Major role of microbes in carbon fluxes during Austral winter in the Southern Drake Passage

Abstract

Carbon cycling in Southern Ocean is a major issue in climate change, hence the need to understand the role of biota in the regulation of carbon fixation and cycling. Southern Ocean is a heterogeneous system, characterized by a strong seasonality, due to long dark winter. Yet, currently little is known about biogeochemical dynamics during this season, particularly in the deeper part of the ocean. We studied bacterial communities and processes in summer and winter cruises in the southern Drake Passage. Here we show that in winter, when the primary production is greatly reduced, Bacteria and Archaea become the major producers of biogenic particles, at the expense of dissolved organic carbon drawdown. Heterotrophic production and chemoautotrophic CO(2) fixation rates were substantial, also in deep water, and bacterial populations were controlled by protists and viruses. A dynamic food web is also consistent with the observed temporal and spatial variations in archaeal and bacterial communities that might exploit various niches. Thus, Southern Ocean microbial loop may substantially maintain a wintertime food web and system respiration at the expense of summer produced DOC as well as regenerate nutrients and iron. Our findings have important implications for Southern Ocean ecosystem functioning and carbon cycle and its manipulation by iron enrichment to achieve net sequestration of atmospheric CO(2).

Figure 1. Sampling area and chlorophyll a distribution.

(Panel A) MODIS-Aqua composite image of the chlorophyll-a gradient during 2004 summer. Red dots: summer 2004 sampling stations; white triangles: winter 2006 sampling stations. (Panel B) Depth profile of chlorophyll-a (Chl a) from summer 2004 cruise (blue circles = ACC water; green circles = mixed water; red circles = shelf water) and winter 2006 cruise (empty squares).

Figure 2. Depth profile of bacterial parameters in summer 2004 and winter 2006.

Winter data are from 32 stations from 5 to 400 m; at 6 stations samples were also taken from 750 m. Summer: blue circles = ACC water; red circles = shelf water; green circles = mixed water; empty square = Winter. (Panel A) Bacterial abundance; (panel B) BCP = bacterial carbon production; BCP calculated from ³H-Leucine incorporation, employing a conversion factor of 3.1 kg C per mol of Leu ; (panel C) μ = bacterial growth rate. Cell-specific growth rate calculations assumed 20 fg C per cell .

Figure 3. Depth distribution of organic matter pools during summer and winter cruises.

Summer 2004: blue circles = ACC water; red circles = shelf water; green circles = mixed water; empty square = Winter 2006. (Panel A) DOC = Dissolved organic carbon. The data presented are measurements of total organic carbon, but since POC represents a negligible contribution to total organic carbon (POC represented only between 7% and 2% of the total pool) we can consider the analysis as DOC values. The shaded area covers the range of winter concentration. For a comparison, a range of variations of DOC from the FRUELA cruise study area in summer (Gerlache Strait, Bransfield Strait and Bellinghausen Strait) has been reported (black triangles). (Panel B) POC = Particulate organic carbon.

Figure 4. Depth profile of chemotrophic carbon production in winter 2006.

DIC = dissolved inorganic carbon.

Figure 5. DGGE fingerprint of Bacteria amplicons from Summer 2004 (A) and Winter 2006 (B) samples.

The numbered bands have been excised for sequencing.

Figure 6. DGGE fingerprint of Archaea amplicons from Summer 2004 (A) and Winter 2006 (B) samples.

The numbered bands have been excised for sequencing.