Plankton assessment across the distribution of West African hake and tuna based on eDNA metabarcoding.

The richness of plankton communities determines the fish productivity in the ocean, including important resources that rely on extractive fisheries, such as hakes (genus Merluccius) and tunas (genus Thunnus). Their preys forage on zooplankton, and the latter feed on phytoplankton. Inventories of plankton communities for scientific advice to sustainable fishing are essential in this moment of climate change. Plankton is generally inventoried using conventional methodologies based on large water volumes and visual morphological analyses of samples. In this study, we have employed metabarcoding on environmental DNA (eDNA) samples extracted from small water volumes for plankton inventory from twelve distant sampling stations in the East Atlantic Ocean. Zones rich in hake and tuna prey were detected from eDNA, and multivariate multiple regression analysis was able to predict those zones from diatom-based indices and planktonic diversity based on functional groups. Salinity was negatively correlated with the proportion of diatoms in phytoplankton, highlighting expected impacts of current global change on marine plankton communities. The results emphasise the importance of the plankton richness for fish productivity and support the utility of environmental DNA as a tool to monitor plankton composition changes.

Particle-attached bacteria act as gatekeepers in the decomposition of complex phytoplankton polysaccharides.

BACKGROUND: Marine microalgae (phytoplankton) mediate almost half of the worldwide photosynthetic carbon dioxide fixation and therefore play a pivotal role in global carbon cycling, most prominently during massive phytoplankton blooms. Phytoplankton biomass consists of considerable proportions of polysaccharides, substantial parts of which are rapidly remineralized by heterotrophic bacteria. We analyzed the diversity, activity, and functional potential of such polysaccharide-degrading bacteria in different size fractions during a diverse spring phytoplankton bloom at Helgoland Roads (southern North Sea) at high temporal resolution using microscopic, physicochemical, biodiversity, metagenome, and metaproteome analyses. RESULTS: Prominent active 0.2-3 µm free-living clades comprised Aurantivirga, "Formosa", Cd. Prosiliicoccus, NS4, NS5, Amylibacter, Planktomarina, SAR11 Ia, SAR92, and SAR86, whereas BD1-7, Stappiaceae, Nitrincolaceae, Methylophagaceae, Sulfitobacter, NS9, Polaribacter, Lentimonas, CL500-3, Algibacter, and Glaciecola dominated 3-10 µm and > 10 µm particles. Particle-attached bacteria were more diverse and exhibited more dynamic adaptive shifts over time in terms of taxonomic composition and repertoires of encoded polysaccharide-targeting enzymes. In total, 305 species-level metagenome-assembled genomes were obtained, including 152 particle-attached bacteria, 100 of which were novel for the sampling site with 76 representing new species. Compared to free-living bacteria, they featured on average larger metagenome-assembled genomes with higher proportions of polysaccharide utilization loci. The latter were predicted to target a broader spectrum of polysaccharide substrates, ranging from readily soluble, simple structured storage polysaccharides (e.g., laminarin, α-glucans) to less soluble, complex structural, or secreted polysaccharides (e.g., xylans, cellulose, pectins). In particular, the potential to target poorly soluble or complex polysaccharides was more widespread among abundant and active particle-attached bacteria. CONCLUSIONS: Particle-attached bacteria represented only 1% of all bloom-associated bacteria, yet our data suggest that many abundant active clades played a pivotal gatekeeping role in the solubilization and subsequent degradation of numerous important classes of algal glycans. The high diversity of polysaccharide niches among the most active particle-attached clades therefore is a determining factor for the proportion of algal polysaccharides that can be rapidly remineralized during generally short-lived phytoplankton bloom events. Video Abstract.

Dissolved storage glycans shaped the community composition of abundant bacterioplankton clades during a North Sea spring phytoplankton bloom.

BACKGROUND: Blooms of marine microalgae play a pivotal role in global carbon cycling. Such blooms entail successive blooms of specialized clades of planktonic bacteria that collectively remineralize gigatons of algal biomass on a global scale. This biomass is largely composed of distinct polysaccharides, and the microbial decomposition of these polysaccharides is therefore a process of prime importance. RESULTS: In 2020, we sampled a complete biphasic spring bloom in the German Bight over a 90-day period. Bacterioplankton metagenomes from 30 time points allowed reconstruction of 251 metagenome-assembled genomes (MAGs). Corresponding metatranscriptomes highlighted 50 particularly active MAGs of the most abundant clades, including many polysaccharide degraders. Saccharide measurements together with bacterial polysaccharide utilization loci (PUL) expression data identified ß-glucans (diatom laminarin) and α-glucans as the most prominent and actively metabolized dissolved polysaccharide substrates. Both substrates were consumed throughout the bloom, with α-glucan PUL expression peaking at the beginning of the second bloom phase shortly after a peak in flagellate and the nadir in bacterial total cell counts. CONCLUSIONS: We show that the amounts and composition of dissolved polysaccharides, in particular abundant storage polysaccharides, have a pronounced influence on the composition of abundant bacterioplankton members during phytoplankton blooms, some of which compete for similar polysaccharide niches. We hypothesize that besides the release of algal glycans, also recycling of bacterial glycans as a result of increased bacterial cell mortality can have a significant influence on bacterioplankton composition during phytoplankton blooms. Video Abstract.

Stress resistance for unraveling potential biopollutants. Insights from ballast water community analysis through DNA.

In marine settings, anthropogenic disturbances and climate change increase the rate of biological invasions. Predicting still undescribed invasive alien species (IAS) is needed for preparing timely management responses. We tested a strategy for discovering new potential IAS using DNA in a trans-equatorial expedition onboard RV Polarstern. During one-month travel, species inside ballast water experienced oxygen depletion, warming, darkness and ammonium stress. Many organisms died but several phytoplankton and zooplankton survivors resisted and were detected through a robust combination of individual sampling, DNA barcoding and metabarcoding, new in ballast water studies. Ammonium was identified as an important influential factor to explain diversity changes in phytoplankton and zooplankton. Some species reproduced until the end of the travel. These species tolerant to travel stress could be targeted as potential IAS and prioritized for designing control measures. Introducing resistance to travel stress in biosecurity risk analysis would be recommended.

Invasion trajectory of Pacific oysters in the northern Wadden Sea.

Invasion trajectories of introduced alien species usually begin with a long establishment phase of low abundance, often followed by exponential expansion and subsequent adjustment phases. We review the first 26 years of feral Pacific oysters Crassostrea gigas around the island of Sylt in the Wadden Sea (North Sea, NE Atlantic), and reveal causal conditions for the invasion phases. Sea-based oyster farming with repeated introductions made establishment of feral oysters almost inevitable. Beds of mussels Mytilus edulis on mud flats offered firm substrate for attachment and ideal growth conditions around low tide level. C. gigas mapped on to the spatial pattern of mussel beds. During the 1990s, cold summers often hampered recruitment and abundances remained low but oyster longevity secured persistence. Since the 2000s, summers were often warmer and recruitment more regular. Young oysters attached to adult oysters and abundances of >1000 m-2 were achieved. However, peak abundance was followed by recruitment failure. The population declined and then was also struck by ice winters causing high mortality. Recovery was fast (>2000 m-2) but then recruitment failed again. We expect adjustment phase will proceed with mean abundance of about 1000 m-2 but density-dependent (e.g., diseases) and density-independent (e.g., weather anomalies) events causing strong fluctuations. With continued global warming, feral C. gigas at the current invasion fronts in British estuaries and Scandinavian fjords may show similar adjustment trajectories as observed in the northern Wadden Sea, and also other marine introductions may follow the invasion trajectory of Pacific oysters.

Plankton-toxin interaction with a variable input nutrient.

A simple model of phytoplankton-zooplankton interaction with a periodic input nutrient is presented. The model is then used to study a nutrient-plankton interaction with a toxic substance that inhibits the growth rate of plankton populations. The effects of the toxin upon the existence, magnitude, and stability of the periodic solutions are discussed. Numerical simulations are also provided to illustrate analytical results and to compare more complicated dynamical behaviour.