Microbial functioning and community structure variability in the mesopelagic and epipelagic waters of the subtropical northeast atlantic ocean

Abstract

We analyzed the regional distribution of bulk heterotrophic prokaryotic activity (leucine incorporation) and selected single-cell parameters (cell viability and nucleic acid content) as parameters for microbial functioning, as well as bacterial and archaeal community structure in the epipelagic (0 to 200 m) and mesopelagic (200 to 1,000 m) subtropical Northeast Atlantic Ocean. We selectively sampled three contrasting regions covering a wide range of surface productivity and oceanographic properties within the same basin: (i) the eddy field south of the Canary Islands, (ii) the open-ocean NE Atlantic Subtropical Gyre, and (iii) the upwelling filament off Cape Blanc. In the epipelagic waters, a high regional variation in hydrographic parameters and bacterial community structure was detected, accompanied, however, by a low variability in microbial functioning. In contrast, mesopelagic microbial functioning was highly variable between the studied regions despite the homogeneous abiotic conditions found therein. More microbial functioning parameters indicated differences among the three regions within the mesopelagic (i.e., viability of cells, nucleic acid content, cell-specific heterotrophic activity, nanoflagellate abundance, prokaryote-to-nanoflagellate abundance ratio) than within the epipelagic (i.e., bulk activity, nucleic acid content, and nanoflagellate abundance) waters. Our results show that the mesopelagic realm in the Northeast Atlantic is, in terms of microbial activity, more heterogeneous than its epipelagic counterpart, probably linked to mesoscale hydrographical variations.

Fig 1

Positions of the stations occupied during the RODA-II cruises in February 2007. Full circles indicate stations located in the eddy field south of the Canaries, squares indicate stations close to the African upwelling, and diamonds represent stations sampled in the open ocean. Surface temperature was obtained from the Met Office (National Centre for Ocean Forecasting) Web Map Service via the Godiva2 interface, corresponding to February 2007.

Fig 2

(A) Principal component analysis (PCA) of the epipelagic physico-chemical and oceanographic data. The first two principal components (PC1 and PC2) explain most (93%) of the variance in the data set. Pot, potential; Sal, salinity. (B) Nonmetric multidimensional scaling with a hierarchical cluster analysis (Euclidian distances) overlain (ellipses showing Euclidian distance of 3) to indicate the clustering of the sampled stations.

Fig 3

(A) Similarity matrix of the fingerprinting pattern in bacterial community composition as revealed by ARISA for epipelagic. (B) Nonmetric multidimensional scaling from the ARISA pattern for mesopelagic. Both peak presence alone and peak height were used for the statistics, yielding similar results. DCM, deep chlorophyll maximum; DSL, deep scattering layer; OMZ, oxygen minimum zone.

Fig 4

Nonmetric multidimensional scaling from the T-RFLP pattern in archaeal community composition. Both peak presence alone and peak height were used for the statistics, yielding similar results. DCM, deep chlorophyll maximum; DSL, deep scattering layer; OMZ, oxygen minimum zone.

Fig 5

Distribution of prokaryotic abundance (PA, cells ml⁻¹) (A), bulk prokaryotic leucine incorporation rates (LIR, pmol Leu l⁻¹ h⁻¹) (B), and cell-specific prokaryotic heterotrophic production (CsPHP, fmol C cell⁻¹ day [d]⁻¹) (C) in the epipelagic (Epi, 0- to 200-m) and mesopelagic (Meso, 200- to 1,000-m) layers of the three oceanic regions: Ef, eddy field region south of the Canary Islands; Uw, influenced by coastal waters of the NW African upwelling; and Oc, open-ocean waters of the NE Atlantic Subtropical Gyre. Letters denote significant differences between regions for the epipelagic and mesopelagic waters (one-way ANOVA mean comparison [alpha = 0.05]).

Fig 6

Distribution of the percentage of high-nucleic-acid-content cells (% HNA) (A) and the percentage of NADS-determined “live” cells (% NADS⁺) (B) in the epipelagic (Epi, 0- to 200-m) and mesopelagic (Meso, 200- to 1,000-m) layers of the three oceanic regions: Ef, eddy field region south of the Canary Islands; Uw, influenced by coastal waters of the NW African upwelling; and Oc, open-ocean waters of the NE Atlantic Subtropical Gyre. Letters denote significant differences between regions for the epipelagic and mesopelagic waters (one-way ANOVA mean comparison [alpha = 0.05]).

Fig 7

Distribution of the abundance of heterotrophic nanoflagellates (HNF, cells ml⁻¹) (A) and the prokaryote:heterotrophic nanoflagellate abundance ratio (PA : HNF) (B) in the deep chlorophyll maximum (DCM) and the deep scattering layer (DSL) of the three oceanic regions: Ef, eddy field region south of the Canary Islands; Uw, influenced by coastal waters of the NW African upwelling; and Oc, open-ocean waters of the NE Atlantic Subtropical Gyre. Letters denote significant differences between regions for the epipelagic and mesopelagic waters (one-way ANOVA mean comparison [alpha = 0.05]).