Nitrogen and phosphorus recycling mediated by copepods in Western Tropical South Pacific. OUTPACE_ZOO

Zooplankton plays a key role in the regeneration of nitrogen and phosphorus in the ocean through grazing and metabolism. In this study, we explore the role of zooplankton release (excretion and leaching of fecal pellets) of organic and inorganic nitrogen and phosphorus compounds on the microbial community, over an oligotrophic gradient in the central/western south Pacific basin. Three microcosm experiments during the long duration stations (LD A, B and C) of OUTPACE cruise were conducted. For this, a using a mix of epipelagic copepods were feed with water from the depth chlorophyll maximum, and incubated with marine microbial communities (filtered seawater 0.7 µm). During the incubation, subsamples for NH4, + NO3-, NO2-, DON, PO43-, DOP, bacterioplankton abundance and RNA samples were obtained at 0 h, 0.5 h, 1 h, 2h and 4h. The response of microbial community from each LD station to the addition of compounds release by copepods showed significantly higher accumulation of ammonium concentration compared to the controls (without copepods) for all LD stations. The highest ammonium release rates were 0.02 µmol L-1 h-1, 0.15 µmol L-1 h-1 and 0.07 µmol L-1 h-1 which were obtained during the last two hours of incubation (T3-T4) in LD A, B, and C, respectively. The major differences in ammonium concentrations between copepods treatments and control were observed at LD C stations and were associated with a higher increment in DON (at initial time) and nitrate (at the final time) concentration compared to the control, suggesting that the release of DON from copepods can stimulate ammonium regeneration by bacteria and also the increment in nitrate concentration suggest a strong of nitrification occurring, providing substrates for microbial growth in LD C station, which correspond to the ultra-oligotrophic station (LD C). In addition, DOP showed significantly higher concentrations respect to the control in LD B and C and the highest release rates (ca. 0.02 µmol L-1 h-1) were found in the last period of incubation (last two hours) and 0.04 µmol L-1 h-1 found during the first period of incubation (30 min of incubation) in LD B and C, respectively. These results suggest that copepods could substantially contribute with phosphorus compounds for Trichodesmium growth, particularly in bloom conditions such as observed at LD B station. With respect to response in community structure of bacterial, the changes in bacterial composition were characterized by an increase in the contribution of Gammaproteobacteria and a decrease in Alphaproteobacteria in all LD stations. The greatest dissimilarities, based on SIMPER analysis, between copepod treatments and controls in LD A, were mainly due to an increase in Alteromonadales and Vibrionales, and a decrease in Rhodobacterales and SAR11. At LD B station smaller dissimilarities between treatment and control were observed and mainly due to a decrease of Alteromonadales, Caulobacterales and Rhodobacterales, while the contribution of SAR11 increased. In the LD C station, a higher increase in Alteromonadales and Vibrionales and a decrease in relative abundance of Oceanospirillales and SAR11 were observed. SAR 11 was one of the major constituent of rRNA (in situ and microcosm incubation) and rDNA in situ. During our incubations SAR11 increased their contribution only in LD B station which coincided with the lowest concentration of inorganic nitrogen and phosphorus (ambient). Meanwhile Alteromonadales became increasingly important at higher substrate concentration (e.g. LD A and LD C). Our results suggest that copepods can provide nitrogen and phosphorus compounds which microbial communities can directly use in a short period of time. Therefore, the major role of copepods in this oligotrophic system is contributing and retaining the nutrients obtained from feeding in the upper layer, preventing the rapid loss of these compounds. Furthermore, the role of copepods in each LD station could be different depending on the oceanographic context involved, such as the presence of cyanobacterial bloom.