Seasonal wind-driven upwelling, high primary production in surface waters, and oxygen deficiency in subsurface waters characterize the coastal ecosystem of the subtropical eastern South Pacific (ESP), and shape the nature and dynamics of the microbial community structure and function. We investigated the diversity, abundance, and transcriptional levels of the gene encoding the large subunit form II of the RuBisCO enzyme (cbbM) in the pelagic microbial community at a continental-shelf site off central Chile over 2 years. We focused on cbbM genes affiliated with the sulfur-oxidizing γ-proteobacteria cluster, whose members are known to dominate in oxygen-deficient marine environments and are highly abundant in the study area. Phylogenetic analysis of cbbM sequences suggests the presence of a novel group of chemolithoautotrophs, closely related to the SUP05/ARCTIC96BD-19 clade. Through (RT)-qPCR, we studied the cbbM gene abundance and transcript dynamics over an annual cycle, finding a significantly higher number of cbbM copies per unit volume in months of active upwelling and at depths in which oxygen was scarce or absent. The same temporal pattern was observed at the transcriptional level. We also analyzed the relative expression of key genes for carbon, nitrogen and sulfur cycling in six metatranscriptomic datasets, for two characteristic periods within the annual cycle: the anoxic upwelling and the suboxic downwelling. Our results indicate that coastal waters of the subtropical ESP contain transcriptionally active populations of carbon fixing pelagic bacteria, whose dynamics is controlled, in large part, by fluctuations in oxygen levels. They also suggest that chemolithoautotrophic processes coupled to the sulfur and nitrogen cycles become increasingly important for the carbon economy of marine coastal waters as oxygen concentrations decline.

ABSTRACT. The Biobío River in central Chile is the third largest watershed and second largest river in Chile in terms of discharge volume. Three sampling campaigns were made in the lower reaches of the river and the adjacent coastal ocean to evaluate the influence of the river plume on the physical/chemical conditions and the abundance/biomass of microphytoplankton during autumn-winter conditions. In addition, a preliminary quantification of riverine nutrients, dissolved silica, and phytoplankton carbon flux to the adjacent ocean was also conducted. High nitrate (NO 3 -) and silicic acid (Si(OH) 4 ) concentration (>18 and 50 µM, respectively) was observed in the lower reaches of the river during all field campaigns. Si(OH) 4 was even high in surface river plume waters. Hydrographic conditions indicate that river plume waters were piled coastward, and they could drove to the south at the Arauco Gulf. In all sampling, highest microphytoplankton biomass (>5000 μgC m -3 ) was associated to the lower reaches of the river and river plume waters as they were moving southward. During autumn-winter conditions a significant flux of phytoplankton carbon and nutrients to the adjacent coastal ocean also played an important role in the high biological productivity of this coastal upwelling area. These preliminary results evidence the need to conduct large-term studies, which should consider the importance of these allochthonous carbon sources in global carbon budgets and coastal food-web models. Keywords: phytoplankton carbon, river plume, nutrient fluxes, silicic acid, coastal food webs, Chile.Caracterización y comparación de la biomasa microfitoplanctónica en el curso inferior del río Biobío y la zona costera adyacente frente a Chile Central durante condiciones de otoño-invierno RESUMEN. El río Biobío, en la zona central de Chile es la tercera cuenca más grande y segundo río más grande de Chile en términos de volumen de descarga de agua dulce. Se realizaron tres campañas de muestreo en el curso inferior del río y la zona costera adyacente para evaluar la influencia de la pluma del río Biobío, en las condiciones físicas/químicas relacionadas con la abundancia/biomasa del microfitoplancton en condiciones de otoño-invierno. Además, se realizó una cuantificación preliminar de los flujos de nutrientes fluviales, sílice disuelta, y el flujo de carbono fitoplanctónico al océano adyacente. Se observó alta concentración de nitrato (NO 3 -) y ácido silícico (Si(OH) 4 ) (>18 y 50 μM, respectivamente) en el curso inferior del río en todas las campañas de terreno. La concentración de Si(OH) 4 fue también alta en las aguas superficiales de la pluma del río. Las condiciones hidrográficas revelaron que las aguas de la pluma del río se apilaron hacia la costa y al sur, en el golfo de Arauco. En todas las muestras, la más alta biomasa microfitoplanctónica (>5000 μgC m -3 ) estuvo asociada a la porción baja del río y las aguas de la pluma que se estaban moviendo hacia el sur. Durante las condiciones de muestreo en otoño-invierno el significat...