Active mesopelagic prokaryotes support high respiration in the subtropical northeast Atlantic Ocean

[EN]Here we provide evidence, based on prokaryote metabolic proxies and direct estimates of oxygen consumption, that the mesopelagic prokaryote assemblage in the subtropical Northeast Atlantic is an active one. It supports a high respiration (0.22 ± 0.05 μmol O2 l−1 d−1, corresponding to 68 ± 8 mmol CO2 m−2 d−1), comparable to that of the epipelagic zone during the same period (64–97 mmol C m−2 d−1). Our findings suggest that mesopelagic prokaryotes in the NE subtropical Ocean, as well as in other eastern boundary regions, are important carbon sinks for organic matter advected from the highly productive coastal systems, and would play a key role in the global carbon cycle of the oceans.

Shedding light on the role of the prokaryotic assemblage in the biogeochemical cycles of the dark ocean

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Pastaje (grazing) del microzooplancton en aguas de las Islas Canarias

[EN]The impact of micrograzers upon primary production was measured north of the Canary Islands during 2010 and 2011 using the dilution technique. Grazing was estimated from chlorophyll a but also taking into account the different phototrophic organisms, that is, Synechococcus (Syn), Prochlorococcus (Pro), autotrophic picoeukaryotes (APE) and autotrophic nanoflagellates (ANF). Some experiments showed significant values of grazing upon Syn, Pro or APE although no significant grazing was measured on chlorophyll a. Furthermore, a positive relationship, this means growth instead of grazing, was observed in Syn and Pro in two experiments. Grazing on heterotrophic prokaryotes was also determined and the obtained values were always highly significant. These results showed that the impact of micrograzers upon primary production is a complex process which involves a different grazing pressure upon phytoplankton groups. Therefore, a detailed analysis of the pico, nano and microplanktonic community is essential to really understand the role of micrograzers and the trophic interactions among these groups in subtropical waters.

Significance of non‐sinking particulate organic carbon and dark CO2 fixation to heterotrophic carbon demand in the mesopelagic northeast Atlantic

[EN] It is generally assumed that sinking particulate organic carbon (POC) constitutes the main source of organic carbon supply to the deep ocean's food webs. However, a major discrepancy between the rates of sinking POC supply (collected with sediment traps) and the prokaryotic organic carbon demand (the total amount of carbon required to sustain the heterotrophic metabolism of the prokaryotes; i.e., production plus respiration, PCD) of deep-water communities has been consistently reported for the dark realm of the global ocean. While the amount of sinking POC flux declines exponentially with depth, the concentration of suspended, buoyant non-sinking POC (nsPOC; obtained with oceanographic bottles) exhibits only small variations with depth in the (sub)tropical Northeast Atlantic. Based on available data for the North Atlantic we show here that the sinking POC flux would contribute only 4–12% of the PCD in the mesopelagic realm (depending on the primary production rate in surface waters). The amount of nsPOC potentially available to heterotrophic prokaryotes in the mesopelagic realm can be partly replenished by dark dissolved inorganic carbon fixation contributing between 12% to 72% to the PCD daily. Taken together, there is evidence that the mesopelagic microheterotrophic biota is more dependent on the nsPOC pool than on the sinking POC supply. Hence, the enigmatic major mismatch between the organic carbon demand of the deep-water heterotrophic microbiota and the POC supply rates might be substantially smaller by including the potentially available nsPOC and its autochthonous production in oceanic carbon cycling models.