the+NP treatment had significantly higher P excretion rate and regeneration rate(3.42±0.35×10^-4?M P ciliate^-1h^-1,27.69±1.66%)than those in the-P?1/20?treatment(1.38±0.06×10^-4?M Pciliate^-1h^-1,15.40±1.17%),suggesting that the fluctuation of phosphorus in resources affects the physiological process of E.vannus.In both experimental groups,more than 90%of consumed carbon was released into envoriment in forms of respiration?CO₂?and excretion?DOC?,and the egestion?particulate organic carbon,POC?and minority were utilized for growth.There was no significant difference in contribution of CO₂?percentage of total ingested carbon?in two experiments,but the contribution of DOC was markedly different.The contribution of DOC in+NP treatment was seven times higher than that of in-P?1/20?treatment.Obviously,most of carbon consumed by the E.vannus returned to the environment by excretion and egestion,supporting activities of other autotrophic and heterotrophic organisms.In addition,the C:P ratio of released matters?+NP,288.00±58.24;-P?1/20?,293.55±20.14?was higher than that of ingested food?+NP,97.64±1.05;-P?1/20?,131.44±16.55?in both treatments.And the C:P ratio of released in the-P?1/20?treatment was slightly higher than that in+NP treament,but there was no significant difference.Our results showed that E.vannus had ability to regulate the stoichiometry imbalance by reducing the excretion of phosphorus and changing the excretion of carbon.Grown under contrasting N conditions,the C:N ratio of P.putida didn't change largely under different nitrogen conditions,revealing homeostasis in terms of N content.The range of ingestion rate of E.vannus was 25.67±10.04 ng C ciliate^-11 h^-132.23±1.22 ng C ciliate^-11 h^-1,there was no significant difference in+NP and-N treaments.While the volume-corrected growth rate in+NP treatment was slightly higher than that in-N treatment.Total nitrogen excretion rate of Euplotes in the+NP treatment(1.52×10^-5?mol N ciliate^-1h^-1)was similar to-N treament(1.09×10^-5?mol N ciliate^-1h^-11.53×10^-5?mol N ciliate^-1h^-1).The regeneration efficiency of ammonium,nitrate and urea in+NP treatment was 23.31±2.05%,3.58±0.12%,6.30±1.00%particularly.All of these values were significantly higher than those in-N treatment.Ammonium was main form of nitrogen excretion in all euplotes,urea and nitrate were minor forms.Similar to P,more than 98%of consumed carbon was released into environment in forms of feces,CO₂ and DOC in both treatments,and minority were utilized for growth.Among them,contributions of CO₂ and defecation in+NP treatment differed significantly from those in-N treatments,but there was no significant difference of contribution of DOC?⁰.5%?.When E.vannus feeding-N bacteria,respiration was enhanced,but the excretion of defecation decreased,which may be caused by the change of biochemical composition of prey.In addition,the C:N ratio of released matters?49.37±3.16⁷9.92±13.01?was significantly higher than that of ingested food in both treatments?4.77±0.21⁵.47±0.45?.And the C:N ratio of released in the-N?1/20?treatmant was significantly higher than that in+NP treatment.Our results indicated that E.vannus showed remarkable regulation for nitrogen and had ability to regulate both C and N by reducing nitrogen excretion and adjusting different forms carbon excretion facing low N bacteria in order to balance the dismatch between resource and themselves.