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 4-8nM leucine d ^-1, but varied among stations by a factor of 2. Generally, heterotrophic potential varied inversely with nitrate implying NO ₃ ^- draw down by nitrate-reducing chemoorganotrophs. Dark dissolved inorganic carbon assimilation (≈chemoautotrophy) was also highly variable with depth and reached a maximum value of 6.4μMC d ^-1 at one station, but only attained maxima of 0.02 and 0.2μMCd ^-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 30μM NH ₄ ⁺ 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 1.0×10 ⁷ and 10.0×10 ⁷cellsL ^-1, respectively) associated with NH ₄ ⁺ minima and NO ₃ ^- maxima. Nitrifying Β-proteobacteria were most abundant in the upper oxycline (1.1-1.8×10 ⁷cellsL ^-1) at three stations and exhibited a secondary peak of 0.3×10 ⁷cellsL ^-1 in the lower oxycline at one station. Anammox bacterial (Planctomycetes) abundances were as high as 2.1×10 ⁶cellsL ^-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 15pgL ^-1 in the upper oxycline and from 51 to 160pgL ^-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.