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Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guano stimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton–bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton–bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria

Organic matter is known to influence community composition and metabolism of marine prokaryotes. However, few studies have addressed this linkage in the deep ocean. We studied the relationship between fluorescent dissolved organic matter and prokaryotic community composition in meso- and bathypelagic water masses along a surface productivity gradient crossing the subtropical and tropical Atlantic Ocean. Four fluorescence components were identified, three humic-like and one protein-like. The distributions of the humic-like components were significantly explained by water mass mixing, apparent oxygen utilisation (AOU) and epipelagic productivity proxies in varying degrees, while the protein-like component was explained only by water mass mixing and epipelagic productivity. The diversity and taxonomic composition of the prokaryotic community differed between water masses: the Nitrosopumilales order dominated in water masses with high AOU and humic-like fluorescence (notably, the SubPolar Mode Water), and tended to co-occur with Marine Group II archaea, the SAR324 clade and Thiomicrospirales, while bathypelagic water masses displayed greater abundances of members of Marinimicrobia, SAR202 and SAR324. Water mass mixing regression models suggested that the distribution of some taxa (e.g., Marinimicrobia, SAR202) was dominated by mixing and selection within the water masses during ageing, while others (chiefly, Alteromonadales) were mostly influenced by local processes. Our results suggest a link between the composition of the prokaryotic community, oxygen utilisation and the signal of fluorescent dissolved organic matter, and has implications for our understanding of the processes that shape carbon cycling and prokaryotic communities in the deep ocean.

Organic matter is known to influence community composition and metabolism of marine prokaryotes. However, few studies have addressed this linkage in the deep ocean. We studied the relationship between fluorescent dissolved organic matter and prokaryotic community composition in meso- and bathypelagic water masses along a surface productivity gradient crossing the subtropical and tropical Atlantic Ocean. Four fluorescence components were identified, three humic-like and one protein-like. The distributions of the humic-like components were significantly explained by water mass mixing, apparent oxygen utilisation (AOU) and epipelagic productivity proxies in varying degrees, while the protein-like component was explained only by water mass mixing and epipelagic productivity. The diversity and taxonomic composition of the prokaryotic community differed between water masses: the Nitrosopumilales order dominated in water masses with high AOU and humic-like fluorescence (notably, the SubPolar Mode Water), and tended to co-occur with Marine Group II archaea, the SAR324 clade and Thiomicrospirales, while bathypelagic water masses displayed greater abundances of members of Marinimicrobia, SAR202 and SAR324. Water mass mixing regression models suggested that the distribution of some taxa (e.g., Marinimicrobia, SAR202) was dominated by mixing and selection within the water masses during ageing, while others (chiefly, Alteromonadales) were mostly influenced by local processes. Our results suggest a link between the composition of the prokaryotic community, oxygen utilisation and the signal of fluorescent dissolved organic matter, and has implications for our understanding of the processes that shape carbon cycling and prokaryotic communities in the deep ocean

Anthropogenic alterations of global biogeochemical cycles are changing not only the magnitude but also the nature of matter entering into the ocean, thus increasing the C/P and N/P ratios. Allochthonous inputs are particularly significant in coastal systems and even may equal autochthonous sources. The interactions between phytoplankton and bacteria likely play a major role in the response of primary producers to anthropogenic matter inputs. However, the magnitude and nature of such interactions, as well as the factors controlling their intensity and sign are largely unknown in marine planktonic ecosystems. A set of 18 microcosms (2 L) experiments were conducted to test the hypothesis that the nature of the nutrient inputs modulates the strength and sign of the interaction between phytoplankton and bacteria. The experiments consisted in the addition of nutrients (nitrate, ammonium an phosphate), nutrients and dissolved organic matter (DOM) (glucose and amino acids), or riverine water (10% final concentration) containing different DOM:P proportions to intact microbial communities and microbial communities where bacteria were selectively blocked with a previously tested combination of antibiotics. Sampling was conducted in different seasons in the Ría de Vigo, a highly productive coastal ecosystem located NW Spain. An interaction index was calculated as the quotient between the change in chlorophyll-a concentration after nutrient amendment in the intact communities and the corresponding change in the same community but treated with antibiotics. An interaction index >1 thus indicates a positive effect of bacteria on phytoplankton, while an interaction index <1 indicates an overall negative impact. The interaction index ranged from 0.51 to 1.24. The index was significantly higher in autumn (1.01±0.03) than in spring (0.91±0.02), and winter (0.90±0.03). Overall, treatments with a high DOM:P ratio promoted a strong negative impact of bacteria on phytoplankton (0.84±0.06)

This dataset contains the results of the fluorescent dissolved organic matter characterisation (FDOM) and water mass optimum multiparameter analysis from the MAFIA cruise (Migrants and Active Flux In the Atlantic ocean). Samples were collected in the tropical and subtropical Atlantic during the MAFIA cruise (April 2015) on board the BIO Hespérides. Seawater samples for biogeochemical analyses were collected at 13 stations (from the Brazilian coast to the Canary Islands), from the surface down to 3500 m, using a General Oceanics oceanographic rosette equipped with 24 l PVC Niskin bottles. Oxygen solubility was computed using the equation of Benson and Krause (1984). AOU (µmol/kg) was calculated by subtracting measured oxygen concentration from the oxygen solubility values at saturation, with respect to the atmosphere. Fluorescence measurements were performed with a Perkin-Elmer LS55 spectrofluorometer and FDOM was characterised by means of a Parallel Factor analysis. The flag denotes if FDOM values were from direct measurement or are interpolated values (some interpolated values were added to match the 16S amplicon data). Integrated values were estimated by multiplying the discrete measurements by the distance, in meters, between samples.The aim of this dataset was to jointly characterise the FDOM and water mass distributions to infer the processes that shape the dissolved organic matter pool in the deep ocean (water mass mixing and history vs. local processes)

This dataset contains the results of the fluorescent dissolved organic matter characterisation (FDOM) and water mass optimum multiparameter analysis from the MAFIA cruise (Migrants and Active Flux In the Atlantic ocean). Samples were collected in the tropical and subtropical Atlantic during the MAFIA cruise (April 2015) on board the BIO Hespérides. Seawater samples for biogeochemical analyses were collected at 13 stations (from the Brazilian coast to the Canary Islands), from the surface down to 3500 m, using a General Oceanics oceanographic rosette equipped with 24 l PVC Niskin bottles. Oxygen solubility was computed using the equation of Benson and Krause (1984). AOU (µmol/kg) was calculated by subtracting measured oxygen concentration from the oxygen solubility values at saturation, with respect to the atmosphere. Fluorescence measurements were performed with a Perkin-Elmer LS55 spectrofluorometer and FDOM was characterised by means of a Parallel Factor analysis. Integrated values were estimated by multiplying the discrete measurements by the distance, in meters, between samples. The contribution of each water mass to each sample was objectively quantified applying an optimum multiparameter analysis (excluding mixed layer samples, here < 100 m). The aim of this dataset was to jointly characterise the FDOM and water mass distributions to infer the processes that shape the dissolved organic matter pool in the deep ocean (water mass mixing and history vs. local processes)