Although free-living (FL) and particle-attached (PA) bacteria are recognized as ecologically distinct compartments of marine microbial food-webs, few, if any, studies have determined their dynamics in abundance, function (production, respiration and substrate utilization) and taxonomy over a yearly cycle. In the Baltic Sea, abundance and production of PA bacteria (defined as the size-fraction >3.0 mu m) peaked over 3 months in summer (6 months for FL bacteria), largely coinciding with blooms of Chitinophagales (Bacteroidetes). Pronounced changes in the growth efficiency (range 0.05-0.27) of FL bacteria (defined as the size-fraction <3.0 mu m) indicated the magnitude of seasonal variability of ecological settings bacteria experience. Accordingly, 16S rRNA gene analyses of bacterial community composition uncovered distinct correlations between taxa, environmental variables and metabolisms, including Firmicutes associated with elevated hydrolytic enzyme activity in winter and Verrucomicrobia with utilization of algal-derived substrates during summer. Further, our results suggested a substrate-controlled succession in the PA fraction, from Bacteroidetes using polymers to Actinobacteria and Betaproteobacteria using monomers across the spring to autumn phytoplankton bloom transition. Collectively, our findings emphasize pronounced seasonal changes in both the composition of the bacterial community in the PA and FL size-fractions and their contribution to organic matter utilization and carbon cycling. This is important for interpreting microbial ecosystem function-responses to natural and human-induced environmental changes.

Marine bacterioplankton are essential in global nutrient cycling and organic matter turnover. Time-series analyses, often at monthly sampling frequencies, have established the paramount role of abiotic and biotic variables in structuring bacterioplankton communities and productivities. However, fine-scale seasonal microbial activities, and underlying biological principles, are not fully understood. We report results from four consecutive years of high-frequency time-series sampling in the Baltic Proper. Pronounced temporal dynamics in most investigated microbial variables were observed, including bacterial heterotrophic production, plankton biomass, extracellular enzyme activities, substrate uptake rate constants of glucose, pyruvate, acetate, amino acids, and leucine, as well as nutrient limitation bioassays. Spring blooms consisting of diatoms and dinoflagellates were followed by elevated bacterial heterotrophic production and abundances. During summer, bacterial productivity estimates increased even further, coinciding with an initial cyanobacterial bloom in early July. However, bacterial abundances only increased following a second cyanobacterial bloom, peaking in August. Uptake rate constants for the different measured carbon compounds varied seasonally and inter-annually and were highly correlated to bacterial productivity estimates, temperature, and cyanobacterial abundances. Further, we detected nutrient limitation in response to environmental conditions in a multitude of microbial variables, such as elevated productivities in nutrient bioassays, changes in enzymatic activities, or substrate preferences. Variations among biotic variables often occurred on time scales of days to a few weeks, yet often spanning several sampling occasions. Such dynamics might not have been captured by sampling at monthly intervals, as compared to more predictable transitions in abiotic variables such as temperature or nutrient concentrations. Our study indicates that high resolution analyses of microbial biomass and productivity parameters can help out in the development of biogeochemical and food web models disentangling the microbial black box.

Measurements of the temporal and depth variability of microbial community respiration (MCR) in the mesopelagic zone (200-1,000 m) at Station ALOHA were made approximately monthly from November 2011 to October 2012 using the in vivo INT method. MCR in the summed and in each of the 0.2-0.8 mu m and >0.8 mu m size-fractions in the upper mesopelagic zone (200-350 m) (mean [+/- SE], 107.1 [+/- 16.3], 57.5 [+/- 8.4], and 49.6 [+/- 9.0] mu mol O-2 m(-3)day(-1), respectively) were higher than in the lower mesopelagic zone (350-1000 m) (56.1 [+/- 4.5], 30.8 [+/- 2.9], and 25.3 [+/- 3.1] mu mol O-2 m(-3)day(-1), respectively). The average > 0.8 mu m/0.2-0.8 m MCR in the mesopelagic zone was 0.97 [+/- 0.16]) and was indistinguishable in the upper and lower portions of the mesopelagic zone. A recurrent local peak in MCR was found at 600-650 m for both non-winter and winter measurements. Total depth-integrated MCR (200-1,000 m) was higher during the non-winter than during winter months (55.7 [+/- 3.4] and 44.8 [+/- 2.4] mmol O-2 m(-2) day(-1)). Variability of total MCR rates was controlled by changes in the >0.8 mu m size fraction, and was possibly related to higher primary production and export from the euphotic zone during the non-winter period. These findings suggest seasonal variability in respiration rates in the mesopelagic zone at Station ALOHA and support the current steady-state model where export flux controls respiration in the mesopelagic zone

Microbial plankton communities do not always respond to inorganic nutrient additions in coastal waters off NW Spain. However, enhanced growth of phytoplankton and heterotrophic bacteria has been observed when exposed to both inorganic (nitrate, ammonium and phosphate) and organic (amino acids and glucose) nutrient amendments, suggesting the existence of a coupling between both microbial compartments. The aim of this paper was to assess the role of the phytoplankton-bacterioplankton coupling in the response of primary producers to nutrient additions. Changes in bacterial production (BP), primary production, and chlorophyll a concentration were measured after different nutrient addition treatments on natural microbial communities where bacterial activity was blocked with antibiotics. The results obtained in this study show that phytoplankton biomass and production respond only to the nutrient inputs when heterotrophic bacteria are active. The response of coastal microbial plankton to nutrient inputs might thus be eventually dependent on the BP of secondary metabolites necessary for phytoplankton growth (e.g. B-12 vitamin).

Anthropogenic pressures are changing the magnitude and nature of matter inputs into the ocean. The Ria de Vigo (NW Spain) is a highly productive and dynamic coastal system that is likely affected by such alterations. Previous nutrient-addition microcosm experiments conducted during contrasting hydrographic conditions suggested that heterotrophic bacteria are limited by organic carbon (C) and occasionally co-limited by inorganic nutrients in this coastal area. In order to assess short-term responses in biomass, production, and respiration of heterotrophic bacteria from the Ria de Vigo to increasing amounts of natural inputs of matter, we conducted 6 microcosm experiments, wherein surface seawater collected in spring, summer, and autumn was mixed with increasing amounts of dissolved natural matter concentrates from riverine and atmospheric origin. Simultaneous experiments with controlled inorganic and/or organic additions indicated that bacteria were co-limited by inorganic nutrients and C in spring and summer and primarily limited by C in autumn. Production responded more than biomass to increasing inputs of matter, whereas respiration did not change. The bacterial production response to increasing dissolved organic C load associated with riverine and atmospheric inputs was strongly related to the relative phosphorus (P) content of the dissolved matter concentrates. Our data suggest that bacterial production might decrease with the increase of P-deficient allochthonous matter inputs, which would have important biogeochemical consequences for C cycling in coastal areas.

The impact of rain water collected at marine, urban and rural sites on coastal phytoplankton biomass, primary production and community composition as well as the effect on microbial plankton metabolism was studied in 3 microcosm experiments conducted under contrasting spring, autumn and winter conditions. The measured responses were highly variable. Rainwater additions increased chlorophyll a (Chl a) concentration (5-68% difference between rainwater treatments relative to the control) in all experiments and reduced or stimulated primary production (PP) depending on the treatment and the experiment (from -10 to +169% relative to the control). Autotrophic stimulation was highest in spring, probably related to the low initial natural nutrient concentrations. Under winter nutrient replete conditions, rainwater inputs changed the phytoplankton community although this change did not promote increases in primary production. Enhancement of net autotrophy (increase of net oxygen production up to 227%) after rainwater inputs were only found during the period of low nutrient availability. Inputs of dissolved organic nitrogen (DON) explained a large fraction of the variability in the response of PP, Chl a, community respiration (CR) and net community production (NCP). Our results suggest that differences in the initial environmental conditions (i.e. nutrient availability), rainwater composition and the ability of the present autotropic communities to utilize the new nutrients result in substantial changes in the microbial responses and associated biologically-mediated carbon fluxes. As atmospheric nutrient inputs into coastal oceans are increasing rapidly, our results help to understand the effects of different inputs on the metabolism of distinct microbial communities. (C) 2014 Elsevier Ltd. All rights reserved.

We simultaneously studied microbial-mediated carbon fluxes at 2 contrasting sites within the coastal upwelling system off Galicia, near A Coruña and Vigo, Spain, over an annual cycle in order to compare the fraction of primary production released as dissolved organic carbon (DOC) and the degree of coupling between bacteria and phytoplankton. A significant fraction of primary production was released as DOC at both sites, averaging ∼30%. DOC release ac counted for, on average, 30% of the total bacterial carbon demand, which is indicative of a moderate trophic dependence of bacteria on phytoplankton in these coastal ecosystems. Nevertheless, differences in hydrographic conditions associated with stronger upwelling pulses off Vigo, and deeper upper mixed layers during the downwelling period off A Coruña, led to significant differences in phytoplankton dynamics and the subsequent direct coupling with heterotrophic bacteria. Strong direct coupling between phytoplankton extracellular release and bacterial production (BP) was found off Vigo, which could be related to the quality of the DOC produced by actively growing phytoplankton. By contrast, DOC release and BP rates were decoupled off A Coruña, likely due to unaccounted DOC associated with indirect trophic processes or to the low availability of freshly produced exudates associated with overflow or photoinhibition mechanisms.

In situ measurements were made to determine oxygen (O<inf>2</inf>) metabolic balance in the upper oligotrophic ocean from diel changes in O<inf>2</inf> to argon (Ar) ratios. The study took place during 13-24 March 2014, at the Hawaii Ocean Time-series Station ALOHA (A Long-term Oligotrophic Habitat Assessment), in the North Pacific Subtropical Gyre. Microbial community respiration and gross O<inf>2</inf> production, estimated from in situ diel changes in O<inf>2</inf>/Ar saturation, agreed well with those calculated using other independent methods. Net oxygen production (NOP), estimated from in situ diel changes in O<inf>2</inf>/Ar saturation, showed large day-to-day variability. However, when averaged over the entire observational period, mean diel NOP was in relatively good agreement with the estimated mean steady state NOP (9.2±9.3mmolO<inf>2</inf>m-2d-1 compared to 11.7±1.1mmolO<inf>2</inf>m-2d-1, respectively). Key Points Diel changes in O<inf>2</inf>/Ar were used to determine mixed layer ecosystem metabolism Estimated metabolic rates agreed well with other independent methods There was a net production of O<inf>2</inf> in the mixed layer during the sampling period.

Methionine (Met) and dimethylsulfoniopropionate (DMSP) are 2 important substrates that can serve as sources of sulfur and carbon to microbial communities in the sea. We studied the vertical and diel distributions and the assimilation rates of dissolved Met (dMet) and dissolved DMSP (dDMSP) into proteins of different microbial groups at Stn ALOHA, in the oligotrophic North Pacific Subtropical Gyre (NPSG). Concentrations of dMet never exceeded 50 pM and were at their daily minimum during the night-time (<0.17 pM). dMet assimilation into proteins accounted for <30% of the dMet lost from the dissolved pool, suggesting that other metabolic pathways were also important. Concentrations of dDMSP ranged from 0.35 to 1.0 nM in surface waters and did not present a distinguishable diel pattern. Cell-sorted Prochlorococcus, high nucleic acid (HNA), and low nucleic acid (LNA) non-pigmented bacteria showed a clear diel pattern for dMet and dDMSP assimilation, with higher rates during the night-time. Among the different groups, HNA bacteria had the highest per-cell assimilation rate for dMet and dDMSP, but when accounting for cell numbers in each group, the HNA and LNA bacterial group assimilation rates were comparable for both dDMSP and dMet. Integrated water column (0 to 125 m) dDMSP assimilation rates by the entire microbial assemblage were 1.7- To 5.3-fold faster than those for dMet, suggesting that dDMSP constitutes a more important source of sulfur than dMet to the microbial community of the NPSG during the time of our study.

Measurements of the temporal and vertical variability of microbial community respiration (MCR) in the euphotic zone (0-200m) at Station ALOHA were made using the in vivo INT method to constrain oxygen and carbon cycling at this oceanic site. Mean (+/- 1 SE) MCR was higher in the upper (0-100m) (0.89 +/- 0.05mmol O-2 m(-3) d(-1)) than in the lower (100-200 m) portion of the euphotic zone (0.52 +/- 0.05mmol O-2 m(-3) d(-1)). Respiration in the 0.8 mu m size-fraction relative to respiration in the 0.2-0.8 mu m size-fraction was on average 1.4 +/- 0.1. Variability in MCR was observed on both daily and monthly time scales, suggesting that respiration is a dynamic process throughout the year at Station ALOHA. MCR in the 0.2-0.8 mu m size fraction was more variable than >0.8 mu m MCR. Despite significant vertical and temporal variability in MCR, the euphotic zone depth-integrated (0-200m) MCR was relatively constant (134.8 +/- 11.8 mmol O-2 m(-2) d(-1)) throughout the period of observation. Oxygen consumption via MCR always exceeded O-2 production extrapolated from C-14-primary production estimation, assuming a photosynthetic quotient of 1.13mol O-2 produced : mol CO2 fixed. MCR plus particulate carbon export from the euphotic zone for the period November 2011-October 2012 at Station ALOHA can be used to set a lower limit of similar to 45mol C m(-2) yr(-1) for gross primary production.