Thaumarchaeota are implicated as the major ammonia oxidizers in the ocean. However, the influence of various abiotic factors in determining their distribution and activity in the upper ocean remain largely unclear. Here, we examined the influence of light, hydrogen peroxide (H2O2), and temperature on ammonia oxidation rates for communities dominated by Thaumarchaeota at the nitrite maximum across two North Pacific transects. In situ ammonia oxidation was almost exclusively driven by Thaumarchaeota, as inferred from ammonia monooxygenase subunit A (amoA) genes, amoA transcripts, and inhibitor studies. A major shift in population structure near the eastern North Pacific Subtropical Front was revealed by sequence variation of amoA genes, showing different Thaumarchaeota community structure in oligotrophic gyre and temperate regions. While the most dominant OTUs were closely related, we found significant differences in physiological responses to light and temperature of incubation. At four stations in different biogeochemical regimes, the impact of sunlight intensity and temperature on activity was evaluated using 15 NH4+‐spiked whole seawater collected from the nitrite maximum and incubated at different depths on a free floating in situ array. Ammonia oxidation was usually completely inhibited by PAR at the surface and 21–45% inhibited at 1% surface PAR, whereas a temperature effect on ammonia oxidation was observed at only two of four stations. While inhibition due to H2O2 cannot be ruled out in surface waters, our findings show that below the mixed layer, photoinhibition, and not H2O2 toxicity, had a greater influence on ammonia oxidation.

Approximately 30% of the ocean's surface is subject to phytoplankton iron (Fe) limitation, especially in the Equatorial Pacific and Southern Oceans where upwelling provides a large flux of nitrate (NO 3 − ) and other nutrients (Moore et al., 2001(Moore et al., , 2013. Elsewhere, stratification of the upper ocean leads to depletion of NO 3 − , ammonia, and other bioavailable forms of nitrogen. In stratified oligotrophic gyres, shallow mixed layers also act to concentrate Fe deposited at the ocean's surface by atmospheric sources (Boyle et al., 2005;Sedwick et al., 2005). The large flux of Fe relative to NO 3 − in these ecosystems results in nitrogen limitation of photosynthesis and selects for phytoplankton like the cyanobacterium Prochlorococcus (Ward et al., 2013;Wu et al., 2000), whose small size allows them to outcompete other phytoplankton for recycled nitrogen species found at nanomolar concentrations (Morel et al., 1991).However, the same stratification that leads to Fe-rich conditions in the surface ocean can also impede Fe supply to the subsurface. Shallow mixed layers ensure that Fe derived from dust deposition does not reach the entirety of the euphotic zone, which can extend below 100 m in subtropical gyres. Stratification also limits the supply of regenerated Fe from below the euphotic zone. Indeed, a common feature of dFe profiles within subtropical gyres is a concentration minimum between 75 and 150 m (Bruland et al., 1994;Fitzsimmons et al., 2015;Sedwick et al., 2005). This subsurface dFe minimum often coincides with the deep chlorophyll maximum (DCM), a unique habitat where low irradiance drives phytoplankton photo-acclimation, increasing chlorophyll per cell to improve photosynthetic light capture (Letelier et al., 2004). Theoretical arguments suggest the increases in chlorophyll per cell should be matched by an equivalent increase in the number of Fe-bearing photosynthetic reaction

Sinking particulate organic matter controls the flux of carbon (C) from the surface ocean to the deep sea. Microorganisms actively colonize particles, but the extent to which microbial metabolism influences particle export remains uncertain. We conducted experiments to quantify rates of bacterial production (derived based on 3 H-leucine incorporation) and dark C-fixation (based on 14 C-bicarbonate assimilation) associated with sinking particles collected from the base of the euphotic zone (175 m) in the subtropical North Pacific Ocean. Seawater was amended with sinking particles and rates of filter size-fractionated (0.2, 2, and 20 μm) bacterial production and dark C-fixation were measured. Sequencing of 16S ribosomal RNA (rRNA) gene amplicons revealed that microorganisms in the particle-amended treatments differed from those in the unamended seawater controls, with the particle treatments enriched in putative copiotrophic bacteria. The addition of sinking particles increased rates of bacterial production (by 6-to 9-fold), and to a lesser extent dark C-fixation (by 1.7-to 4.6-fold), relative to unamended controls, with most of the production associated with filter pore sizes < 20 μm. Normalizing production to concentrations of particulate C yielded rates that were statistically indistinguishable between particle-amended treatments and unamended controls. We then examined possible relationships between sinking particulate C flux attenuation and its supply to the mesopelagic waters, revealing that flux attenuation was positively related to increases in particulate C supply. Together with results from our experiments, we suggest processes that contribute to sinking particle disaggregation both increase flux attenuation and favor microbial mineralization of particle-derived organic matter.

No abstract

Abstract. We analyzed net-tow samples of natural assemblages of plankton, and associated particulate matter, from the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. Size-fractionated suspended particles were collected using net tows with mesh sizes of 50 and 200 µm to examine the composition of small- and large-size plankton populations. Samples were collected in two different years (11 offshore sites in October 2012 and 6 nearshore sites in April 2014) to examine temporal and spatial variabilities. We calculated the excess metal concentrations by correcting the bulk composition for inputs from atmospheric dust using aluminum (Al) as a lithogenic tracer and the metal∕Al ratios for average Qatari dust. Atmospheric dust in Qatar is depleted in Al and enriched in calcium (Ca), in the form of calcium carbonate (CaCO3), relative to the global average Upper Continental Crust (UCC). To evaluate the fate of this carbonate fraction when dust particles enter seawater, we leached a subset of dust samples using an acetic acid–hydroxylamine hydrochloride (HAc–HyHCl) procedure that should solubilize CaCO3 minerals and associated elements. As expected, we found that Ca was removed in Qatari dust; however, the concentrations (ppm) for most elements actually increased after leaching because the reduction in sample mass resulting from the removal of CaCO3 by the leach was more important than the loss of metals solubilized by the leach. Because surface seawater is supersaturated with respect to CaCO3 and acid-soluble Ca is abundant in the particulate matter, we only used unleached dust for the lithogenic correction. Statistical analysis showed that for many elements the excess concentrations were indistinguishable from zero. This suggested that the concentrations of these elements in net-tow plankton samples were mostly of lithogenic (dust) origin. These elements include Al, Fe, Cr, Co, Mn, Ni, Pb, and Li. For several other elements (Cd, Cu, Mo, Zn, and Ca) the excess concentrations present after lithogenic correction are most likely of biogenic/anthropogenic origin. The excess concentrations, relative to average dust, for most elements (except Cd) decreased with distance from the shore, which may be due to differences in biology, currents, proximity to the coast, or interannual processes.