Limnology and Oceanography

The raphidophyte Chattonella marina is a harmful algal bloom (HAB) species known for its distinct diurnal vertical migration (DVM), a behavior important for its survival and bloom formation. However, the single-cell mechanisms governing this migration remain unclear. In this study, we investigated the swimming characteristics of individual C. marina cells during day (light) and night (dark) phases. We observed a strong positive correlation between the length of the propulsive anterior flagellum and the cells swimming speed. We discovered that the length distribution of the anterior flagellum is different during the day and at night. We also found that the beat frequency of the anterior flagellum was significantly higher during the day compared to the night. This resulted in faster mean swimming speeds during the light phase. To investigate the mechanism of length regulation, we tested the role of intraflagellar transport (IFT) using the IFT dynein inhibitor, ciliobrevin D. Treatment with ciliobrevin D induced a time- and concentration-dependent shortening of the anterior flagellum. This is the first pharmacological evidence to suggest that an IFT-like mechanism may actively control motile flagellar length in C. marina. These findings suggest that C. marina modulates its swimming speed through diurnal changes in both flagellar length and beat frequency, likely as an energy-saving strategy coupled to its DVM.

The Mississippi River (MR) is the largest source of freshwater and nutrients to the Gulf of Mexico (GoM), strongly influencing stratification, primary production, and plankton organization. The interaction between buoyant plume waters and denser shelf waters in the northern Gulf of Mexico (nGoM) generates sharp density gradients that can promote fine-scale biological aggregation. We investigated how hydrographic structure associated with the MR plume controls the vertical distribution of plankton during May 2017 using an integrated instrumentation suite that included an in situ digital holographic imaging system (HOLOCAM) coupled with CTD and optical sensors. Phytoplankton thin layers were repeatedly detected at plume-edge stations within or immediately above a compressed pycnocline formed by bottom-trapped saline wedges. These layers were 1.2-3.5 m thick and exhibited chlorophyll-a concentrations up to threefold higher than background levels. The assemblage was dominated by chain-forming diatoms, particularly Chaetoceros debilis and C. socialis, whose local abundance maxima coincided with chlorophyll peaks. In contrast, copepods, appendicularians, and other zooplankton were broadly distributed throughout the upper water column and rarely aggregated within the layers. Redundancy analysis indicated that chlorophyll concentration and stratification intensity were primary drivers of community structure across stations. Satellite imagery revealed rapid short-term variability in plume extent, helping explain differences in stratification and thin layer development among sampling days. Our results demonstrate that salt-wedge dynamics at the plume-shelf interface constitute a key physical mechanism governing transient phytoplankton thin layer formation in the nGoM, while zooplankton responses remain weakly coupled at the temporal scales resolved here.

O_LIPhytoplankton are the major primary producers in the Southern Ocean, participate in the global carbon cycle, nutrient cycles, and are at the base of the food-web. These polar ecosystems are unique in their extended periods of darkness in the winter. C_LIO_LIProlonged darkness has the potential to exert selection that affects the composition of diatom communities if there is differential survival of diatoms in the dark, variation in population growth rates in subsequent light periods, or both. C_LIO_LIWe tested whether prolonged darkness has the potential to exert within-species selection on a model polar diatom species by exposing 5 strains of the polar diatom Porosira glacialis to prolonged darkness at two different temperatures in the laboratory. We measured population survival in the dark, growth rate upon re-illumination, and between strain variability in these traits. C_LIO_LIWe found a pronounced decline in survival and growth rate with time spent in the dark, as well as important intraspecific variation in these. C_LIO_LIHigher temperature exacerbated declines in growth and survival. C_LIO_LIOur results show that the darkness of polar night can exert selection within diatom species, with implications for phytoplankton community composition and subsequent impacts on Southern Ocean biogeochemical cycles. C_LI

As B-vitamins are organic cofactors required by prokaryotic and eukaryotic planktonic cells, their availability impacts aquatic microbial communities and associated biogeochemistry. Contrary to inorganic nutrients, measurements of B-vitamins from brackish systems are scarce and relationships between B-vitamins and plankton composition in estuaries are unclear, limiting our understanding of estuary biology in general as well as how B-vitamins are distributed and dispersed in marine systems. Here, we quantify multiple B-vitamins and their vitamers in particulate and dissolved phases, and characterize microbial community composition, across fresh to polyhaline zones of the Neuse River Estuary (NRE), North Carolina, USA. We uncover elevated concentrations of B-vitamins within the mid-estuary, Chlorophyll a maximum along with a unique suite of dissolved B-vitamin associated with sporadic surges in pico- and microplankton populations. The dynamics of both dissolved and particulate B-vitamin concentrations in space and time were striking - from subpicomolar to high picomolar levels observed and strong short-term (weeks) variability. We find notable autochtonous B-vitamin production in the estuary, but we expect the ability of the system to deliver these micronutrients to the ocean will depend on flushing as well as changes in microbial community. We identify vitamin B1, B12, psB12 (pseudocobalamin), and B3 as key explanatory variables for change in prokaryotic and eukaryotic NRE plankton, providing new evidence of B-vitamin influence upon estuarine plankton community composition. Our work reveals new complexities in B-vitamin production and consumption within zones of estuaries while underscoring these micronutrients as key drivers of microbial plankton composition.

While gene expression in bacteria has been shown to be affected by near-zero or extremely high gravity, a mechanism has not been established to date. In larger organisms, gravity-sensing mechanisms usually rely on a dense body applying directional pressure which can be detected by the cell. Herein we demonstrate a means of observing the effect of gravity on cyanobacteria by differential expression of native pigments in response to both gravity and light. We observe that in the cyanobacterium Synechococcus sp. PCC 7002, the distribution of pigmentation within the cell, and across cell colonies, is regulated by combined directional sensing of incoming light, adhesion to a surface via extracellular matrix, and applied external force, including the normal force of gravity applied to the cell. Cells grown on a substrate orient their thylakoids on the cell faces proximal and distal to the substrate and locate both chlorophyll and phycobilins in both of these membrane regions; phycobilins are primarily targeted to the membrane region nearest to the light source, while chlorophyll is preferentially expressed in the region opposite the overall external force applied to the cell. The mechanism for distribution of pigments appears to be regulated by presence of polyphosphate bodies within the cell, and removal of polyphosphate negates the cells ability to sense external forces. Furthermore, colonial morphology is affected by application of force, with cells responding to the secretions of other cells along a gradient along the expected response to shading. These results represent a critical step toward understanding basal phototrophic regulatory mechanisms of light use and demonstrate the first known intracellular directional gravity response mechanism in a prokaryote. Statement of SignificanceTo date, no directionally sensitive gravity response mechanism has been observed in any prokaryote. We demonstrate the first evidence of a directional response to external force in a cyanobacterium. This pigment distribution force-directed response interacts with the conventional response to directional light. Furthermore, the cells appear to be able to respond to the presence of other cells above them via intercellular signaling which is not simply due to shading by light.

Climate change is driving species range shifts and population change in density and location globally. Two theories behind these shifts, that species in the ocean are largely tracking climate velocities, and the concept of long-term temporal turnover, have garnered increased attention recently. However, research in marine ecosystems has largely focused on mobile species, namely commercially important fishes. Here we examine changes in sessile invertebrate and algal species on vertical surfaces, subtidal rock walls, in the southern Gulf of Maine (GOM), and to what extent these changes might have been driven by 42 years of warming. In part due to ocean circulation patterns in the GOM, the thermally-sensitive species in this community are unlikely to track climate velocities by moving laterally, and are therefore disappearing, moving into deeper water, or adapting to novel thermal conditions. We find that some species, including one of the previously competitive dominants, Alcyonium siderium, have become exceedingly rare at these sites. Two other competitive dominants, Metridium senile and Aplidiiuam glabrum, have also declined precipitously. Meanwhile, the blue mussel, Mytilus edulis, the non-native tunicate Didemnum vexillum, and a complex of erect bryozoans have become dominant space holders. Over the same period of time, average summer temperatures in the southern GOM increased by more than 3{degrees}C. Using occupancy derived thermal affinities, we find warm-affinity species increasing, while generally, cool and cold-affinity species are decreasing. All species which decreased in abundance normally occupy sites with temperatures below a mean of 17.4{degrees}C maximum summer temperatures. A few species did not change abundance despite the rapidly warming surface waters, indicating their broad tolerances and the importance of other biological processes in mediating community structure in the GOM. Overall, sessile rock wall communities in the southern GOM are transitioning to more thermally-tolerant species, most of which are not native to the Atlantic coast of North America.

European tidal flats that host non-native Magallana gigas reefs contribute to several ecosystem functions. Among others, they provide a habitat for a large variety of associated fauna. However, we often lack detailed information about any trophic interactions of the associated macrozoobenthos species with the oysters, and about their role in the carbon and nutrient cycle. Therefore, we performed ex-situ pulse-chase tracer experiments in the Eastern Scheldt (Southwest Dutch Delta, Netherlands) in summer and autumn 2020, where we fed M. gigas and their associated fauna 13C- and 15N-enriched bacterioplankton while the macrozoobenthos was incubated in water containing deuterium oxide (2H2O; enrichment: 1 - 2.5%). The aim was (1) to assess differences in short-term (<12h) processing of bacterioplankton in summer and autumn, and (2) to study differences in 2H incorporation - a proxy for metabolic activity - of M. gigas and its associated fauna in summer and autumn. In summer, all macrozoobenthos species combined consumed significantly less bacterioplankton-derived 13C and 15N than in autumn, while all macrozoobenthos species combined incorporated comparable amounts of 2H into their tissue in both seasons. Most bacterioplankton-derived 13C and 15N was taken up by sponges (Halichondria panicea, Hymeniacidon perlevis), crabs (Carcinus maenas, Eriocheir sinensis, Rhithropanopeus harrisii), and limpets (Crepidula fornicata). Most 2H was taken up by crabs (C. maenas, E. sinensis), sponges (H. perlevis), and snails (Littorina littorea), implying that these species were the most metabolically active ones. Overall, the metabolic activity was linked to feeding activity in summer 2020, whereas in autumn 2020, the link was weaker and the most metabolically active species were not necessarily the species that had incorporated most 13C and/or 15N.

Many benthic marine invertebrates disperse by releasing microscopic larvae carried by ocean currents to new sites, where they must settle into appropriate habitats and metamorphose to recruit. Species whose larvae settle in response to water-borne chemical cues live in topographically complex habitats. To study whether sinking in response to dissolved cues affects retention of larvae within complex habitats exposed to ambient water flow moving faster than larvae sink, we used the reef-dwelling sea slug, Phestilla sibogae, whose competent larvae stop swimming and sink in response to dissolved cue from their prey coral, Porites compressa. We conducted field experiments where dye-labelled water, neutrally buoyant particles, and larval mimics (particles that sank at the velocity of larvae of P. sibogae) were released together upstream of reefs of branching corals to determine if larval sinking in water above and within a reef affects larval retention within the reef. Wave-driven water flow measured above a reef in the field had instantaneous velocities peaking at 0.3 m s-1, driving slow net advection of water shoreward at [~]0.02 m s-1. Much slower wave-driven flow moved through the interstices within the reef. In this field flow, sinking by larval mimics caused their retention within a reef after dye-labelled water and neutrally buoyant particles had left. Such retention of sinking larvae within topographically complex benthic communities enhances successful recruitment by exposing larvae to high concentrations of cue for long periods, allowing them time to sink to surfaces, adhere, and undergo metamorphosis.

Seamounts are prominent deep-ocean features that strongly influence geological processes, ocean circulation, and benthic biodiversity. Despite their importance, most seamounts remain unmapped and poorly characterized, particularly in the southeast Pacific Ocean, a region recognized for high marine endemism and ecological isolation. In this study, we present a quantitative habitat characterization of a previously undocumented seamount, informally named Solito Seamount, located between the Nazca-Desventuradas Marine Park and the Juan Fernandez Archipelago. High-resolution multibeam bathymetry and backscatter intensity data were integrated with in situ observations from two remotely operated vehicle (ROV) dives (SO643 and SO645) to investigate how geomorphology and substrate distribution influence benthic community patterns. An automated and hierarchical quantitative mapping framework incorporating objective terrain analysis and multivariate statistical techniques, including principal component analysis and clustering, was applied to delineate five distinct megahabitat types: flat, basal slope, valley, ridge slope, and ridge crest. ROV video transects traversing these megahabitats revealed five associated substrate type forming macrohabitats: bedrock, bedrock with sediment veneer, sediment-rock transition, sediment, and coral rubble. Outputs were used to investigate how environmental heterogeneity structures megafaunal assemblages of Solito Seamount. Multivariate analysis revealed a combined effect of megahabitat type and substrate type on benthic megafaunal assemblages across the depth gradient. These compositional dissimilarities were primarily driven by habitat-forming taxa. In the deeper dive (SO643), a broad suite of taxa contributed to dissimilarities, and assemblages were primarily organised by megahabitat. The ridge crest hosted a distinct reef-building scleractinian community, whereas the ridge slope hosted mixed antipatharian, gorgonian and actiniarian assemblages. In contrast, the shallower dive exhibited simpler patterns with few taxa driving dissimilarities. Substrate effects were most pronounced with coral rubble forming a distinct habitat characterised by sponges (Stelletta sp.). Pronounced biological differences between dives may also represent depth-dependent structuring resulting from differences in oxygen regimes associated with water masses, underscoring the role of oceanographic forcing. This study provides the first quantitative habitat map of this previously undocumented seamount, delivering essential baseline information for this largely unexplored region of the southeast Pacific. The integrated multi-scale geophysical and biological approach presented here offers a robust framework for advancing seamount ecosystem understanding and supporting future biodiversity assessments and conservation planning.

Biologists aim to predict where species will survive and thrive as the planet warms. To do so, we often rely on data-hungry species distribution models (SDMs) that use associations between species occurrences and environmental predictors to capture the realised niche. An alternative basis for predictions is to experimentally quantify the effect of environmental drivers on performance, which captures the fundamental niche. We presently do not know which of these approaches represents a better path towards accurate forecasts. SDMs may depend too strongly on present-day environmental covariation, which will change in the future. In contrast, a major shortcoming of experiments is that they ignore most environmental drivers to focus on one or two. Quantifying how well fundamental and realised niches agree today would help establish how useful both SDMs and experiments are likely to be. We therefore compared both niches in 39 relatively common marine phytoplankton species. The temperature-dependence of population growth rate was characterised with a thermal performance curve model applied to lab experimental data, and the temperature-dependence of species occurrence probability estimated with SDMs applied to a global compilation of marine presence records. We found a fairly strong, near 1:1 relationship between measures of thermal niche centre: the median growth temperature in the lab and the median occurrence temperature in the field (R2 = 0.49). We also found a modest positive relationship between measures of thermal niche width, the growth niche width and the occurrence niche width (R2 = 0.24). This agreement should increase our confidence in environmental preferences inferred with SDMs. It also suggests that simple experiments can reliably constrain species ranges and help forecast range shifts. This has important implications for forecasting community composition and ecosystem processes, as we ought to be able to predict range shifts in biogeochemically-important taxa such as diatoms and nitrogen-fixing cyanobacteria.

Ocean Alkalinity Enhancement (OAE) is a marine carbon dioxide removal (mCDR) strategy that involves adding alkaline substances to surface waters to enhance CO2 uptake and storage. The dispersal of alkaline materials such as sodium hydroxide (NaOH) into seawater can cause rapid increases in pH and total alkalinity (TA) that substantially exceeds natural variability in marine environments. Such fluctuations may negatively affect marine life, especially small animals like copepods who cannot avoid OAE plumes and whose physiological processes could be disrupted by large and rapid shifts in seawater pH. To address knowledge gaps regarding potential biological impacts of OAE, we studied these effects in Calanus finmarchicus, a keystone copepod species in the Northwest Atlantic Ocean. We exposed C. finmarchicus from the late juvenile copepodite stages and adult females to NaOH-dosed seawater at pH 10.5 ([~]5,000 {micro}mol kg-1 TA) and pH 9.0 ([~]3,150 {micro}mol kg-1 TA) for durations that reflect expected short-term exposure times during field OAE deployments (pH 10.5: 1, 5, 10 minutes; pH 9.0: 1, 15, 30 minutes). None of the treatment combinations resulted in mortality immediately after the initial exposure. Individuals were monitored for survival for 72 hours post-exposure (hpe), and only one treatment group (juveniles exposed to pH 10.5 for 10 minutes) showed a significant reduction in final survival; no other pH-duration combination showed increased mortality. Effects on the ability to initiate an escape response were more substantial. Adult females treated with pH 10.5 for 5 or 10 minutes showed a significant reduction in escape response immediately after exposure. In contrast, juveniles showed no immediate change in escape response following exposure to pH 10.5 or pH 9.0, although juveniles exposed to pH 10.5 for 10 minutes exhibited reduced escape response at 24 hpe. Using microrespirometry, we measured oxygen consumption following a 10-minute exposure to pH 10.5 and detected no effect on routine metabolic rate immediately post-exposure or at 12 hpe. Overall, our results suggest that C. finmarchicus is relatively tolerant to short-term exposures to very high pH and alkalinity. Future work should prioritize longer-term exposure under more moderate ocean OAE conditions.

Diatoms significantly contribute to aquatic primary productivity and biogeochemical cycles, with motility playing a crucial role in their ecological success. While several factors influence their motility, the effect of water flow remains poorly understood. This study used a digital holographic microscope to investigate the locomotion of the pennate diatom Navicula cf parapontica under varying flow rates. It demonstrates, for the first time, that Navicula perceives and actively counteracts water flows. As flow rates increased up to 500 nL/s, cells consistently moved against the current and frequently adjusted their orientation to maximize resistance. This behaviour allowed the diatoms to maintain a stable locomotion velocity despite a 6.7-fold increase in flow rate. This active rheotaxis likely serves as a strategy to resist resuspension and passive dispersal. These findings reveal a behavioural trait that might play significant role in the way benthic diatom communities maintain their position in the sediments, influencing bentho-pelagic coupling and biogeochemical processes.

Using linear inverse ecosystem modeling as a data assimilation tool, we compare spawning grounds of Atlantic and Southern Bluefin Tuna (ABT and SBT, respectively) based on results from field campaigns in the Gulf of Mexico (GoM) and eastern Indian Ocean off northwest Australia (Argo Basin). Both regions are warm, stratified, low-nutrient waters dominated by cyanobacteria (Prochlorococcus). Despite these similarities, the Argo Basin is more productive, with [~]1.5X higher net primary production and nearly 2X higher production of top trophic levels in the model (tuna larvae, planktivorous fish, and predatory gelatinous zooplankton). Higher primary production in the Argo Basin is mainly driven by higher N2 fixation and storm mixing of new nutrients in the upper and lower euphotic zone, respectively. Increased ecosystem efficiency (secondary production of top trophic levels / primary production) results from differences in plankton food web organization. In the GoM, protistan zooplankton are the direct consumers of nearly all phytoplankton production. In contrast, higher rates of herbivory by crustaceans feeding on nanophytoplankton combines with a higher impact of appendicularians on cyanobacteria to convert plankton production into larval tuna prey more efficiently in the Argo Basin. Despite similarities in the proportions of phytoplankton production mediated by cyanobacteria and other picoplankton in both systems, food web pathways to larval tuna and other planktivorous fish are substantially shorter in the Argo Basin. Our results highlight the impact of distinct zooplankton ecological niches on ecosystem efficiency and suggest a need for better inclusion of plankton food-web structure in models simulating climate impacts on fisheries production. HIGHLIGHTSO_LIDeveloped food web models of tuna spawning habitat (Indian Ocean & Gulf of Mexico) C_LIO_LISpawning habitats in the Argo Basin and Gulf of Mexico (GoM) are both oligotrophic C_LIO_LIArgo Basin had higher net primary production in part as a result of nitrogen fixation C_LIO_LIArgo Basin had higher rates of direct herbivory by metazoan zooplankton C_LIO_LIThis resulted in greater ecosystem efficiency in the Argo Basin. C_LI

Quagga mussels (Dreissena rostriformis bugensis) are among the most impactful invaders in freshwaters of the Northern Hemisphere. As filter-feeders, they can reduce harmful algal blooms (HABs), but their effects are expected to be dependent on cyanobacteria species and water temperature. However, conclusive studies on these traits and their combination are lacking. Here, we combined laboratory experiments with an analysis of long-term data from a temperate shallow lake 10 years before and after quagga mussel invasion, respectively. We tested the hypotheses that quagga mussel filtration rates in the laboratory would 1) vary among common cyanobacteria species and 2) decrease above a critical temperature. Regarding the field data, we expected that 3) quagga mussels can reduce the summer biovolume of palatable cyanobacteria, but that 4) this effect disappears above a critical temperature. Our results support all four hypotheses. In laboratory experiments, Dolichospermum flos-aquae was classified as palatable to quagga mussels, while Aphanizomenon flos-aquae, Anabaenopsis elenkinii and Microcystis aeruginosa were less-palatable cyanobacteria. Filtration rates decreased above 28.9{degrees}C (CI: 27.6-30.2{degrees}C) with mussels dying at 32{degrees}C. Our long-term lake data show that cyanobacteria biovolumes were lower after quagga mussel invasion, but only below 27.7{degrees}C (CI: 26.9-28.4{degrees}C), confirming a critical thermal window for quagga mussel filtration. Global warming will therefore facilitate HABs by increasing the growth rates of cyanobacteria and reducing the filtration rates of quagga mussels above critical summer water temperatures, which are increasingly being reached in invaded lakes. This critical thermal window must be considered when making HAB predictions. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=93 SRC="FIGDIR/small/707163v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@175851eorg.highwire.dtl.DTLVardef@76a481org.highwire.dtl.DTLVardef@12a3965org.highwire.dtl.DTLVardef@11e3e7d_HPS_FORMAT_FIGEXP M_FIG C_FIG

Tropical coral reef ecosystems worldwide are being impacted by combined pressures of climate change and human activities that introduce large quantities of nutrients and sediments into coastal areas. In this context, phytoplankton represent a critical link between dissolved inorganic nutrients and coral reef food webs, yet their role in these ecosystems remains understudied. We investigated ecological responses of the summer phytoplankton community of Shiraho Reef (Ishigaki Island, Okinawa, Japan) to nutrient enrichment using field-based microcosm experiments under natural light and temperature conditions in September 2022 and 2023. Treatments included single and combined additions of nitrogen, phosphorus, and silicon. Chlorophyll a (Chl a) concentrations increased after three days under combined nutrient conditions, whereas single-nutrient additions produced limited responses, indicating a strong co-limitation by nitrogen and phosphorus in the reef. Analysis of size-fractionated Chl a revealed shifts from picophytoplankton that typically dominate tropical oligotrophic ecosystems toward larger groups supported by enhanced nutrient availability. Our results show short-term impacts of nutrient enrichment events on phytoplankton size structure and biogeochemical cycling in coral reefs, and highlight the importance of pelagic processes in coral reef carbon dynamics under nutrient-enrichment.

The post-detachment drifting phase of macrophytes, during which they can be alive, dead, or senescent, plays a crucial ecological and biogeochemical role by influencing long-range dispersal, transporting rafting species, affecting carbon sequestration, promoting blooms, and leading to beaching events. In order to predict the dispersal of macrophytes and macroplastic particles and where they will affect the ecosystem, it is important to be able to model how their drift velocities are influenced by hydrodynamic and aerodynamic factors. In this study, we investigated the drift velocity of macrophytes with diverse morphologies and macroplastic particles in a racetrack flume under different current conditions, in combination with and without wind in the same direction as the water current. Our data show that the drift velocity of macrophytes is highly dependent on their buoyancy and affected by morphological characteristics. Wind increased the velocity of the surface water, which in turn increased the drift velocity of both macrophytes and macroplastic particles. However, wind-induced turbulences reduced the overall effect, especially for macrophytes, which protruded minimally above the water surface in comparison to macroplastic particles. For positively buoyant specimens, an existing particle model was experimentally confirmed to predict macrophyte and macroplastic particle drift velocities reliably, irrespective of shape. For negatively buoyant species, we propose a novel equation to predict drift velocity, incorporating the diverse shapes of macrophytes, as well as their interaction with the bottom. These results represent the first step toward the development of trait-based models that represent macrophytes more realistically in dispersal simulations. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=135 SRC="FIGDIR/small/709487v1_ufig1.gif" ALT="Figure 1"> View larger version (54K): org.highwire.dtl.DTLVardef@1ab9f6aorg.highwire.dtl.DTLVardef@6ef75dorg.highwire.dtl.DTLVardef@132334forg.highwire.dtl.DTLVardef@c6a3d8_HPS_FORMAT_FIGEXP M_FIG C_FIG

Cyanobacterial colonies often exploit their buoyancy and large size to float upwards rapidly and form dense surface blooms, which can degrade water quality, threaten ecosystems and public health, and impose substantial economic costs. Yet, how the morphology of cyanobacterial colonies controls their vertical velocity remains poorly understood. We conducted detailed three-dimensional morphological characterization of colonies of the cyanobacterium Microcystis in lake samples at the single-colony level and performed controlled flotation experiments in stratified flows. Using particle tracking in a vertical density gradient, we separately quantified the contributions of colony shape and buoyant density at the level of individual colonies. Our results show that the shape factor in Stokes law varies systematically with colony size. Consequently, the vertical velocity of colonies does not scale with the square of colony size but only with a power of 1.13, as larger colonies have a more irregular shape and therefore experience enhanced drag. We therefore correct the commonly used Stokes law to account for the size-dependent change in the shape factor. Interestingly, implementation of this power law relationship in a vertical migration model shows widespread chaotic dynamics in the migration trajectories of Microcystis colonies. These results highlight the importance of morphological plasticity in cyanobacterial colonies and can inform predictive models and hydrodynamic control strategies for toxic blooms. Our methodology to simultaneously determine the density, shape factor and velocity is broadly applicable to suspended aggregates with complex shapes in freshwater and marine systems.

Sea star wasting disease (SSWD) is one of the most severe marine epidemics recorded, affecting numerous asteroid species and causing widespread population declines. Although a bacterial pathogen has recently been proposed for one species, its generality across taxa and regions remains unresolved. Here, we report the first year-round field assessment of SSWD in the Baltic Sea, a rapidly warming, low-salinity ecosystem hosting a single keystone sea star predator, Asterias rubens. Using field surveys, image-based monitoring, and laboratory experiments, we characterised disease dynamics and potential drivers in Kiel Fjord (western Baltic Sea). SSWD-like symptoms were present throughout 2024, with mean prevalence exceeding 40% across seasons and elevated levels during summer and early autumn, when sea surface temperatures approached 22 {degrees}C and salinity fluctuated between 11 and 17. The mean body radius of asymptomatic individuals declined from 5.6 cm in spring to 2.3 cm in early summer before partially recovering in autumn, consistent with high recruitment of juveniles and selective loss of larger, symptomatic individuals. In a complementary laboratory experiment, survival analyses identified body size as the strongest predictor of SSWD-associated mortality (hazard ratio = 50.8, p < 0.001), with large individuals far more likely to die than small ones. This size-selective mortality, together with environmental constraints on recruitment, suggests that SSWD may be reshaping population size structure and reducing predation pressure on blue mussels, with potential consequences for benthic community dynamics. Continued monitoring will be essential to assess the long-term impacts of SSWD on A. rubens populations and associated benthic ecosystems under ongoing climate change.

Microphytobenthos (MPB) contributes significantly to the marine primary production in estuarine ecosystems. MPB is mainly composed of benthic motile diatoms inhabiting intertidal and shallow subtidal sediments. Unlike intertidal small-sized diatom models, subtidal ([&ge;] 10 m depth) MPB and large-sized (>100 {micro}m) species, have comparatively received much less attention, especially as regards to their photosynthetic productivity. Yet, the subtidal light environment shows unusual (very) low intensities and a green-blue light spectrum at high tide. The present study investigates the light-dependent green-blue responses of the subtidal giant diatom Pleurosigma strigosum, combining in situ monitoring with laboratory experiments. In both MPB and P. strigosum, we documented a strong photophysiological plasticity, and a striking alignment with the photophysiology of (very) low light-adapted polar diatoms. Altogether, our results highlight the nature of subtidal photoadaptation: a very low green-blue light sensitive response, which explains the early blooming of P. strigosum at the very beginning of spring and underpins the ecological success of MPB in colonizing coastal subtidal sediments. The general coherence between subtidal P. strigosum and MPB light-responses offers a unique model species and growth form to further decipher the specific light-driven metabolism of subtidal MPB under precisely controlled environmental conditions.

Although bacterial symbiosis is well-documented in chemosynthetic-based ecosystems, associations with microeukaryotes remain overlooked. In this study, using scanning electron microscopy and 18S rDNA barcoding, we investigate the presence, diversity, and biogeographic patterns of ciliate epibionts associated with two deep-sea caridean shrimp families: Alvinocarididae, and Thoridae. We identified a widespread lineage of ciliates colonizing the gills of different alvinocaridid species, extending their previously known distribution in freshwater and coastal habitats to deep-sea areas down to 3388 m. These ciliates form a distinct clade related to coastal Chonotrichia, but showing clear genetic divergence from the previously-described species. Geographic divergence of these ciliate populations was observed across the Pacific Ocean, with no evident structure related to their host species. These chonotrichian ciliates exhibited variation occurrence across host species, individuals, and regions, indicating a facultative association with their hosts. In contrast, the thorid shrimps harbored rare and phylogenetically diverse ciliates. More rarely, we found ciliates related to known parasitic lineages hosted by both shrimp families, with signs of immune response - black gills - in some individuals colonized by these ciliates. Our results reveal previously overlooked protist-crustacean associations in chemosynthetic ecosystems and highlight the ecological and biogeographic importance of this group in the deep ocean.