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Oxygen minimum zones (OMZs) are oceanographic features that affect ocean productivity and biodiversity, and contribute to ocean nitrogen loss and greenhouse gas emissions. Here we describe the viral communities associated with the Eastern Tropical South Pacific (ETSP) OMZ off Iquique, Chile for the first time through abundance estimates and viral metagenomic analysis. The viral-to-microbial ratio (VMR) in the ETSP OMZ fluctuated in the oxycline and declined in the anoxic core to below one on several occasions. The number of viral genotypes (unique genomes as defined by sequence assembly) ranged from 2040 at the surface to 98 in the oxycline, which is the lowest viral diversity recorded to date in the ocean. Within the ETSP OMZ viromes, only 4.95% of genotypes were shared between surface and anoxic core viromes using reciprocal BLASTn sequence comparison. ETSP virome comparison with surface marine viromes (Sargasso Sea, Gulf of Mexico, Kingman Reef, Chesapeake Bay) revealed a dissimilarity of ETSP OMZ viruses to those from other oceanic regions. From the 1.4 million non-redundant DNA sequences sampled within the altered oxygen conditions of the ETSP OMZ, more than 97.8% were novel. Of the average 3.2% of sequences that showed similarity to the SEED non-redundant database, phage sequences dominated the surface viromes, eukaryotic virus sequences dominated the oxycline viromes, and phage sequences dominated the anoxic core viromes. The viral community of the ETSP OMZ was characterized by fluctuations in abundance, taxa and diversity across the oxygen gradient. The ecological significance of these changes was difficult to predict; however, it appears that the reduction in oxygen coincides with an increased shedding of eukaryotic viruses in the oxycline, and a shift to unique viral genotypes in the anoxic core.

The distribution of picoplankton ( 75% total picoplanktonic abundance). As expected, Prochlorococcus was the most numerically abundant phytoplankter under very oligotrophic (chlorophyll a concentration ≤0.1 mg m -3 ) and mesotrophic (>0.1 and ≤1 mg m -3 ) conditions. However, in contrast to other subtropical regions, picophytoeukaryotes appear to dominate the

Variability of several bio-optical and photosynthetic properties of phytoplankton in the highly productive coast off Concepcion, central Chile (∼36°S) was quantified during contrasting seasons at offshore, shelf, and nearshore sites. During October 1998 (upwelling) and July 1999 (non-upwelling), 21 stations were sampled for chlorophyll- a , pigment spectral light absorption, primary production, and attenuation coefficients ( kλ ) of downwelling irradiance ( E d ( λ )); kλ was used to estimate the depth of the euphotic zone ( Z eu ). Primary production was measured through 14 C-fixation experiments using simulated in situ and in situ incubations with water from six depths within the Z eu . The photosynthetic parameters ( α B , P B max ) were obtained through photosynthesis-irradiance experiments with samples from two Z eu depths. Bio-optical properties showed large spatial variability during both seasons, including spectral in vivo phytoplankton absorption ( a ph ( λ )), pigment specific phytoplankton absorption ( a *( λ )) within the upper 25 m, and water-column transmittance ( T ( λ )), calculated for the upper 10 m using E d ( λ ) at four wavelengths. Under non-upwelling conditions, a *( λ ), T ( λ ), and α B presented higher values whereas a ph ( λ ) values decreased accordingly. Under upwelling conditions, primary productivity ranged from 0.7 to 7.5 g C m −2 d −1 at the coastal stations and was −2 d −1 at the oceanic stations; in the non-upwelling season, values over the entire area ranged from 0.2 to 1.9 g C m −2 d −1 . The significant correlations found among bio-optical properties and physiological parameters highlight both the short- and long-term changes in the area's environmental conditions. The combination of three independent biological parameters— α B , P B max , and a ph ( 442 )—accounted for most of the horizontal and vertical variability among and within stations for each cruise. These results support a distinction of different functional zones for the two contrasting upwelling conditions.

Oxygen minimum zones (OMZs) are oceanographic features that affect ocean productivity and biodiversity, and contribute to ocean nitrogen loss and greenhouse gas emissions. Here we describe the viral communities associated with the Eastern Tropical South Pacific (ETSP) OMZ off Iquique, Chile for the first time through abundance estimates and viral metagenomic analysis. The viral-to-microbial ratio (VMR) in the ETSP OMZ fluctuated in the oxycline and declined in the anoxic core to below one on several occasions. The number of viral genotypes (unique genomes as defined by sequence assembly) ranged from 2040 at the surface to 98 in the oxycline, which is the lowest viral diversity recorded to date in the ocean. Within the ETSP OMZ viromes, only 4.95% of genotypes were shared between surface and anoxic core viromes using reciprocal BLASTn sequence comparison. ETSP virome comparison with surface marine viromes (Sargasso Sea, Gulf of Mexico, Kingman Reef, Chesapeake Bay) revealed a dissimilarity of ETSP OMZ viruses to those from other oceanic regions. From the 1.4 million non-redundant DNA sequences sampled within the altered oxygen conditions of the ETSP OMZ, more than 97.8% were novel. Of the average 3.2% of sequences that showed similarity to the SEED non-redundant database, phage sequences dominated the surface viromes, eukaryotic virus sequences dominated the oxycline viromes, and phage sequences dominated the anoxic core viromes. The viral community of the ETSP OMZ was characterized by fluctuations in abundance, taxa and diversity across the oxygen gradient. The ecological significance of these changes was difficult to predict; however, it appears that the reduction in oxygen coincides with an increased shedding of eukaryotic viruses in the oxycline, and a shift to unique viral genotypes in the anoxic core.

The role of potential factors limiting bacterial growth was investigated along vertical and longitudinal gradients across the South Eastern Pacific Gyre. The effects of glucose, nitrate, ammonium and phosphate additions on heterotrophic bacterial production (using leucine technique) were studied in parallel in unfiltered seawater samples incubated under natural daily irradiance. The enrichments realized on the subsurface showed three types of responses. From 141° W (Marquesas plateau) to approx 125° W, bacteria were not bottom-up controlled, as confirmed by the huge potential of growth in non-enriched seawater (median of enhancement factor×39 in 24 h). Within the Gyre (125° W–95° W), nitrogen alone stimulated leucine incorporation rates (median×4.2), but rapidly labile carbon (glucose) became a second limiting factor (median×37) when the two elements were added. Finally from the border of the gyre to the Chilean upwelling (95° W–73° W), labile carbon was the only factor stimulating heterotrophic bacterial production. Interaction between phytoplankton and heterotrophic bacterial communities and the direct versus indirect effect of iron and macronutrients on bacterial production were also investigated in four selected sites: two sites on the vicinity of the Marquesas plateau, the centre of the gyre and the Eastern border of the gyre. Both phytoplankton and heterotrophic bacteria were limited by availability of nitrogen within the gyre, but not by iron. Iron limited phytoplankton at Marquesas plateau and at the eastern border of the gyre. However 48 h enrichment experiments were not sufficient to show any clear limitation of heterotrophic bacteria within Marquesas plateau and showed a limitation of these organisms by labile carbon in the eastern border of the Gyre.

Prochlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration, derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which was not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particle beam attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both total chlorophyll a (i.e. mono + divinyl chlorophyll a) concentration and cp. Finally, picophytoeukaryotes contributed ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by size measurements (i.e. equivalent spherical diameter) on cells sorted by flow cytometry and optical theory. Although there are some uncertainties associated with these estimates, the new approach used in this work further supports the idea that picophytoeukaryotes play a dominant role in carbon cycling in the upper open ocean, even under hyper-oligotrophic conditions.

La composicion de pigmentos y las caracteristicas de absorcion fueron analizadas en muestras de fitoplancton recolectadas en dos cruceros frente a las costas Chile. Se encontraron altas concentraciones de pigmentos (hasta 20 mg chl-a m-3) en aguas de surgencia sobre el quiebre de la plataforma frente a Concepcion durante la primavera, octubre de 1998; sin embargo, se encontraron condiciones relativamente oligotroficas costa afuera. De manera similar, estaciones mas al norte (entre Coquimbo e Iquique), muestreadas durante el verano austral (febrero de 1999), tambien mostraron bajas concentraciones de pigmentos, caracterizados por la presencia de primnesioficeas y cianobacterias (Synechococccus sp. y Prochlorococcus sp.). El coeficiente de absorcion especifico del fitoplancton a 443 nm (a*ph(443)) fue mucho mayor para las poblaciones costa afuera que para las poblaciones costeras, las cuales estaban dominadas por diatomeas. Esta diferencia fue atribuida a cambios en el empaquetamiento y composicion de pigmentos. La proporcion relativa entre carotenoides no-fotosinteticos y chl-a, junto con la razon de la altura del pico de las bandas Gausianas en las regiones azul y roja del espectro p(435)/p(676) (un indice del efecto de empaquetamiento), representa hasta el 92 % de la variacion total en a*ph(443). Las razones de absorcion azul/verde estuvieron relacionadas fuertemente a la concentracion relativa de 19'-hexanoiloxifucoxantina y fucoxantina. Se encontro una relacion aceptable entre las estimaciones de chl-a (datos de SeaWiFS) medidas in situ y las obtenidas con el satelite, a pesar de la gran variabilidad en los coeficientes de absorcion especifica del fitoplancton, sugiriendo que las relaciones de absorcion de clorofila "global" abarcan las variaciones regionales observadas a la altura de las costas de Chile. Las mediciones de chl-a satelitales fueron sobreestimadas en aguas oligotroficas cuando se compararon con determinaciones HPLC de chl- a, aparentemente por los coeficientes especificos de absorcion altos del fitoplancton en mar abierto. Ademas, los datos obtenidos por barco y de satelite estuvieron estrechamente relacionados cuando se utilizaron datos de chl- a in situ y fluorometricos. Probablemente la correlacion entre la chl- a in situ y satelital podria ser mejorada utilizando algoritmos regionales