No abstract

Eddies are mesoscale oceanographic features (∼ 200 km diameter) that can cause transient blooms of phytoplankton by shifting density isoclines in relation to light and nutrient resources. To better understand how bacterioplankton respond to eddies, we examined depth-resolved distributions of bacterial populations across an anticyclonic mode-water eddy in the Sargasso Sea. Previous work on this eddy has documented elevated phytoplankton productivity and diatom abundance within the eddy centre with coincident bacterial productivity and biomass maxima. We illustrate bacterial community shifts within the eddy centre, differentiating populations uplifted along isopycnals from those enriched or depleted at horizons of enhanced bacterial and primary productivity. Phylotypes belonging to the Roseobacter, OCS116 and marine Actinobacteria clades were enriched in the eddy core and were highly correlated with pigment-based indicators of diatom abundance, supporting developing hypotheses that members of these clades associate with phytoplankton blooms. Typical mesopelagic clades (SAR202, SAR324, SAR406 and SAR11 IIb) were uplifted within the eddy centre, increasing bacterial diversity in the lower euphotic zone. Typical surface oligotrophic clades (SAR116, OM75, Prochlorococcus and SAR11 Ia) were relatively depleted in the eddy centre. The biogeochemical context of a bloom-inducing eddy provides insight into the ecology of the diverse uncultured bacterioplankton dominating the oligotrophic oceans.

We investigated the patterns and controls of dissolved organic carbon (DOC) production by the giant kelp (Macrocystis pyrifera) using data from short-term in situ incubations of entire blades and portions of stipes. These data were incorporated into an empirical model of reef-scale net primary production (NPP) at Mohawk Reef in southern California, U.S.A. for an 8-yr period. Rates of DOC release of incubated blades varied unpredictably with time of year, but were significantly related to the irradiance at the sea surface during the incubations. The growth stage, C/N ratio, and epiphyte load of the blades and the temperature of the ocean during the incubations had no discernable effect on rates of DOC release. Blades produced on average 2-3 times more DOC than stipes, and stipes and blades produced on average 30% and 80% more DOC respectively during the day compared to the night. Modeled DOC NPP at the reef scale was on average highest in summer and spring (0.5 g C m 22 d 21) and lowest in winter and autumn (0.31 g C m 22 d 21

Marine dissolved organic matter (DOM) varies in its recalcitrance to rapid microbial degradation. DOM of varying recalcitrance can be exported from the ocean surface to depth by subduction or convective mixing and oxidized over months to decades in deeper seawater. Carboxyl-rich alicyclic molecules (CRAM) are characterized as a major component of recalcitrant DOM throughout the oceanic water column. The oxidation of CRAM-like compounds may depend on specific bacterioplankton lineages with oxidative enzymes capable of catabolizing complex molecular structures like long-chain aliphatics, cyclic alkanes, and carboxylic acids. To investigate the interaction between bacteria and CRAM-like compounds, we conducted microbial remineralization experiments using several compounds rich in carboxyl groups and/or alicyclic rings, including deoxycholate, humic acid, lignin, and benzoic acid, as proxies for CRAM. Mesopelagic seawater (200 m) from the northwest Sargasso Sea was used as media and inoculum and incubated over 28 d. All amendments demonstrated significant DOC removal (2-11 μmol C L −1 ) compared to controls. Bacterioplankton abundance increased significantly in the deoxycholate and benzoic acid treatments relative to controls, with fast-growing Spongiibacteracea, Euryarcheaota, and slowgrowing SAR11 enriched in the deoxycholate treatment and fast-growing Alteromonas, Euryarcheaota, and Thaumarcheaota enriched in the benzoic acid treatment. In contrast, bacterioplankton grew slower in the lignin and humic acid treatments, with oligotrophic SAR202 becoming significantly enriched in the lignin treatment. Our results indicate that the character of the CRAM proxy compounds resulted in distinct bacterioplankton removal rates of DOM and affected specific lineages of bacterioplankton capable of responding.Carboxyl-rich alicyclic molecules (CRAM), defined as a diverse array of organic compounds enriched with carboxylated and fused alicyclic rings, comprise some of the more recalcitrant compounds in the ocean (Hertkorn et al. 2006). Similar in structure to steroids and hopanoids, CRAM compounds are resistant to rapid microbial degradation (Hertkorn et al. 2006;

Seasonal trends in organic matter (OM) partitioning (between the dissolved and particulate phases), dissolved inorganic nitrogen (DIN) distributions, and bacterioplankton dynamics were examined across a 3 km stretch of the Santa Barbara Channel continental shelf from January 2008 to April 2009. OM partitioning was assessed as the percentage of 14 C-primary production (PP) released as 14 C-labeled dissolved organic carbon (DOC), i.e. percent extracellular release (PER), and as the ratio of DOC and particulate organic carbon (POC) concentrations. Spring upwelling raised surface DIN concentrations, stimulating phytoplankton blooms and OM accumulation. During upwelling, PER accounted for 10 to 30%, with ~60% of OM accumulating as POC versus ~40% as DOC. After stratification and macronutrient depletion, PER increased to between 35 and 45%, and the proportion of seasonally produced DOC to total accumulated organic carbon increased to > 50%. Experiments revealed a seasonal maximum in DOC bioavailability to heterotrophic bacterioplankton during upwelling: 9 to 18% of DOC was consumed in <1 wk at bacterial growth efficiencies (BGE) of 5 to 12%. Throughout the stratified period, ≤5% of DOC was consumed in <1 wk, and the BGE increased to between 12 and 52%. Bacterial carbon demand (BCD) ranged from 0.2 to 24 µmol C l, and for most of the sampling period, PP was sufficient to meet BCD. However, at times, BCD exceeded phytoplankton PP, indicating that sources of DOC other than daily PP were used to support BCD. Similar to other coastal systems, OM is partitioned mainly into the particulate phase under nutrient-replete conditions and into the dissolved phase following stratification and the onset of oligotrophic conditions. We discuss how the partitioning of OM has consequences for bacterial metabolism and carbon cycling of this coastal system.