Persistent equatorial Pacific iron limitation under ENSO forcing.

Projected responses of ocean net primary productivity to climate change are highly uncertain1. Models suggest that the climate sensitivity of phytoplankton nutrient limitation in the low-latitude Pacific Ocean plays a crucial role1-3, but this is poorly constrained by observations4. Here we show that changes in physical forcing drove coherent fluctuations in the strength of equatorial Pacific iron limitation through multiple El Niño/Southern Oscillation (ENSO) cycles, but that this was overestimated twofold by a state-of-the-art climate model. Our assessment was enabled by first using a combination of field nutrient-addition experiments, proteomics and above-water hyperspectral radiometry to show that phytoplankton physiological responses to iron limitation led to approximately threefold changes in chlorophyll-normalized phytoplankton fluorescence. We then exploited the >18-year satellite fluorescence record to quantify climate-induced nutrient limitation variability. Such synoptic constraints provide a powerful approach for benchmarking the realism of model projections of net primary productivity to climate changes.

Spatial and biological oceanographic insights into the massive fish-killing bloom of the haptophyte Chrysochromulina leadbeateri in northern Norway.

A bloom of the fish-killing haptophyte Chrysochromulina leadbeateri in northern Norway during May and June 2019 was the most harmful algal event ever recorded in the region, causing massive mortalities of farmed salmon. Accordingly, oceanographic and biodiversity aspects of the bloom were studied in unprecedented detail, based on metabarcoding and physico-chemical and biotic factors related with the dynamics and distribution of the bloom. Light- and electron-microscopical observations of nanoplankton samples from diverse locations confirmed that C. leadbeateri was dominant in the bloom and the primary cause of associated fish mortalities. Cell counts by light microscopy and flow cytometry were obtained throughout the regional bloom within and adjacent to five fjord systems. Metabarcoding sequences of the V4 region of the 18S rRNA gene from field material collected during the bloom and a cultured isolate from offshore of Tromsøy island confirmed the species identification. Sequences from three genetic markers (18S, 28S rRNA gene and ITS region) verified the close if not identical genetic similarity to C. leadbeateri from a previous massive fish-killing bloom in 1991 in northern Norway. The distribution and cell abundance of C. leadbeateri and related Chrysochromulina species in the recent incident were tracked by integrating observations from metabarcoding sequences of the V4 region of the 18S rRNA gene. Metabarcoding revealed at least 14 distinct Chrysochromulina variants, including putative cryptic species. C. leadbeateri was by far the most abundant of these species, but with high intraspecific genetic variability. Highest cell abundance of up to 2.7 × 107 cells L - 1 of C. leadbeateri was found in Balsfjorden; the high cell densities were associated with stratification near the pycnocline (at ca. 12 m depth) within the fjord. The cell abundance of C. leadbeateri showed positive correlations with temperature, negative correlation with salinity, and a slightly positive correlation with ambient phosphate and nitrate concentrations. The spatio-temporal succession of the C. leadbeateri bloom suggests independent initiation from existing pre-bloom populations in local zones, perhaps sustained and supplemented over time by northeastward advection of the bloom from the fjords.

Niche partitioning by photosynthetic plankton as a driver of CO2-fixation across the oligotrophic South Pacific Subtropical Ocean.

Oligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1-5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO2 fixation were low (0.1 µmol C l-1 d-1) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell 13CO2 uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65-88% of the areal CO2 fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO2 fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

Distribution and abundance of azaspiracid-producing dinophyte species and their toxins in North Atlantic and North Sea waters in 2018.

Representatives of the marine dinophyte family Amphidomataceae produce lipophilic phycotoxins called azaspiracids (AZA) which may cause azaspiracid shellfish poisoning (AZP) in humans after consumption of contaminated seafood. Three of the four known toxigenic species are observed frequently in the eastern North Atlantic. In 2018, a research survey was performed to strengthen knowledge on the distribution and abundance of toxigenic Amphidomataceae and their respective toxins in Irish coastal waters and in the North Sea. Species-specific quantification of the three toxigenic species (Azadinium spinosum, Azadinium poporum and Amphidoma languida) was based on recently developed qPCR assays, whose performance was successfully validated and tested with specificity tests and spike experiments. The multi-method approach of on-board live microscopy, qPCR assays and chemical AZA-analysis revealed the presence of Amphidomataceae in the North Atlantic including the three targeted toxigenic species and their respective AZA analogues (AZA-1, -2, -33, -38, -39). Azadinium spinosum was detected at the majority of Irish stations with a peak density of 8.3 x 104 cells L-1 and AZA (AZA-1, -2, -33) abundances up to 1,274 pg L-1. Amphidoma languida was also present at most Irish stations but appeared in highest abundance in a bloom at a central North Sea station with a density of 1.2 x 105 cells L-1 and an AZA (AZA-38, -39) abundances of 618 pg L-1. Azadinium poporum was detected sporadically at the Irish south coast and North Sea and was rather low in abundance during this study. The results confirmed the wide distribution and frequent occurrence of the target species in the North Atlantic area and revealed, for the first time, bloom abundances of toxigenic Amphidomataceae in this area. This emphasizes the importance of future studies and monitoring of amphidomatacean species and their respective AZA analogues in the North Atlantic.

Occurrence and distribution of Amphidomataceae (Dinophyceae) in Danish coastal waters of the North Sea, the Limfjord and the Kattegat/Belt area.

Some species of the dinophytes Azadinium and Amphidoma (Amphidomataceae) produce azaspiracids (AZA), lipophilic polyether compounds responsible for Azaspiracid Shellfish Poisoning (AZP) in humans after consumption of contaminated seafood. Toxigenic Amphidomataceae are known to occur in the North Atlantic and the North Sea area, but little is known about their importance in Danish coastal waters. In 2016, 44 Stations were sampled on a survey along the Danish coastline, covering the German Bight, Limfjord, the Kattegat area, Great Belt and Kiel Bight. Samples were analysed by live microscopy, liquid chromatography-tandem mass spectrometry (LC-MS/MS) and quantitative polymerase-chain-reaction (qPCR) on the presence of Amphidomataceae and AZA. Amphidomataceae were widely distributed in the area, but were below detection limit on most of the inner Limfjord stations. Cell abundances of the three toxigenic species, determined with species-specific qPCR assays on Azadinium spinosum, Az. poporum and Amphidoma languida, were generally low and restricted to the North Sea and the northern Kattegat, which was in agreement with the distribution of the generally low AZA abundances in plankton samples. Among the toxigenic species, Amphidoma languida was dominant with highest cell densities up to 3 × 103 cells L-1 on North Sea stations and at the western entrance of the Limfjord. Azaspiracids detected in plankton samples include low levels of AZA-1 at one station of the North Sea, and higher levels of AZA-38 and -39 (up to 1.5 ng L-1) in the North Sea and the Limfjord entrance. Furthermore, one new AZA (named AZA-63) was discovered in plankton of two North Sea stations. Morphological, molecular, and toxinological characterisation of 26 newly established strains from the area confirmed the presence of four amphidomatacean species (Az. obesum, Az. dalianense, Az. poporum and Am. languida). The single new strain of Az. poporum turned out as a member of Ribotype A2, which was previously only known from the Mediterranean. Consistent with some of these Mediterranean A2 strains, but different to the previously established AZA-37 producing Az. poporum Ribotype A1 strains from Denmark, the new strain did not contain any AZA. Azaspiracids were also absent in all Az. obesum and Az. dalianense strains, but AZA-38 and -39 were found in all Am. languida strains with total AZA cell quotas ranging from 0.08 up to 94 fg cell-1. In conclusion, AZA-producing microalgae and their respective toxins were low in abundance but widely present in the area, and thus might be considered in local monitoring programs to preserve seafood safety in Danish coastal waters.

Distributions of three Alexandrium species and their toxins across a salinity gradient suggest an increasing impact of GDA producing A. pseudogonyaulax in shallow brackish waters of Northern Europe.

Blooms of Alexandrium spp. are a well-known phenomenon in Northern European waters. While A. tamarense/catenella, and A. pseudogonyaulax have been reported from marine waters, high densities of A. ostenfeldii are mainly observed at lower salinities in North Sea estuaries and the Baltic Sea, suggesting salinity as a driver of Alexandrium species composition and toxin distribution. To investigate this relationship, an oceanographic expedition through a natural salinity gradient was conducted in June 2016 along the coasts of Denmark. Besides hydrographic data, phytoplankton and sediment samples were collected for analyses of Alexandrium spp. cell and cyst abundances, for toxin measurement and cell isolation. Plankton data revealed the predominance of A. pseudogonyaulax at all transect stations while A. ostenfeldii and A. catenella generally contributed a minor fraction to the Alexandrium community. High abundances of A. pseudogonyaulax in the shallow enclosed Limfjord were accompanied by high amounts of goniodomin A (GDA). This toxin was also detected at low abundances along with A. pseudogonyaulax in the North Sea and the Kattegat. Genetic and morphological characterization of established strains showed high similarity of the Northern European population to distant geographic populations. Despite low cell abundances of A. ostenfeldii, different profiles of cycloimines were measured in the North Sea and in the Limfjord. This field survey revealed that salinity alone does not determine Alexandrium species and toxin distribution, but emphasizes the importance of habitat conditions such as proximity to seed banks, shelter, and high nutrient concentrations. The results show that A. pseudogonyaulax has become a prominent member of the Alexandrium spp. community over the past decade in the study area. Analyses of long term monitoring data from the Limfjord confirmed a recent shift to A. pseudogonyaulax dominance. Cyst and toxin records of the species in Kiel Bight suggest a spreading potential into the brackish Baltic Sea, which might lead to an expansion of blooms under future climate conditions.

On-Site Analysis of Bacterial Communities of the Ultraoligotrophic South Pacific Gyre.

The South Pacific Gyre (SPG) covers 10% of the ocean's surface and is often regarded as a marine biological desert. To gain an on-site overview of the remote, ultraoligotrophic microbial community of the SPG, we developed a novel onboard analysis pipeline, which combines next-generation sequencing with fluorescence in situ hybridization and automated cell enumeration. We tested the pipeline during the SO-245 "UltraPac" cruise from Chile to New Zealand and found that the overall microbial community of the SPG was highly similar to those of other oceanic gyres. The SPG was dominated by 20 major bacterial clades, including SAR11, SAR116, the AEGEAN-169 marine group, SAR86, Prochlorococcus, SAR324, SAR406, and SAR202. Most of the bacterial clades showed a strong vertical (20 m to 5,000 m), but only a weak longitudinal (80°W to 160°W), distribution pattern. Surprisingly, in the central gyre, Prochlorococcus, the dominant photosynthetic organism, had only low cellular abundances in the upper waters (20 to 80 m) and was more frequent around the 1% irradiance zone (100 to 150 m). Instead, the surface waters of the central gyre were dominated by the SAR11, SAR86, and SAR116 clades known to harbor light-driven proton pumps. The alphaproteobacterial AEGEAN-169 marine group was particularly abundant in the surface waters of the central gyre, indicating a potentially interesting adaptation to ultraoligotrophic waters and high solar irradiance. In the future, the newly developed community analysis pipeline will allow for on-site insights into a microbial community within 35 h of sampling, which will permit more targeted sampling efforts and hypothesis-driven research.IMPORTANCE The South Pacific Gyre, due to its vast size and remoteness, is one of the least-studied oceanic regions on earth. However, both remote sensing and in situ measurements indicated that the activity of its microbial community contributes significantly to global biogeochemical cycles. Presented here is an unparalleled investigation of the microbial community of the SPG from 20- to 5,000-m depths covering a geographic distance of ∼7,000 km. This insight was achieved through the development of a novel onboard analysis pipeline, which combines next-generation sequencing with fluorescence in situ hybridization and automated cell enumeration. The pipeline is well comparable to onshore systems based on the Illumina platforms and yields microbial community data in less than 35 h after sampling. Going forward, the ability to gain on-site knowledge of a remote microbial community will permit hypothesis-driven research, through the generation of novel scientific questions and subsequent additional targeted sampling efforts.

Diversity, distribution, and azaspiracids of Amphidomataceae (Dinophyceae) along the Norwegian coast.

Azaspiracids (AZA) are a group of lipophilic polyether compounds which have been implicated in shellfish poisoning incidents around Europe. They are produced by a few species of the dinophycean genera Azadinium and Amphidoma (Amphidomataceae). The presence of AZA toxins in Norway is well documented, but knowledge of the distribution and diversity of Azadinium and other Amphidomataceae along the Norwegian coast is rather limited and poorly documented. On a research survey along the Norwegian coast in 2015 from the Skagerrak in the South to Trondheimsfjorden in the North, plankton samples from 67 stations were analysed for the presence of Azadinium and Amphidoma and their respective AZA by on-board live microscopy, real-time PCR assays specific for Amphidomataceae, and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Microscopy using live samples and positive real-time PCR assays using a general family probe and two species specific probes revealed the presence of Amphidomataceae distributed throughout the sampling area. Overall abundance was low, however, and was in agreement with a lack of detectable AZA in plankton samples. Single cell isolation and morphological and molecular characterisation of established strains revealed the presence of 7 amphidomatacean species (Azadiniun spinosum, Az. poporum, Az. obesum, Az. dalianense, Az. trinitatum, Az. polongum, Amphidoma languida) in the area. Azaspiracids were produced by the known AZA producing species Az. spinosum, Az. poporum and Am. languida only. LC-MS/MS analysis further revealed that Norwegian strains produce previously unreported AZA for Norway (AZA-11 by Az. spinosum, AZA-37 by Az. poporum, AZA-38 and AZA-39 by Am. languida), and also four novel compounds (AZA-50, -51 by Az. spinosum, AZA-52, -53 by Am. languida), whose structural properties are described and which now can be included in existing analytical protocols. A maximum likelihood analysis of concatenated rDNA regions (SSU, ITS1-ITS2, partial LSU) showed that the strains of Az. spinosum fell in two well supported clades, where most but not all new Norwegian strains formed the new Ribotype B. Ribotype differentiation was supported by a minor morphological difference with respect to the presence/absence of a rim around the pore plate, and was consistently reflected by different AZA profiles. Strains of Az. spinosum from ribotype A produce AZA-1, -2 and -33, whereas the new strains of ribotype B produce mainly AZA-11 and AZA-51. Significant sequence differences between both Az. spinosum ribotypes underline the need to redesign the currently used qPCR probes in order to detect all AZA producing Az. spinosum. The results generally underline the conclusion that for the Norwegian coast area it is important that amphidomatacean species are taken into account in future studies and monitoring programs.

Classifying Natural Waters with the Forel-Ule Colour Index System: Results, Applications, Correlations and Crowdsourcing.

Societal awareness of changes in the environment and climate has grown rapidly, and there is a need to engage citizens in gathering relevant scientific information to monitor environmental changes due to recognition that citizens are a potential source of critical information. The apparent colour of natural waters is one aspect of our aquatic environment that is easy to detect and an essential complementary optical water quality indicator. Here we present the results and explore the utility of the Forel-Ule colour index (FUI) scale as a proxy for different properties of natural waters. A FUI scale is used to distinguish the apparent colours of different natural surface water masses. Correlation analysis was completed in an effort to determine the constituents of natural waters related to FUI. Strong correlations with turbidity, Secchi-disk depth, and coloured dissolved organic material suggest the FUI is a good indicator of changes related to other constituents of water. The increase in the number of tools capable of determining the FUI colours, (i) ocean colour remote sensing products; (ii) a handheld scale; and (iii) a mobile device app, make it a versatile relative measure of water quality. It has the potential to provide higher spatial and temporal resolution of data for a modernized classification of optical water quality. This FUI colour system has been favoured by several scientists in the last century because it is affordable and easy to use and provides indicative information about the colour of water and the water constituents producing that colour. It is therefore within the scope of a growing interest in the application and usefulness of basic measurement methodologies with the potential to provide timely benchmark information about the environment to the public, scientists and policymakers.

New azaspiracids in Amphidomataceae (Dinophyceae).

Azaspiracids (AZAs) are a group of lipophilic polyether toxins implicated in incidents of shellfish poisoning in humans, particularly in northern Europe, which are produced by the small marine dinoflagellate Azadinium spinosum. Other related species/strains of the Amphidomataceae have not been proven to date to contain any of the known azaspiracids. Closer analyses of these species/strains by triple quadrupole mass spectrometry in the precursor and product ion mode now revealed four new compounds with high similarity to azaspiracids, all of them with a characteristic m/z 348 fragment but with absence of the m/z 362 fragment. These compounds were detected in three species/strains, i.e. in North Sea isolates of Azadinium poporum (molecular mass: 845.5 Da), in a Korean isolate which has been designated as A. cf. poporum (molecular mass: 857.5 Da) and in Amphidoma languida isolated from Bantry Bay, Ireland (molecular masses: 815.5 and 829.5 Da). Cell quotas of roughly 2-20 fg per cell were in the same range as found for AZA-1/-2 in A. spinosum. Structures for all compounds were proposed by interpretation of fragmentation patterns and high resolution mass measurements using Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS).