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Today we know that diazotrophs are common and active in nitrogen (N) replete regions, however the factors controlling their distribution remain elusive. Previous studies in upwelling regions revealed that the composition of diazotrophs responded to changes in hydrodynamic forcing over seasonal scales. Here we used high-frequency observations collected during a 3-week cruise in the upwelling region off NW Iberia to describe changes in the activity and composition of diazotrophs over shorter temporal scales. The cruise started after a strong upwelling event followed by a few days of relaxation-downwelling, and soon after another upwelling pulse. Higher N2 fixation rates (2.2 ± 0.7 µmol m-3 d-1) were measured during relaxation-downwelling, when surface nitrate concentration was low. During the fertilization associated with the upwelling, N2 fixation dramatically decreased to 0.10 ± 0.09 µmol m-3 d-1. The comparison with nitrate consumption and diffusion confirmed the minor role of N2 fixation (<1%) as a source of new N for primary production. The unicellular cyanobacterium UCYN-A2 was the dominant diazotroph during the cruise. UCYN-A2 abundance was four times higher during relaxation-downwelling (4x104 copies L-1) compared to upwelling conditions (0.2x104 copies L-1), when the unusual Epsilonproteobacteria increased their relative abundance. These results indicate that diazotrophs can respond rapidly to changes in the environment, and point out to the availability of N as a key factor controlling the activity, composition and distribution of diazotrophs in eutrophic regions.

The communication architecture required to provide a bidirectional communication between a central command node and a full set of fiber positioners feeding a spectrograph is studied. Six different architectures have been analyzed in terms of communication time and power consumption. These architectures are the result of the combination of three different communication protocols: transmission control protocol/internet protocol (TCP/IP) over ethernet, interintegrated circuit (I2C), and controller area network. The design of communication architecture must prioritize between communication time and power consumption. The fastest architecture is the hybrid TCP/IP over ethernet-I2C. This architecture requires the least time to provide a full set of coordinates to every fiber positioner less than 50 ms. The most power efficient solution is the I2C - I2C with demultiplexers. This architecture solves a bidirectional communication between a central node and a full set of fiber positioners requiring only an addition of 27 mW. © 2019 Society of Photo-Optical Instrumentation Engineers (SPIE).

Anthropogenic pressures are changing the magnitude and nature of matter inputs into the ocean. The Ría 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 Ría 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.

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.

The effects of inorganic and/or organic (glucose+AAs) inputs on phytoplankton and heterotrophic bacteria were assessed, using a microcosm approach, in two contrasting marine environments: an open ocean oligotrophic site (North Atlantic Subtropical Gyre) and a highly productive coastal embayment (Ría de Vigo, NW Spain). Overall, changes in microbial plankton biomass were smaller than those of metabolic rates. The largest increases in primary production, bacterial production and community respiration were measured in response to mixed () nutrient additions in both sites. Primary production responded to additions only in oligotrophic waters. The distinct autotrophic responses to nutrient additions measured in these environments were related to the different initial composition of phytoplankton populations and, presumably, also to differences in grazing pressures in both marine ecosystems. Heterotrophic bacteria were limited by organic substrates in both ecosystems, although mixed additions further enhanced bacterial growth in the subtropical gyre. The differences detected in bacterial responses to nutrient additions may be related to differences in nutrient limitations and to the prevalence of different relationships between components of the microbial food web (e.g. coupling between heterotrophic bacteria and phytoplankton and predation pressure) in both environments. We found a more relevant role of inorganic nutrients in controlling the efficiency of bacterial growth in oligotrophic regions as compared with highly productive systems. Our results suggest that organic matter inputs into both ecosystems might result in a tendency towards heterotrophy and in increases in bacterial growth efficiency.

The short-term effects of inorganic N and P (nitrate, ammonium, phosphate) and organic C and N (glucose, amino acids) inputs, added separately as well as jointly, on phytoplankton and bacterioplankton community composition were studied in 6 microcosm experiments conducted in a eutrophic coastal embayment under contrasting hydrographic conditions. The responses of the different bacterioplankton and phytoplankton groups to the distinct nutrient inputs were highly variable among experiments, which was partially related to changes in the initial environmental conditions. Gammaproteobacteria and nanoflagellates were the most responsive groups to nutrient additions. Inorganic nutrients did not promote important changes in the microbial plankton community structure but did significantly reduce diatom diversity. In contrast, organic additions promoted changes mainly in bacterioplankton groups, whilst mixed additions provoked changes in both bacterial and phytoplankton groups. While nanoflagellates increased equally in abundance after inorganic and mixed additions (2.9-fold), dinoflagellates and diatoms increased their abundances more in the mixed treatment (2.3-fold and 2.2-fold, respectively) than in the inorganic treatment. Organic and mixed additions did not provoke changes in diatom or dinoflagellate diversity. The magnitude of response of Gammaproteobacteria largely explained changes in bulk bacterial biomass and activity, whereas changes in bulk phytoplankton biomass and primary production associated to nutrient enrichment were mostly explained by the response of diatoms and large picoeukaryotes. Our results demonstrate that the type of nitrogen inputs (inorganic and/or organic) strongly affects the microbial plankton community composition and functioning in this coastal ecosystem.

Bacterioplankton are responsible for a large fraction of respiration and dissolved organic matter (DOM) remineralisation in the ocean; therefore, changes in their taxonomic composition due to changes in nutrient inputs may have important biogeochemical implications. The impact of nutrient loading will depend on the type of input and the initial environmental conditions. We conducted a set of 5 microcosm experiments along a broad latitudinal transect in the Atlantic Ocean (26°N to 29°S) in order to assess the effects of inorganic (nitrate, ammonium, phosphate, silica) and organic (glucose, amino acids) inputs, added separately as well as jointly, on the bacterioplankton community composition at the level of major groups. We followed changes in the relative abundance of important bacterial phylogenetic (Roseobacter spp., SAR11, Gammaproteobacteria, Bacteroidetes) and cytometric (low, high and very high nucleic acid content) groups. We observed that distinct groups responded differently to nutrient additions: SAR11 and Bacteroidetes responded negatively to organic and mixed additions, while Roseobacter spp. and Gammaproteobacteria responded positively. Only the group Roseobacter spp. responded positively to inorganic additions. The abundance of very high nucleic acid content bacteria was low in the control and the inorganic treatment; however, they reached very high numbers in the organic and mixed treatments. There was a clear increasing north to south gradient in the magnitude of response of Gammaproteobacteria to organic inputs (containing carbon and nitrogen), likely related to an increasing north to south gradient in phosphate concentration. The magnitude of response of Gammaproteobacteria explained 89% of the observed variability in the magnitude of bacterial production response, which suggests a strong link between bacterial community composition and carbon cycling in the oligotrophic ocean.

The short-term effects of inorganic N and P (nitrate, ammonium, phosphate) and organic C and N (glucose, amino acids) input, added separately as well as jointly, on microplankton community structure and metabolism were studied in 6 microcosm experiments conducted from February to July 2008 in a eutrophic coastal embayment in NW Spain under contrasting hydrographic conditions. The responses of planktonic communities were highly variable. Strong positive effects of combined inorganic and organic nutrient enrichments for phytoplankton and heterotrophic bacteria were found. In most experiments enhanced phytoplankton biomass and productivity (primary production increased up to 5.5-fold) was measured after mixed (inorganic plus organic) additions but phytoplankton responded only in one of the experiments after inorganic additions. Heterotrophic bacteria responded faster than phytoplankton to the additions but were never affected by inorganic nutrient additions, suggesting a C limitation of bacterial activity in this coastal environment. Heterotrophic bacteria biomass, productivity, and respiration responded to mixed additions (containing C, N, and P) in all the experiments (bacterial production increased from 14- to 34-fold) whereas its response to organic (containing C and N) additions greatly differed among experiments (bacterial production increased from 0.6- to 21-fold), which could be partially related to P availability. A general increase in bacterial growth efficiency occurred only after mixed additions (up to 2-fold). The tendency towards heterotrophy of the microbial plankton community was weaker after the mixed (Production/Respiration (P/R) ratio changes from 0.34- to 1.6-fold) than after the organic additions (P/R changes from 0.16- to 0.8-fold). Our results suggest that phytoplankton requirememnts for organic nutrients for growth and P deficiency of bacterial populations intermittently occur in this coastal area.