Growth and mortality of aerobic anoxygenic phototrophs in the North Pacific Subtropical Gyre.

Aerobic anoxygenic phototrophic (AAP) bacteria harvest light energy using bacteriochlorophyll-containing reaction centers to supplement their mostly heterotrophic metabolism. While their abundance and growth have been intensively studied in coastal environments, much less is known about their activity in oligotrophic open ocean regions. Therefore, we combined in situ sampling in the North Pacific Subtropical Gyre, north of O'ahu island, Hawaii, with two manipulation experiments. Infra-red epifluorescence microscopy documented that AAP bacteria represented approximately 2% of total bacteria in the euphotic zone with the maximum abundance in the upper 50 m. They conducted active photosynthetic electron transport with maximum rates up to 50 electrons per reaction center per second. The in situ decline of bacteriochlorophyll concentration over the daylight period, an estimate of loss rates due to predation, indicated that the AAP bacteria in the upper 50 m of the water column turned over at rates of 0.75-0.90 d-1. This corresponded well with the specific growth rate determined in dilution experiments where AAP bacteria grew at a rate 1.05 ± 0.09 d-1. An amendment of inorganic nitrogen to obtain N:P = 32 resulted in a more than 10 times increase in AAP abundance over 6 days. The presented data document that AAP bacteria are an active part of the bacterioplankton community in the oligotrophic North Pacific Subtropical Gyre and that their growth was mostly controlled by nitrogen availability and grazing pressure.IMPORTANCEMarine bacteria represent a complex assembly of species with different physiology, metabolism, and substrate preferences. We focus on a specific functional group of marine bacteria called aerobic anoxygenic phototrophs. These photoheterotrophic organisms require organic carbon substrates for growth, but they can also supplement their metabolic needs with light energy captured by bacteriochlorophyll. These bacteria have been intensively studied in coastal regions, but rather less is known about their distribution, growth, and mortality in the oligotrophic open ocean. Therefore, we conducted a suite of measurements in the North Pacific Subtropical Gyre to determine the distribution of these organisms in the water column and their growth and mortality rates. A nutrient amendment experiment showed that aerobic anoxygenic phototrophs were limited by inorganic nitrogen. Despite this, they grew more rapidly than average heterotrophic bacteria, but their growth was balanced by intense grazing pressure.

Seasonality of biogeochemically relevant microbial genes in a coastal ocean microbiome.

Microbes drive the biogeochemical cycles of marine ecosystems through their vast metabolic diversity. While we have a fairly good understanding of the spatial distribution of these metabolic processes in various ecosystems, less is known about their seasonal dynamics. We investigated the annual patterns of 21 biogeochemical relevant functions in an oligotrophic coastal ocean site by analysing the presence of key genes, analysing high-rank gene taxonomy and the dynamics of nucleotide variants. Most genes presented seasonality: photoheterotrophic processes were enriched during spring, phosphorous-related genes were dominant during summer, coinciding with potential phosphate limitation, and assimilatory nitrate reductases appeared mostly during summer and autumn, correlating negatively with nitrate availability. Additionally, we identified the main taxa driving each function at each season and described the role of underrecognized taxa such as Litoricolaceae in carbon fixation (rbcL), urea degradation (ureC), and CO oxidation (coxL). Finally, the seasonality of single variants of some families presented a decoupling between the taxonomic abundance patterns and the functional gene patterns, implying functional specialization of the different genera. Our study unveils the seasonality of key biogeochemical functions and the main taxonomic groups that harbour these relevant functions in a coastal ocean ecosystem.

A Metagenomic and Amplicon Sequencing Combined Approach Reveals the Best Primers to Study Marine Aerobic Anoxygenic Phototrophs.

Studies based on protein-coding genes are essential to describe the diversity within bacterial functional groups. In the case of aerobic anoxygenic phototrophic (AAP) bacteria, the pufM gene has been established as the genetic marker for this particular functional group, although available primers are known to have amplification biases. We review here the existing primers for pufM gene amplification, design new ones, and evaluate their phylogenetic coverage. We then use samples from contrasting marine environments to evaluate their performance. By comparing the taxonomic composition of communities retrieved with metagenomics and with different amplicon approaches, we show that the commonly used PCR primers are biased towards the Gammaproteobacteria phylum and some Alphaproteobacteria clades. The metagenomic approach, as well as the use of other combinations of the existing and newly designed primers, show that these groups are in fact less abundant than previously observed, and that a great proportion of pufM sequences are affiliated to uncultured representatives, particularly in the open ocean. Altogether, the framework developed here becomes a better alternative for future studies based on the pufM gene and, additionally, serves as a reference for primer evaluation of other functional genes.

Global diversity and distribution of aerobic anoxygenic phototrophs in the tropical and subtropical oceans.

The aerobic anoxygenic phototrophic (AAP) bacteria are common in most marine environments but their global diversity and biogeography remain poorly characterized. Here, we analyzed AAP communities across 113 globally-distributed surface ocean stations sampled during the Malaspina Expedition in the tropical and subtropical ocean. By means of amplicon sequencing of the pufM gene, a genetic marker for this functional group, we show that AAP communities along the surface ocean were mainly composed of members of the Halieaceae (Gammaproteobacteria), which were adapted to a large range of environmental conditions, and of different clades of the Alphaproteobacteria, which seemed to dominate under particular circumstances, such as in the oligotrophic gyres. AAP taxa were spatially structured within each of the studied oceans, with communities from adjacent stations sharing more taxonomic similarities. AAP communities were composed of a large pool of rare members and several habitat specialists. When compared to the surface ocean prokaryotic and picoeukaryotic communities, it appears that AAP communities display an idiosyncratic global biogeographical pattern, dominated by selection processes and less influenced by dispersal limitation. Our study contributes to the understanding of how AAP communities are distributed in the horizontal dimension and the mechanisms underlying their distribution across the global surface ocean.

Taxonomic differences shape the responses of freshwater aerobic anoxygenic phototrophic bacterial communities to light and predation.

Aerobic anoxygenic phototrophic (AAP) bacteria are a phylogenetically diverse and ubiquitous group of prokaryotes that use organic matter but can harvest light using bacteriochlorophyll a. Although the factors regulating AAP ecology have long been investigated through field surveys, the few available experimental studies have considered AAPs as a group, thus disregarding the potential differential responses between taxonomically distinct AAP assemblages. Here, we used sequencing of the pufM gene to describe the diversity of AAPs in 10 environmentally distinct temperate lakes, and to investigate the taxonomic responses of AAP communities in these lakes when subjected to similar experimental manipulations of light and predator removal. The studied communities were clearly dominated by Limnohabitans AAP but presented a clear taxonomic segregation between lakes presumably driven by local conditions, which was maintained after experimental manipulations. Predation reduction (but not light exposure) caused significant compositional shifts across most assemblages, but the magnitude of these changes could not be clearly related to changes in bulk AAP abundances or taxonomic richness of AAP assemblages during experiments. Only a few operational taxonomic units, which differed taxonomically between lakes, were found to respond positively during experimental treatments. Our results highlight that different freshwater AAP communities respond differently to similar control mechanisms, highlighting that in-depth knowledge on AAP diversity is essential to understand the ecology and potential role of these photoheterotrophs.

Impact of grazing, resource availability and light on prokaryotic growth and diversity in the oligotrophic surface global ocean.

The impact of grazing, resource competition and light on prokaryotic growth and taxonomic composition in subtropical and tropical surface waters were studied through 10 microcosm experiments conducted between 30°N and 30°S in the Atlantic, Pacific and Indian oceans. Under natural sunlight conditions, significant changes in taxonomic composition were only observed after the reduction of grazing by sample filtration in combination with a decrease in resource competition by sample dilution. Sunlight exposure significantly reduced prokaryote growth (11 ± 6%) and community richness (14 ± 4%) compared to continuous darkness but did not significantly change community composition. The largest growth inhibition after sunlight exposure occurred at locations showing deep mixed layers. The reduction of grazing had an expected and significant positive effect on growth, but caused a significant decrease in community richness (16 ± 6%), suggesting that the coexistence of many different OTUs is partly promoted by the presence of predators. Dilution of the grazer-free prokaryotic community significantly enhanced growth at the level of community, but consistently and sharply reduced the abundance of Prochlorococcus and SAR11 populations. The decline of these oligotrophic bacterial taxa following an increase in resource availability is consistent with their high specialization for exploiting the limited resources available in the oligotrophic warm ocean.

Primer Design for an Accurate View of Picocyanobacterial Community Structure by Using High-Throughput Sequencing.

High-throughput sequencing (HTS) of the 16S rRNA gene has been used successfully to describe the structure and dynamics of microbial communities. Picocyanobacteria are important members of bacterioplankton communities, and, so far, they have predominantly been targeted using universal bacterial primers, providing a limited resolution of the picocyanobacterial community structure and dynamics. To increase such resolution, the study of a particular target group is best approached with the use of specific primers. Here, we aimed to design and evaluate specific primers for aquatic picocyanobacterial genera to be used with high-throughput sequencing. Since the various regions of the 16S rRNA gene have different degrees of conservation in different bacterial groups, we therefore first determined which hypervariable region of the 16S rRNA gene provides the highest taxonomic and phylogenetic resolution for the genera Synechococcus, Prochlorococcus, and Cyanobium An in silico analysis showed that the V5, V6, and V7 hypervariable regions appear to be the most informative for this group. We then designed primers flanking these hypervariable regions and tested them in natural marine and freshwater communities. We successfully detected that most (97%) of the obtained reads could be assigned to picocyanobacterial genera. We defined operational taxonomic units as exact sequence variants (zero-radius operational taxonomic units [zOTUs]), which allowed us to detect higher genetic diversity and infer ecologically relevant information about picocyanobacterial community composition and dynamics in different aquatic systems. Our results open the door to future studies investigating picocyanobacterial diversity in aquatic systems.IMPORTANCE The molecular diversity of the aquatic picocyanobacterial community cannot be accurately described using only the available universal 16S rRNA gene primers that target the whole bacterial and archaeal community. We show that the hypervariable regions V5, V6, and V7 of the 16S rRNA gene are better suited to study the diversity, community structure, and dynamics of picocyanobacterial communities at a fine scale using Illumina MiSeq sequencing. Due to its variability, it allows reconstructing phylogenies featuring topologies comparable to those generated when using the complete 16S rRNA gene sequence. Further, we successfully designed a new set of primers flanking the V5 to V7 region whose specificity for picocyanobacterial genera was tested in silico and validated in several freshwater and marine aquatic communities. This work represents a step forward for understanding the diversity and ecology of aquatic picocyanobacteria and sets the path for future studies on picocyanobacterial diversity.

Quantifying long-term recurrence in planktonic microbial eukaryotes.

How much temporal recurrence is present in microbial assemblages is still an unanswered ecological question. Even though marked seasonal changes have been reported for whole microbial communities, less is known on the dynamics and seasonality of individual taxa. Here, we aim at understanding microbial recurrence at three different levels: community, taxonomic group and operational taxonomic units (OTUs). For that, we focused on a model microbial eukaryotic community populating a long-term marine microbial observatory using 18S rRNA gene data from two organismal size fractions: the picoplankton (0.2-3 µm) and the nanoplankton (3-20 µm). We have developed an index to quantify recurrence in particular taxa. We found that community structure oscillated systematically between two main configurations corresponding to winter and summer over the 10 years studied. A few taxonomic groups such as Mamiellophyceae or MALV-III presented clear recurrence (i.e., seasonality), whereas 13%-19% of the OTUs in both size fractions, accounting for ~40% of the relative abundance, featured recurrent dynamics. Altogether, our work links long-term whole community dynamics with that of individual OTUs and taxonomic groups, indicating that recurrent and non-recurrent changes characterize the dynamics of microbial assemblages.

Delineation of ecologically distinct units of marine Bacteroidetes in the Northwestern Mediterranean Sea.

Bacteroidetes is one of the dominant phyla of ocean bacterioplankton, yet its diversity and population structure is poorly understood. To advance in the delineation of ecologically meaningful units within this group, we constructed near full-length 16S rRNA gene clone libraries from contrasting marine environments in the NW Mediterranean. Based on phylogeny and the associated ecological variables (depth and season), 24 different Bacteroidetes clades were delineated. By considering their relative abundance (from iTag amplicon sequencing studies), we described the distribution patterns of each of these clades, delimiting them as Ecologically Significant Taxonomic Units (ESTUs). Spatially, there was almost no overlap among ESTUs at different depths. In deep waters there was predominance of Owenweeksia, Leeuwenhoekiella, Muricauda-related genera, and some depth-associated ESTUs within the NS5 and NS2b marine clades. Seasonally, multi-annual dynamics of recurring ESTUs were present with dominance of some ESTUs within the NS4, NS5 and NS2b marine clades along most of the year, but with variable relative frequencies between months. A drastic change towards the predominance of Formosa-related ESTUs and one ESTU from the NS5 marine clade was typically present after the spring bloom. Even though there are no isolates available for these ESTUs to determine their physiology, correlation analyses identified the environmental preference of some of them. Overall, our results suggest that there is a high degree of niche specialisation within these closely related clades. This work constitutes a step forward in disentangling the ecology of marine Bacteroidetes, which are essential players in organic matter processing in the oceans.

Electrochemical performance and microbial community profiles in microbial fuel cells in relation to electron transfer mechanisms.

BACKGROUND: Microbial fuel cells (MFCs) operating with complex microbial communities have been extensively reported in the past, and are commonly used in applications such as wastewater treatment, bioremediation or in-situ powering of environmental sensors. However, our knowledge on how the composition of the microbial community and the different types of electron transfer to the anode affect the performance of these bioelectrochemical systems is far from complete. To fill this gap of knowledge, we designed a set of three MFCs with different constrains limiting direct and mediated electron transfer to the anode. RESULTS: The results obtained indicate that MFCs with a naked anode on which a biofilm was allowed unrestricted development (MFC-A) had the most diverse archaeal and bacterial community, and offered the best performance. In this MFC both, direct and mediated electron transfer, occurred simultaneously, but direct electron transfer was the predominant mechanism. Microbial fuel cells in which the anode was enclosed in a dialysis membrane and biofilm was not allowed to develop (MFC-D), had a much lower power output (about 60% lower), and a prevalence of dissolved redox species that acted as putative electron shuttles. In the anolyte of this MFC, Arcobacter and Methanosaeta were the prevalent bacteria and archaea respectively. In the third MFC, in which the anode had been covered by a cation selective nafion membrane (MFC-N), power output decreased a further 5% (95% less than MFC-A). In this MFC, conventional organic electron shuttles could not operate and the low power output obtained was presumably attributed to fermentation end-products produced by some of the organisms present in the anolyte, probably Pseudomonas or Methanosaeta. CONCLUSION: Electron transfer mechanisms have an impact on the development of different microbial communities and in turn on MFC performance. Although a stable current was achieved in all cases, direct electron transfer MFC showed the best performance concluding that biofilms are the major contributors to current production in MFCs. Characterization of the complex microbial assemblages in these systems may help us to unveil new electrogenic microorganisms and improve our understanding on their role to the functioning of MFCs.

Spatial variability of marine bacterial and archaeal communities along the particulate matter continuum.

Biotic and abiotic particles shape the microspatial architecture that defines the microbial aquatic habitat, being particles highly variable in size and quality along oceanic horizontal and vertical gradients. We analysed the prokaryotic (bacterial and archaeal) diversity and community composition present in six distinct particle size classes ranging from the pico- to the microscale (0.2 to 200 µm). Further, we studied their variations along oceanographic horizontal (from the coast to open oceanic waters) and vertical (from the ocean surface into the meso- and bathypelagic ocean) gradients. In general, prokaryotic community composition was more variable with depth than in the transition from the coast to the open ocean. Comparing the six size-fractions, distinct prokaryotic communities were detected in each size-fraction, and whereas bacteria were more diverse in the larger size-fractions, archaea were more diverse in the smaller size-fractions. Comparison of prokaryotic community composition among particle size-fractions showed that most, but not all, taxonomic groups have a preference for a certain size-fraction sustained with depth. Species sorting, or the presence of diverse ecotypes with distinct size-fraction preferences, may explain why this trend is not conserved in all taxa.

Validation of a new catalysed reporter deposition-fluorescence in situ hybridization probe for the accurate quantification of marine Bacteroidetes populations.

Catalysed reporter deposition-fluorescence in situ hybridization (CARD-FISH) is a powerful approach to quantify bacterial taxa. In this study, we compare the performance of the widely used Bacteroidetes CF319a probe with the new CF968 probe. In silico analyses and tests with isolates demonstrate that CF319a hybridizes with non-Bacteroidetes sequences from the Rhodobacteraceae and Alteromonadaceae families. We test the probes' accuracy in 37 globally distributed marine samples and over two consecutive years at the Blanes Bay Microbial Observatory (NW Mediterranean). We also compared the CARD-FISH data with the Bacteroidetes 16S rRNA gene sequences retrieved from 27 marine metagenomes from the TARA Oceans expedition. We find no significant differences in abundances between both approaches, although CF319a targeted some unspecific sequences and both probes displayed different abundances of specific Bacteroidetes phylotypes. Our results demonstrate that quantitative estimations by using both probes are significantly different in certain oceanographic regions (Mediterranean Sea, Red Sea and Arabian Sea) and that CF968 shows seasonality within marine Bacteroidetes, notably large differences between summer and winter that is overlooked by CF319a. We propose CF968 as an alternative to CF319a for targeting the whole Bacteroidetes phylum since it has better coverage, greater specificity and overall better quantifies marine Bacteroidetes.

Sample dilution and bacterial community composition influence empirical leucine-to-carbon conversion factors in surface waters of the world's oceans.

The transformation of leucine incorporation rates to prokaryotic carbon production rates requires the use of either theoretical or empirically determined conversion factors. Empirical leucine-to-carbon conversion factors (eCFs) vary widely across environments, and little is known about their potential controlling factors. We conducted 10 surface seawater manipulation experiments across the world's oceans, where the growth of the natural prokaryotic assemblages was promoted by filtration (i.e., removal of grazers [F treatment]) or filtration combined with dilution (i.e., also relieving resource competition [FD treatment]). The impact of sunlight exposure was also evaluated in the FD treatments, and we did not find a significant effect on the eCFs. The eCFs varied from 0.09 to 1.47 kg C mol Leu(-1) and were significantly lower in the FD than in the F samples. Also, changes in bacterial community composition during the incubations, as assessed by automated ribosomal intergenic spacer analysis (ARISA), were more pronounced in the FD than in the F treatments, compared to unmanipulated controls. Thus, we discourage the common procedure of diluting samples (in addition to filtration) for eCF determination. The eCFs in the filtered treatment were negatively correlated with the initial chlorophyll a concentration, picocyanobacterial abundance (mostly Prochlorococcus), and the percentage of heterotrophic prokaryotes with high nucleic acid content (%HNA). The latter two variables explained 80% of the eCF variability in the F treatment, supporting the view that both Prochlorococcus and HNA prokaryotes incorporate leucine in substantial amounts, although this results in relatively low carbon production rates in the oligotrophic ocean.

Biological support media influence the bacterial biofouling community in reverse osmosis water reclamation demonstration plants.

The diversity of the bacterial community developed in different stages of two reverse osmosis (RO) water reclamation demonstration plants designed in a wastewater treatment plant (WWTP) in Tarragona (Spain) was characterized by applying 454-pyrosequencing of the 16S rRNA gene. The plants were fed by secondary treated effluent to a conventional pretreatment train prior to the two-pass RO system. Plants differed in the material used in the filtration process, which was sand in one demonstration plant and Scandinavian schists in the second plant. The results showed the presence of a highly diverse and complex community in the biofilms, mainly composed of members of the Betaproteobacteria and Bacteroidetes in all stages, with the presence of some typical wastewater bacteria, suggesting a feed water origin. Community similarities analyses revealed that samples clustered according to filter type, highlighting the critical influence of the biological supporting medium in biofilm community structure.

Marked seasonality of aerobic anoxygenic phototrophic bacteria in the coastal NW Mediterranean Sea as revealed by cell abundance, pigment concentration and pyrosequencing of pufM gene.

The abundance and diversity of aerobic anoxygenic phototrophs (AAPs) were studied for a year cycle at the Blanes Bay Microbial Observatory (NW Mediterranean) and their potential links to an array of environmental variables were explored. Cell numbers were low in winter and peaked in summer, showing a marked seasonality that positively correlated with day length and light at the surface. Bacteriochlorophyll a concentration, their light-harvesting pigment, was only detected between April and October, and pigment cell quota showed large variations during this period. Pyrosequencing analysis of the pufM gene revealed that the most abundant operational taxonomic units (OTUs) were affiliated to phylogroup K (Gammaproteobacteria) and uncultured phylogroup C, although they were outnumbered by alphaproteobacterial OTUs in spring. Overall, richness was higher in winter than in summer, showing an opposite trend to abundance and day length. Clustering of samples by multivariate analyses showed a clear seasonality that suggests a succession of different AAP subpopulations over time. Temperature, chlorophyll a and day length were the environmental drivers that best explained the distribution of AAP assemblages. These results indicate that AAP bacteria are highly dynamic and undergo seasonal variations in diversity and abundance mostly dictated by environmental conditions as exemplified by light availability.

Metagenomic 16S rDNA Illumina tags are a powerful alternative to amplicon sequencing to explore diversity and structure of microbial communities.

Sequencing of 16S rDNA polymerase chain reaction (PCR) amplicons is the most common approach for investigating environmental prokaryotic diversity, despite the known biases introduced during PCR. Here we show that 16S rDNA fragments derived from Illumina-sequenced environmental metagenomes (mi tags) are a powerful alternative to 16S rDNA amplicons for investigating the taxonomic diversity and structure of prokaryotic communities. As part of the Tara Oceans global expedition, marine plankton was sampled in three locations, resulting in 29 subsamples for which metagenomes were produced by shotgun Illumina sequencing (ca. 700 Gb). For comparative analyses, a subset of samples was also selected for Roche-454 sequencing using both shotgun (m454 tags; 13 metagenomes, ca. 2.4 Gb) and 16S rDNA amplicon (454 tags; ca. 0.075 Gb) approaches. Our results indicate that by overcoming PCR biases related to amplification and primer mismatch, mi tags may provide more realistic estimates of community richness and evenness than amplicon 454 tags. In addition, mi tags can capture expected beta diversity patterns. Using mi tags is now economically feasible given the dramatic reduction in high-throughput sequencing costs, having the advantage of retrieving simultaneously both taxonomic (Bacteria, Archaea and Eukarya) and functional information from the same microbial community.

Prevalence of potentially thermophilic microorganisms in biofilms from greenhouse-enclosed drip irrigation systems.

Drip irrigation systems using reclaimed water often present clogging events of biological origin. Microbial communities in biofilms from microirrigation systems of an experimental greenhouse in Almería, SE Spain, which used two different qualities of water (treated wastewater and reclaimed water), were analyzed by denaturing gradient gel electrophoresis and subsequent sequencing of amplified 16S rRNA gene bands. The most remarkable feature of all biofilms was that regardless of water origin, sequences belonging to Firmicutes were prevalent (53.5 % of total mean band intensity) and that almost all sequences recovered had some similarity (between 80.2 and 97 %) to thermophilic microorganisms. Mainly, sequences were closely related to potentially spore-forming organisms, suggesting that microbial communities able to grow at high temperatures were selected from the microbiota present in the incoming water. These pioneer results may contribute to improve management strategies to minimize the problems associated to biofouling in irrigation systems.

Growth rates of marine prokaryotes are extremely diverse, even among closely related taxa.

Marine prokaryotes play crucial roles in ocean biogeochemical cycles, being their contribution strongly influenced by their growth rates. Hence, elucidating the variability and phylogenetic imprint of marine prokaryotes' growth rates are crucial for better determining the role of individual taxa in biogeochemical cycles. Here, we estimated prokaryotic growth rates at high phylogenetic resolution in manipulation experiments using water from the northwestern Mediterranean Sea. Experiments were run in the four seasons with different treatments that reduced growth limiting factors: predators, nutrient availability, viruses, and light. Single-amplicon sequence variants (ASVs)-based growth rates were calculated from changes in estimated absolute abundances using total prokaryotic abundance and the proportion of each individual ASV. The trends obtained for growth rates in the different experiments were consistent with other estimates based on total cell-counts, catalyzed reporter deposition fluorescence in situ hybridization subcommunity cell-counts or metagenomic-operational taxonomic units (OTUs). Our calculations unveil a broad range of growth rates (0.3-10 d-1) with significant variability even within closely related ASVs. Likewise, the impact of growth limiting factors changed over the year for individual ASVs. High numbers of responsive ASVs were shared between winter and spring seasons, as well as throughout the year in the treatments with reduced nutrient limitation and viral pressure. The most responsive ASVs were rare in the in situ communities, comprising a large pool of taxa with the potential to rapidly respond to environmental changes. Essentially, our results highlight the lack of phylogenetic coherence in the range of growth rates observed, and differential responses to the various limiting factors, even for closely related taxa.

Impacts of a newly formed lava delta on the marine environment: Lava-induced upwelling and abrupt chlorophyll depletion during the Tajogaite eruption (La Palma, 2021).

The 2021 Tajogaite eruption in La Palma (Canary Islands, Spain) emitted vast volumes of lava during 85 days, which reached the ocean in several occasions at the western flank of the island. Most of these flows merged to create a primary lava delta, covering an area of 48 ha, with an additional 30 ha underwater. Here we characterize the effects of the lava-seawater interaction on the surrounding marine environment. The area was sampled during two multidisciplinary oceanographic cruises: the first one comprised the days before the lava reached the ocean and after the first contact; and the second took place a month later, when the lava delta was already formed but still receiving lava inputs. Physical-chemical anomalies were found in the whole water column at different depths up to 300 m in all measured parameters, such as turbidity (+9 NTU), dissolved oxygen concentration (-17.17 µmol kg-1), pHT25 (-0.1), and chlorophyll-a concentration (-0.33 mg m-3). Surface temperature increased up to +2.3 °C (28.5 °C) and surface salinity showed increases and decreases of -1.01 and +0.70, respectively, in a radius of 4 km around the lava delta. In the water column, the heated waters experimented a lava-induced upwelling, bringing deeper, nutrient-rich waters to shallower depths; however, this feature did not trigger any phytoplankton bloom. In fact, integrated chlorophyll-a showed an abrupt decrease of -41 % in just two days and -69 % a month later, compared to prior conditions. The chlorophyll-a depletion reached a distance larger than 2.5 km (not delimited).

Phytoplankton dynamics in the Mar Menor, a Mediterranean coastal lagoon strongly impacted by eutrophication.

The Mar Menor hypersaline coastal lagoon has suffered serious degradation in the last three decades attributable to nutrient pollution. In 2015, the lagoon experienced an intensive bloom of cyanobacteria that triggered a drastic change of its ecosystem. Our analyses indicate that phytoplankton in 2016-2021 did not present a seasonal variability pattern; the community was mainly dominated by diatoms and punctually reached abundance peaks above 107 cell L-1 along with chlorophyll a concentrations exceeding 20 µg L-1. The predominant diatom genera during these blooms were different as well as the nutrient conditions under which they were produced. These high diatom abundances are unprecedented in the lagoon; in fact, our data indicate that the taxonomic composition, time variation patterns and cell abundance of phytoplankton in 2016-2021 differ notably in comparison to the data published before 2015. Consequently, our results support the finding that the trophic status of the lagoon has changed profoundly.