Microbial community structure and productivity in the oxygen minimum zone of the eastern tropical North Pacific

Microbial abundances and activities (heterotrophic and chemoautotrophic) were profiled in the fall of 2007 along a transect of four stations in the upwelling region between the Tehuantepec Bowl, off the coast of southern Mexico, and the Costa Rica Dome. Heterotrophic potential varied with depth exhibiting relatively low maximum values of 4–8 nM leucine d−1, but varied among stations by a factor of 2. Generally, heterotrophic potential varied inversely with nitrate implying NO3− draw down by nitrate-reducing chemoorganotrophs. Dark dissolved inorganic carbon assimilation (≈chemoautotrophy) was also highly variable with depth and reached a maximum value of 6.4 μM C d−1 at one station, but only attained maxima of 0.02 and 0.2 μM C d−1 at other stations. Elevated values of chemoautotrophy corresponded with enrichments in nitrite concentrations suggesting nitrification as well as with depletions in nitrite and ammonium concentrations suggesting anammox processes. Additions of 30 μM NH4+ at selected depths stimulated chemoautotrophic activity up to 33-fold. Fluorescence in situ hybridization (FISH) analyses revealed elevated abundances of Crenarchaeota and β-proteobacteria (up to 1.0×107 and 10.0×107 cells L−1, respectively) associated with NH4+ minima and NO3- maxima. Nitrifying β-proteobacteria were most abundant in the upper oxycline (1.1–1.8×107 cells L−1) at three stations and exhibited a secondary peak of 0.3×107 cells L−1 in the lower oxycline at one station. Anammox bacterial (Planctomycetes) abundances were as high as 2.1×106 cells L−1, but accounted for less than 1% of total DAPI counts. The concentration of diploptene (a hopanoid biomarker found in bacteria) varied from 2 to 15 pg L−1 in the upper oxycline and from 51 to 160 pg L−1 in the lower oxycline. Delta 13C values of particulate organic carbon (POC) varied from −30 to −21‰ in the water column (avg. −25‰). Distributions of isotopically light POC coincided with enrichments in diploptene, Planctomycetes cells, nitrite, and chemoautotrophy as well as with nitrate and ammonium depletions, suggesting that anammox and nitrification are important for secondary biological production in this system.