Role of bacterial community composition as a driver of the small-sized phytoplankton community structure in a productive coastal system

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGWe present here the first detailed description of the seasonal patterns in bacterial community composition (BCC) in shelf waters off the Ría de Vigo (Spain), based on monthly samplings during 2 years. Moreover, we studied the relationship between bacterial and small-sized eukaryotic community composition to identify potential biotic interactions among components of these two communities. Bacterial operational taxonomic unit (OTU) richness and diversity systematically peaked in autumn–winter, likely related to low resource availability during this period. BCC showed seasonal and vertical patterns, with Rhodobacteraceae and Flavobacteriaceae families dominating in surface waters, and SAR11 clade dominating at the base of the photic zone (30 m depth). BCC variability was significantly explained by environmental variables (e.g., temperature of water, solar radiation, or dissolved organic matter). Interestingly, a strong and significant correlation was found between BCC and small-sized eukaryotic community composition (ECC), which suggests that biotic interactions may play a major role as structuring factors of the microbial plankton in this productive area. In addition, co-occurrence network analyses revealed strong and significant, mostly positive, associations between bacteria and small-sized phytoplankton. Positive associations likely result from mutualistic relationships (e.g., between Dinophyceae and Rhodobacteraceae), while some negative correlations suggest antagonistic interactions (e.g., between Pseudo-nitzchia sp. and SAR11). These results support the key role of biotic interactions as structuring factors of the small-sized eukaryotic community, mostly driven by positive associations between small-sized phytoplankton and bacteria.Xunta de Galicia | Ref. EM2013/023Xunta de Galicia | Ref. ED481A-2019/290Xunta de Galicia | Ref. ED431I 2020/03Ministerio de Economía y Competitividad | Ref. CTM2017-83362-RMinisterio de Ciencia e Innovación | Ref. PID2019-110011RB-C3

Co-occurrence and diversity patterns of benthonic and planktonic communities in a shallow marine ecosystem

Marine microorganisms are involved in a variety of biogeochemical cycles and live in diverse ecological communities where they interact with each other and with other organisms to guarantee ecosystem functions. The present study focused on a shallow marine environment located in Ría de Vigo (NW, Spain), where sediment and size-fractionated plankton samples were collected from 2016 to 2018. DNA metabarcoding was used to describe the eukaryote and prokaryote composition and diversity in sediments and plankton and to depict possible associations among the most frequent and abundant organisms by co-occurrence network analysis. High eukaryote and prokaryote diversity indices were obtained in all compartments. Significant differences among eukaryote and prokaryote communities were found between sediment and plankton samples, with a high percentage of exclusive operational taxonomic units (OTUs) associated with each compartment, especially from sediment. Despite these differences, shared taxa between water and sediment were also obtained, suggesting a relatively meaningful exchange of organisms between both environmental compartments. Significant co-occurrences were mainly obtained between prokaryotes (41%), followed by eukaryotes–prokaryotes (32%) and between eukaryotes (27%). The abundant and strong positive correlations between organisms, including representatives from the sediment and the water column, suggested an essential role of biotic interactions as community-structuring factors in shallow waters where beneficial associations likely prevail. This study provides a novel approach for the detailed description of the eukaryote and prokaryote diversity and co-occurrence patterns in a shallow marine area, including both the sediment and different water-size fractions. The high diversity obtained and the detection of predominantly coexisting interactions among organisms from sediment and the overlying water column suggest a movement of species between both habitats and therefore confirm the importance of integratively studying shallow marine ecosystems.Xunta de Galicia | Ref. IN606A-2018/020Xunta de Galicia | Ref. IN607B 2019/01Agencia Estatal de Investigación | Ref. CTM2017-83362-RInterreg España-Portugal | Ref. 20200474_BLUEBIOLA

Linking the impact of bacteria on phytoplankton growth with microbial community composition and co-occurrence patterns

The interactions between microalgae and bacteria have recently emerged as key control factors which might contribute to a better understanding on how phytoplankton communities assemble and respond to environmental disturbances. We analyzed partial 16S rRNA and 18S rRNA genes from a total of 42 antibiotic bioassays, where phytoplankton growth was assessed in the presence or absence of an active bacterial community. A significant negative impact of bacteria was observed in 18 bioassays, a significant positive impact was detected in 5 of the cases, and a non-detectable effect occurred in 19 bioassays. Thalasiossira spp., Chlorophytes, Vibrionaceae and Alteromonadales were relatively more abundant in the samples where a positive effect of bacteria was observed compared to those where a negative impact was observed. Phytoplankton diversity was lower when bacteria negatively affect their growth than when the effect was beneficial. The phytoplankton-bacteria co-occurrence subnetwork included many significant Chlorophyta-Alteromonadales and Bacillariophyceae-Alteromonadales positive associations. Phytoplankton-bacteria co-exclusions were not detected in the network, which contrasts with the negative effect of bacteria on phytoplankton growth frequently detected in the bioassays, suggesting strong competitive interactions. Overall, this study adds strong evidence supporting the key role of phytoplanktonbacteria interactions in the microbial communities.Agencia Estatal de Investigación | Ref. CTM2017-83362-RAgencia Estatal de Investigación | Ref. PID2019-110011RB-C33Xunta de Galicia | Ref. ED481A-2019/290Xunta de Galicia | Ref. ED481A-2018/288Universidade de Vigo/CISU

Potential overestimation of bacterial respiration rates in oligotrophic plankton communities

Adequate bacterial respiration (BR) estimations are necessary to understand the flow of carbon through marine plankton food webs. A considerably higher bacterial contribution to total microbial plankton community respiration (CR) has been observed in oligotrophic systems compared to more productive systems. Classical BR estimation procedures, typically oxygen consumption measurements, comprise pre-incubation filtration to separate bacteria from the rest of the plankton community and long incubation times (24 h). The resulting disruption of the community linkages over long time periods might affect BR measurements, especially in oligotrophic systems characterized by tightly coupled microbial food webs. In this study, BR data were compiled from 2 contrasting environments: the highly productive Ría de Vigo (NW Spain) and the North Atlantic oligotrophic gyre. Standard procedures or in vivo electron transport system (ETS) activity (non pre-filtered and short-time incubation) procedures were used to obtain a total of 209 BR estimations. Bacterial contribution to plankton CR was significantly higher in oligotrophic than in highly productive conditions (155% and 42%, respectively) when using standard procedures, while similar values were observed (31% and 30%) with in vivo ETS procedures. The relation between plankton CR and BR along the studied productivity gradient suggests that bacterial contribution to total CR varies less than previously assumed with an average value approximating 30% through different trophic situations.Ministerio de Educación y Ciencia | Ref. REN2003-09532-C03-03Ministerio de Educación y Ciencia | Ref. VEM2003-20021Ministerio de Educación y Ciencia | Ref. CTM2007- 61983/MA

Response of prokaryote community composition to riverine and atmospheric nutrients in a coastal embayment: role of organic matter on Vibrionales

The responses of coastal microbial communities to nutrient inputs depend on the magnitude and nature of those inputs. To understand the response of prokaryote communities to contrasting levels of nutrient inputs from different sources, three nutrient addition experiments were performed in the Ría de Vigo in spring, summer, and autumn. The experiments consisted of controlled inorganic (nitrate, ammonium and phosphate), organic (glucose and a mix of 18 amino acids), and mixed (inorganic and organic); as well as natural (atmospheric and riverine) nutrient additions to surface water samples taken from the middle of the embayment and incubated for 48h, under in situ temperature and irradiance conditions. Prokaryote community DNA was collected and analyzed using high-throughput amplicon sequencing of the 16S rRNA gene. The joint addition of inorganic and organic nutrients provoked the most pronounced changes in community composition, systematically leading to great increases in the proportion of sequence reads belonging to Vibrionales, while natural amendments only caused subtle alterations. The proportion of reads assigned to the Planktomarina genus, bacteria belonging to the order Rhodobacterales, tended to increase when natural atmospheric nutrients were added. Overall, the fraction of reads belonging to the order Flavobacteriales did not show relevant changes, although the contribution of sequences affiliated to Polaribacter and Aurantivirga genera increased after atmospheric and riverine inputs in summer, and autumn. The proportion of Vibrionales reads also increased after controlled organic amendments in autumn, and to a lesser extent after riverine additions in spring and summer. In general, our findings suggest that an increasing input of nitrogenous organic matter from allochthonous sources into this coastal environment might promote the growth of potentially pathogenic bacteria belonging to the order Vibrionales.Financiado para publicación en acceso aberto: Universidade de Vigo/CISUGMinisterio de Ciencia e Innovación | Ref. CTM2011-30155- C03-01Xunta de Galicia | Ref. EM2013/02

Spatial and temporal variability in the response of phytoplankton and prokaryotes to B-vitamin amendments in an upwelling system

We experimentally evaluated the temporal (interday and interseason) and spatial variability in microbial plankton responses to vitamin B12 and/or B1 supply (solely or in combination with inorganic nutrients) in coastal and oceanic waters of the northeast Atlantic Ocean. Phytoplankton and, to a lesser extent, prokaryotes were strongly limited by inorganic nutrients. Interday variability in microbial plankton responses to B vitamins was limited compared to interseason variability, suggesting that B-vitamin availability might be partially controlled by factors operating at seasonal scale. Chlorophyll a (Chl a) concentration and prokaryote biomass (PB) significantly increased after B-vitamin amendments in 13 % and 21 %, respectively, of the 216 cases (36 experiments × 6 treatments). Most of these positive responses were produced by treatments containing either B12 solely or B12 combined with B1 in oceanic waters, which was consistent with the significantly lower average vitamin-B12 ambient concentrations compared to that in the coastal station. Negative responses, implying a decrease in Chl a or PB, represented 21 % for phytoplankton and 26 % for prokaryotes. Growth stimulation by B1 addition was more frequent on prokaryotes than in phytoplankton, suggesting that B1 auxotrophy in the sampling area could be more widespread in prokaryotes than in phytoplankton. Negative responses to B vitamins were generalized in coastal surface waters in summer and were associated with a high contribution of Flavobacteriales to the prokaryote community. This observation suggests that the external supply of B12 and/or B1 may promote negative interactions between microbial components when B-vitamin auxotrophs are abundant. The microbial response patterns to B12 and/or B1 amendments were significantly correlated with changes in the prokaryotic community composition, highlighting the pivotal role of prokaryotes in B-vitamin cycling in marine ecosystems.ENVISION (CTM2014-59031-P)INTERES (CTM2017-83362-R

Role of vitamin B12 in the microbial plankton response to nutrient enrichment

In order to evaluate the role of vitamin B12 availability in microbial plankton communities, 10 microcosm experiments were performed in which inorganic nutrients (nitrate, ammonium and phosphate) and vitamin B12 were added, separately or in combination, to surface sea - water samples drawn from an on-shelf station over an annual cycle. The responses of both autotrophic and heterotrophic microbial plankton were highly variable. Vitamin B12 addition enhanced phytoplankton biomass in early spring (March and April) and autumn (October) and bacterial biomass in winter (January) and summer (June and September). Our data reinforce the idea that inorganic nutrient limitation experienced by autotrophic and heterotrophic microbial communities is mostly dependent on the initial background nutrient conditions. The single (addition of vitamin B12 alone) or secondary vitamin B12 limitation (combined with inorganic nutrients) of phytoplankton growth implies that the availability of this molecule might modulate the increase in primary production associated with enhanced nitrogen loads to the ocean derived from human activities. These findings may have significant implications for our understanding of carbon and nutrient cycling through the planktonic microbial compartments of coastal ecosystems.Ministerio de Economía y Competitividad | Ref. CTM2014-59031-PXunta de Galicia | Ref. EM2013/02

Inputs of seabird guano alter microbial growth, community composition and the phytoplankton–bacterial interactions in a coastal system

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.Universidade de Vigo/CISUGAgencia Estatal de Investigación | Ref. CTM2017‐83362‐