Abstract
The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a, particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in these gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a, POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions.
Highlights
Heterotrophic prokaryotes are major players in decomposing and cycling of organic matter fixed by phytoplankton primary production in the global oceans (Azam and Malfatti, 2007)
We addressed the following questions: (i) how is prokaryotic abundance, heterotrophic prokaryotic production (HPP) and bulk growth related to phytoplankton and particulate organic matter, the major substrate sources? (ii) how is HPP related to the hydrolysis of major dissolved biopolymers and the turnover of monomeric substrate classes, dissolved free amino acids (DFAA), dissolved free monosaccharides and acetate? (iii) how do the patterns of these variables differ along the transect and vertically in the upper 200 m? (iv) how does the composition of the bacterial communities vary along the transect and reflect the latitudinal patterns of the microbial organic matter cycling and the biogeochemical features? (v) do the microbial and biogeochemical properties assessed reflect the different biogeographic provinces?
Our study further shows that at 20 m depth, numbers of Bacteroidetes were closely correlated to chlorophyll a (Chl a), particulate organic carbon (POC), PON, nitrate and leucine aminopeptidase (Leu-AMP) and numbers of the SAR11 clade and the Roseobacter group to POC and PON
Summary
Heterotrophic prokaryotes are major players in decomposing and cycling of organic matter fixed by phytoplankton primary production in the global oceans (Azam and Malfatti, 2007). There are surprisingly few studies along transects in ocean basins that allow a detailed examination of the manner in which HPP, bacterial growth, and their potentially controlling variables in the epipelagic zone differ in biogeographic provinces (Longhurst, 2007), and across oceanic fronts. These few studies include two transects between the North and South Atlantic and the Southern Ocean (Zubkov et al, 2000; Hoppe et al, 2002). In the Pacific Ocean, microbial abundance and HPP have been investigated between the subarctic region and the Antarctic province (Baldwin et al, 2005; Yokokawa et al, 2013), Yokokawa et al (2013) focused mostly on the bathypelagic zone
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