Phytoplankton Community Structure Is Driven by Stratification in the Oligotrophic Mediterranean Sea

Seasonal dynamics and key drivers of phytoplankton community structure in the Eastern Indian Ocean

To fully understand variation in phytoplankton community structure in the Eastern Indian Ocean (EIO), two research cruises were carried out during September-November 2020, and March-May 2021. The phytoplankton community in the EIO was mainly composed of cyanobacteria and diatoms in 2020, cyanobacteria in 2021. Trichodesmium thiebaultii was the dominant specie in both years. The phytoplankton community of the EIO was classified into two provinces based on cluster. The RDA results indicated that in 2020, DIP was the predominant factor influencing phytoplankton abundance, but DIN as the main factor affecting phytoplankton abundance in 2021. Meanwhile, the SEM demonstrated that temperature and salinity indirectly influenced phytoplankton abundance by affecting nutrient concentrations (DIN and DIP) and vertical stratification index (VSI). Changes in nutrient concentrations had a greater impact on phytoplankton communities, especially diatoms and dinoflagellates. The phytoplankton community shifted from cyanobacteria to diatoms when VSI decrease, the composition of the community was also more complex. The composition and assembly mechanisms of phytoplankton communities indicated that seasonal seawater mixing enhanced growth and interspecific interactions, thus increasing network complexity and significantly influencing community structure.

Marine plankton community and net primary production responding to island-trapped waves in a stratified oligotrophic ecosystem

The oligotrophic Adriatic Sea is characterized during a typical summer by low productivity caused by strong water column stratification, which inhibits vertical mixing and nutrient supply to the euphotic zone. These conditions can be disrupted by transient physical forcing, which enhances nutrient fluxes and creates localized hotspots of relatively high net primary production. In this study, plankton abundance and diversity were investigated in relation to the physical forcing and nutrient concentrations in an area affected by island-trapped waves (ITWs) near Lastovo Island (Adriatic Sea). The episodic ITW events resulted in enhanced uplift and vertical excursion of the thermocline, marked by anomalously higher nutrient concentrations and a corresponding increase in net primary production in the thermocline layer. Physicochemical properties explained 11.7 % (p = 0.002) of the variability in micro- and nanophytoplankton and 88.9 % (p = 0.001) in the picoplankton community. A significant response to the ITW phenomenon in the plankton community composition (p = 0.001) was observed for bacterioplankton. Among the identified amplicon sequence variances, primary producers were scarce and mainly represented cyanobacteria (Synechococcus strain CC9902), stramenopiles (Pelagomonas), and chlorophytes (Ostreococcus). The remaining amplicon sequence variances were assigned to the classes Copepoda, parasitic fungi (Meyerozyma spp.), mixotrophic dinoflagellates (family Peridiniales, mostly the genus Blastodinium), and parasitic Ciliophora (Scuticociliata). Bacterial ecological functions corresponded to chemoheterotrophic, degradation, and fermentation processes, whereas samples collected after the most intense ITW episode also showed abundant bacteria linked to microplastic degradation and parasitosis. These results highlight the ecological role of localized physical phenomena in enhancing nearshore primary productivity and fine shifts in plankton taxa in oligotrophic systems.

Water stratification alters phytoplankton assemblages in scallop farming waters of the North Yellow Sea in China

As evaluation indicators of the primary productivity, the phytoplankton biomass and community structure are of great significance to the fishery industry, which can be driven by ocean currents, nutrients and water stratification. In the present study, the characteristics of phytoplankton assemblages in different water layers of a typical Yesso scallop farming area in Zhangzi Island, the North Yellow Sea were investigated from March 2021 to January 2022. According to the vertical distribution of temperature, water stratification was observed from June to August (stratification period), and disappeared in March, October and the following January with vertical homogeneity (mixing period). 18S rRNA gene sequencing results revealed that Pyrrophyta was the most dominant phylum during the sampling period, with high gene proportions in the stratification (63.36%) and mixing periods (77.35%). The gene proportion of Bacillariophyta in the stratification period was 5.44%, which was significantly lower than that in the mixing period of 8.93% (p < 0.05). Moreover, Pseudo-nitzschia, a toxin-producing taxon affiliated with Bacillariophyta, exhibited a significantly higher proportion in the stratification period than in the mixing period. During the stratification period, a number of toxin-producing taxa such as Pseudo-nitzschia and Karlodinium were enriched in the bottom layer, which was 1.29-fold and 1.37-fold of that in the surface layer, respectively. Redundancy analysis showed that phosphate and water temperature were major environmental factors driving the vertical distribution of phytoplankton assemblages. The phosphate (0.11 μM) and silicate (2.09 μM) concentrations in the surface layer approached the minimum threshold for phytoplankton growth, and the stoichiometric limitation of phosphate was detected in the surface and middle layers. Collectively, these results indicated that the decreased proportion ratio of Bacillariophyta to Pyrrophyta and unfavorable community composition of Bacillariophyta for scallops were observed during summer, which might result from the phosphate limitation driven by water stratification. The results will further our understanding of the dynamics of phytoplankton communities under the background of intensifying ocean stratification and provide ecological guidance for mollusc mariculture.

Response of two temperate scleractinian corals to projected ocean warming and marine heatwaves

The Mediterranean Sea is a hotspot of global change, particularly exposed to ocean warming and the increasing occurrence of marine heatwaves (MHWs). However, experiments based on long-term temperature data from the field are scarce. Here, we investigate the response of the zooxanthellate coral Cladocora caespitosa and the azooxanthellate coral Astroides calycularis to future warming and MHWs based on 8 years of in situ data. Corals were maintained in the laboratory for five months under four temperature conditions: Warming (3.2°C above the in situ mean from 2012 to 2020), Heatwave (temperatures of 2018 with two heatwaves), Ambient (in situ mean) and Cool (deeper water temperatures). Under the Warming treatment, some C. caespitosa colonies severely bleached and A. calycularis colonies presented necrosis. Cladocora caespitosa symbiosis was impaired by temperature with a decrease in the density of endosymbiotic algae and an increase in per cent whiteness in all the treatments except for the coolest. Recovery for both species was observed through different mechanisms such as regrowth of polyps of A. calycularis and recovery of pigmentation for C. caespitosa. These results suggest that A. calycularis and C. caespitosa may be resilient to heat stress and can recover from physiological stresses caused by heatwaves in the laboratory.

The apparent quantum yield matrix (AQY-M) of CDOM photobleaching in estuarine, coastal, and oceanic surface waters

The photochemical degradation of chromophoric dissolved organic matter (CDOM) upon solar exposure, known as photobleaching, can significantly alter the optical properties of the surface ocean. By leading to the breakdown of UV- and visible-radiation-absorbing moieties within dissolved organic matter, photobleaching regulates solar heating, the vertical distribution of photochemical processes, and UV exposure and light availability to the biota in surface waters. Despite its biogeochemical and ecological relevance, this sink of CDOM remains poorly quantified. Efforts to quantify photobleaching globally have long been hampered by the inherent challenge of determining representative apparent quantum yields (AQYs) for this process, and by the resulting lack of understanding of their variability in natural waters. Measuring photobleaching AQY is made challenging by the need to determine AQY matrices (AQY-M) that capture the dual spectral dependency of this process (i.e., magnitude varies with both excitation wavelength and response wavelength). A new experimental approach now greatly facilitates the quantification of AQY-M for natural waters, and can help address this problem. Here, we conducted controlled photochemical experiments and applied this new approach to determine the AQY-M of 27 contrasting water samples collected globally along the land-ocean aquatic continuum (i.e., rivers, estuaries, coastal ocean, and open ocean). The experiments and analyses revealed considerable variability in the magnitude and spectral characteristics of the AQY-M among samples, with strong dependencies on CDOM composition/origin (as indicated by the CDOM 275-295-nm spectral slope coefficient, S275-295), solar exposure duration, and water temperature. The experimental data facilitated the development and validation of a statistical model capable of accurately predicting the AQY-M from three simple predictor variables: 1) S275-295, 2) water temperature, and 3) a standardized measure of solar exposure. The model will help constrain the variability of the AQY-M when modeling photobleaching rates on regional and global scales.

Temperature driving vertical stratification regulates phytoplankton community structure in the Bohai Sea and Yellow Sea

To analyse the effects of physicochemical factors on the phytoplankton community in the Bohai Sea (BS) and Yellow Sea (YS), a investigation was conducted during 27 July to 10 August 2020. A sum of 156 species were identified in the BS and YS, including Bacillariophyta (69 species), Pyrrophyta (85 species) and Chrysophyta (2 species). The phytoplankton community were divided into four provinces according to Bray-Curtis similarity. In order to study the phytoplankton community in the BS and YS, we studied the phytoplankton community composition and their assembly mechanisms. The results showed that stochastic ecological processes had a greater effect on the province C community structure. The Raup-crick dissimilarity showed that deterministic factors had a greater effect on the province A, B and D communities structure. The habitat niche width results indicated that niche was larger in the province D, compared to the province A, B and C. Based on a structural equation model (SEM), we analyzed the effects of physicochemical factors on phytoplankton community structure and temperature was found to affect the phytoplankton community composition and structure by the vertical stratification. The result showed that temperature was an important parameter for phytoplankton abundance and revealed that temperature affected phytoplankton community structure by influencing the vertical stratification index (VSI) in the BS and YS.

Long-term warming and human-induced plankton shifts at a coastal Eastern Mediterranean site

Plankton are key ecological indicators for assessing the impacts of human-induced pressures like climate change and waste-water discharge. Here, 26 years (1988-2015) of biweekly in-situ chlorophyll-a concentration, mesozooplankton biomass and remotely-sensed sea surface temperature (SST) data are utilized to investigate long-term changes of plankton biomass and timing of growth (phenology) in relation to warming, in a coastal region of the Saronikos Gulf (Aegean Sea). A Waste-Water Treatment Plant (WWTP) was established in 1995, leading to decreased nutrient concentrations circa 2004. Overall, the results indicate an interplay between warming and changes in ecological status. During higher nutrient input (1989-2004), a temporal mismatch between zooplankton and phytoplankton, and a positive zooplankton growth-SST association, are evident. Conversely, in the warmer, less mesotrophic period 2005-2015, an earlier timing of zooplankton growth (related to copepod abundance) synchronizes with phytoplankton growth, including a secondary autumn growth period. Concurrently, an abrupt negative interannual relationship between SST and mesozooplankton, and a summer biomass decrease (linked with cladoceran abundance) are observed. This work provides evidence that current warming could alter plankton abundance and phenology in nearshore Eastern Mediterranean ecosystems, suggesting shifts in plankton community composition that could trigger potential cascading effects on higher trophic levels.

Water column stratification governs picophytoplankton community structure in the oligotrophic eastern Indian ocean

Under the background of global warming, the area extent of the oligotrophic tropical oceans has growing due to increased water-column stratification over the past decades. Picophytoplankton is usually the most dominant phytoplankton group in oligotrophic tropical oceans and substantially contribute to carbon biomass and primary production three. Understanding how vertical stratification governs the community structure of picophytoplankton communities in oligotrophic tropical oceans is important for comprehensively understanding the plankton ecology and biogeochemical cycle in these areas. In this study, the distribution of the picophytoplankton communities in the eastern Indian Ocean (EIO) was investigated during a period of thermal stratification in the spring of 2021. Prochlorococcus contributed most (54.9%) to picophytoplankton carbon biomass, followed by picoeukaryotes (38.5%) and Synechococcus (6.6%). Vertically, the three picophytoplankton groups showed quite different distribution pattern: the abundance of Synechococcus was highest in the surface layer, while Prochlorococcus and picoeukaryotes were usually located between 50 m and 100 m. The relationship between the abundance of picophytoplankton and environmental factors was analyzed, and the results revealed that picophytoplankton distribution was strongly correlated with the degree of vertical stratification of the water column. The density of Synechococcus was higher in strongly stratified waters, while Prochlorococcus was more abundant in regions of weaker stratification. This is mainly attributed to variation of physicochemical parameters such as nutrient structures and temperature resulted from water column stratification. Understanding the distribution patterns of these organisms and their relationship with stratification in the oligotrophic EIO is essential for comprehensive understanding on oligotrophic tropical ecosystem with increasing stratification in future.

From the light blue sky to the dark deep sea: Trophic and resource partitioning between epipelagic and mesopelagic layers in a tropical oceanic ecosystem

The connection between epipelagic and deep-sea mesopelagic realms controls a variety of ecosystem processes including oceanic carbon storage and the provision of harvestable fish stocks. So far, these two layers have been mostly addressed in isolation and the ways they connect remain poorly understood. Furthermore, both systems are affected by climate change, exploitation of resources, and increasing pervasion of pollutants. Here we use bulk isotopes of δ¹³C and δ¹⁵N of 60 ecosystem components to evaluate the trophic linkage between epipelagic and mesopelagic ecosystems in warm oligotrophic waters. Additionally, we we conducted a comparison of isotopic niche sizes and overlaps across multiple species to evaluate how environmental gradients between epipelagic and mesopelagic ecosystems shape ecological patterns of resource use and competition between species. Our database comprises siphonophores, crustaceans, cephalopods, salpas, fishes, and seabirds. It also includes five zooplankton size classes, two groups of fish larvae, and particulate organic matter collected at different depths. Through this wide taxonomic and trophic variety of epipelagic and mesopelagic species, we show that pelagic species access resources originating from different food sources, mostly autotrophic-based (epipelagics) and microbial heterotrophic-based (mesopelagics). This leads to a sharp trophic dissimilarity between vertical layers. Additionally, we show that trophic specialization increases in deep-sea species and argue that food availability and environmental stability are among the main drivers of this pattern. Finally, we discuss how the ecological traits of pelagic species highlighted in this study can respond to human impacts and increase their vulnerability in the Anthropocene.

Genomic differentiation of three pico-phytoplankton species in the Mediterranean Sea

For more than a decade, high-throughput sequencing has transformed the study of marine planktonic communities and has highlighted the extent of protist diversity in these ecosystems. Nevertheless, little is known relative to their genomic diversity at the species-scale as well as their major speciation mechanisms. An increasing number of data obtained from global scale sampling campaigns is becoming publicly available, and we postulate that metagenomic data could contribute to deciphering the processes shaping protist genomic differentiation in the marine realm. As a proof of concept, we developed a findable, accessible, interoperable and reusable (FAIR) pipeline and focused on the Mediterranean Sea to study three a priori abundant protist species: Bathycoccus prasinos, Pelagomonas calceolata and Phaeocystis cordata. We compared the genomic differentiation of each species in light of geographic, environmental and oceanographic distances. We highlighted that isolation-by-environment shapes the genomic differentiation of B. prasinos, whereas P. cordata is impacted by geographic distance (i.e. isolation-by-distance). At present time, the use of metagenomics to accurately estimate the genomic differentiation of protists remains challenging since coverages are lower compared to traditional population surveys. However, our approach sheds light on ecological and evolutionary processes occurring within natural marine populations and paves the way for future protist population metagenomic studies.

Phytoplankton and Bacterial Communities’ Patterns in a Highly Dynamic Ecosystem (Central Mediterranean Sea)

The Straits of Messina (Southern Italy, Mediterranean Sea) are a very complex area: they connect two basins (Tyrrhenian and Ionian) with different hydrographic features and is characterised by upwelling and mixing phenomena. The aim of the study was to evaluate if and how the physical and chemical water conditions and hydrodynamics influenced the phytoplankton and bacterial patterns and the functioning of this ecosystem. During a late winter survey, size-fractionated phytoplankton (from 0.2 to 200 μm) biomass (chlorophyll a), cell densities and species composition as well as total picoplankton abundances, morphotype composition, and activity levels of the enzymes leucine aminopeptidase, β-glucosidase, and alkaline phosphatase were investigated. The obtained results showed a marked diversification among the water masses identified within the Straits area. The analyses of the phytoplankton diversity indices, particularly those based on phylogenetic relationships between species (indices of taxonomic diversity and distinctness), confirmed our findings. In conclusion, the patterns of phytoplankton and bacterial communities provide a suitable approach to evaluate how microbial communities respond to changing environmental scenarios. This tool could be applied to other temperate Mediterranean ecosystems.

Diversity and ecological footprint of Global Ocean RNA viruses

DNA viruses are increasingly recognized as influencing marine microbes and microbe-mediated biogeochemical cycling. However, little is known about global marine RNA virus diversity, ecology, and ecosystem roles. In this study, we uncover patterns and predictors of marine RNA virus community- and "species"-level diversity and contextualize their ecological impacts from pole to pole. Our analyses revealed four ecological zones, latitudinal and depth diversity patterns, and environmental correlates for RNA viruses. Our findings only partially parallel those of cosampled plankton and show unexpectedly high polar ecological interactions. The influence of RNA viruses on ecosystems appears to be large, as predicted hosts are ecologically important. Moreover, the occurrence of auxiliary metabolic genes indicates that RNA viruses cause reprogramming of diverse host metabolisms, including photosynthesis and carbon cycling, and that RNA virus abundances predict ocean carbon export.

Climate change and n-3 LC-PUFA availability

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are essential fatty acids for the growth, development and survival of virtually all organisms. There is increasing evidence that anthropogenic climate change has a direct and indirect impact on the availability of natural n-3 LC-PUFA. However, this information is fragmented and not well organized. Therefore, this article reviewed published data from laboratory experiments, field experiments and model simulations to reveal the impact of climate change on the global supply of natural n-3 LC-PUFA and how this will limit the availability of n-3 LC-PUFA in the future food web. In general, climate change can significantly reduce the availability of natural n-3 LC-PUFA in grazing food webs in the following ways: 1) decrease the total biomass of phytoplankton and shift the plankton community structure to a smaller size, which also reduce the biomass of animals in higher trophics; 2) reduce the n-3 LC-PUFA content and/or quality (n-3: n-6 ratio) of all marine organisms; 3) reduce the transfer efficiency of n-3 LC-PUFA in grazing food web. In addition, as an anthropogenic climate adaptation measure, this review also proposed some alternative sources of n-3 LC-PUFA and determined the direction of future research. The information in this article is very useful for providing a critical analysis of the impact of climate change on the supply of natural n-3 LC-PUFA. Such information will aid to establish climate adaptation or management measures, and determine the direction of future research.

Phytoplankton dynamics in Louisiana estuaries: Building a baseline to understand current and future change

Louisiana estuaries are important habitats in the northern Gulf of Mexico, a region undergoing significant and sustained human- and climate-driven changes. This paper synthesizes data collected over multiple years from four Louisiana estuaries - Breton Sound, Terrebonne Bay, the Atchafalaya River Delta Estuary, and Vermilion Bay - to characterize trends in phytoplankton biomass, community composition, and the environmental factors influencing them. Results highlight similarities in timing and composition of maximum chlorophyll, with salinity variability often explaining biomass trends. Distinct drivers for biomass versus community structure were observed in all four estuarine systems. Systems shared a lack of significant correlation between river discharge and overall phytoplankton biomass, while discharge was important for understanding community composition. Temperature was a significant explanatory variable for both biomass and community composition in only one system. These results provide a regional view of phytoplankton dynamics in Louisiana estuaries critical to understanding and predicting the effects of ongoing change.

Atlantic bluefin tuna spawn early to avoid metabolic meltdown in larvae

To predict shifts in phenology and distribution of organisms we need to understand how survival through early life stages depends on environmental conditions. Here, we present a mechanistic model of development, feeding and bioenergetics of early life stages in bluefin tuna and predict the optimal time of the year for them to be born. We find that the availability of prey, particularly nauplii, is sufficient for fast growth in tuna larvae while temperature is moderate during midsummer, but not when temperatures increase later in summer. High temperatures benefit egg and yolk-sac stages, but the metabolic needs of feeding larvae are hard to sustain during the warmest periods. Heatwaves, such as the one in 2003, increase larval survival potential, but shorten the viable part of the season for the larvae. Atlantic bluefin tuna is a large, highly migratory marine top predator that spawns while temperatures are rising, but before the heat leads to a metabolic meltdown in larvae. This means that food resources modulate how temperature change shifts optimal phenology.

Evolutionarily Stable Coevolution Between a Plastic Lytic Virus and Its Microbial Host

Hosts influence and are influenced by viral replication. Cell size, for example, is a fundamental trait for microbial hosts that can not only alter the probability of viral adsorption, but also constrain the host physiological processes that the virus relies on to replicate. This intrinsic connection can affect the fitness of both host and virus, and therefore their mutual evolution. Here, we study the coevolution of bacterial hosts and their viruses by considering the dependence of viral performance on the host physiological state (viral plasticity). To this end, we modified a standard host-lytic phage model to include viral plasticity, and compared the coevolutionary strategies emerging under different scenarios, including cases in which only the virus or the host evolve. For all cases, we also obtained the evolutionary prediction of the traditional version of the model, which assumes a non-plastic virus. Our results reveal that the presence of the virus leads to an increase in host size and growth rate in the long term, which benefits both interacting populations. Our results also show that viral plasticity and evolution influence the classic host quality-quantity trade-off. Poor nutrient environments lead to abundant low-quality hosts, which tends to increase viral infection time. Conversely, richer nutrient environments lead to fewer but high-quality hosts, which decrease viral infection time. Our results can contribute to advancing our understanding of the microbial response to changing environments. For instance, both cell size and viral-induced mortality are essential factors that determine the structure and dynamics of the marine microbial community, and therefore our study can improve predictions of how marine ecosystems respond to environmental change. Our study can also help devise more reliable strategies to use phage to, for example, fight bacterial infections.

The global explosion of eukaryotic algae: The potential role of phosphorus?

There arose one of the most important ecological transitions in Earth's history approximately 750 million years ago during the middle Neoproterozoic Era (1000 to 541 million years ago, Ma). Biomarker evidence suggests that around this time there was a rapid shift from a predominantly bacterial-dominated world to more complex ecosystems governed by eukaryotic primary productivity. The resulting 'Rise of the algae' led to dramatically altered food webs that were much more efficient in terms of nutrient and energy transfer. Yet, what triggered this ecological shift? In this study we examined the theory that it was the alleviation of phosphorus (P) deficiency that gave eukaryotic alga the prime opportunity to flourish. We performed laboratory experiments on the cyanobacterium Synechocystis salina and the eukaryotic algae Tetraselmis suecica and examined their ability to compete for phosphorus. Both these organisms co-occur in modern European coastal waters and are not known to have any allelopathic capabilities. The strains were cultured in mono and mixed cultures in chemostats across a range of dissolved inorganic phosphorus (DIP) concentrations to reflect modern and ancient oceanic conditions of 2 μM P and 0.2 μM P, respectively. Our results show that the cyanobacteria outcompete the algae at the low input (0.2 μM P) treatment, yet the eukaryotic algae were not completely excluded and remained a constant background component in the mixed-culture experiments. Also, despite their relatively large cell size, the algae T. suecica had a high affinity for DIP. With DIP input concentrations resembling modern-day levels (2 μM), the eukaryotic algae could effectively compete against the cyanobacteria in terms of total biomass production. These results suggest that the availability of phosphorus could have influenced the global expansion of eukaryotic algae. However, P limitation does not seem to explain the complete absence of eukaryotic algae in the biomarker record before ca. 750 Ma.

Seasonal Niche Partitioning of Surface Temperate Open Ocean Prokaryotic Communities

Surface microbial communities are exposed to seasonally changing environmental conditions, resulting in recurring patterns of community composition. However, knowledge on temporal dynamics of open ocean microbial communities remains scarce. Seasonal patterns and associations of taxa and oligotypes from surface and chlorophyll maximum layers in the western Mediterranean Sea were studied over a 2-year period. Summer stratification versus winter mixing governed not only the prokaryotic community composition and diversity but also the temporal dynamics and co-occurrence association networks of oligotypes. Flavobacteriales, Rhodobacterales, SAR11, SAR86, and Synechococcales oligotypes exhibited contrasting seasonal dynamics, and consequently, specific microbial assemblages and potential inter-oligotype connections characterized the different seasons. In addition, oligotypes composition and dynamics differed between surface and deep chlorophyll maximum (DCM) prokaryotic communities, indicating depth-related environmental gradients as a major factor affecting association networks between closely related taxa. Taken together, the seasonal and depth specialization of oligotypes suggest temporal dynamics of community composition and metabolism, influencing ecosystem function and global biogeochemical cycles. Moreover, our results indicate highly specific associations between microbes, pointing to keystone ecotypes and fine-tuning of the microbes realized niche.