Plankton Population Dynamics Research Articles

Analytical and Numerical Study of the Problem of the Plankton Population Dynamics in the Presence of Microplastics

Analytical and Numerical Study of the Problem of the Plankton Population Dynamics in the Presence of Microplastics

Interactions between Noctiluca scintillans and Three Co-Occurring Microalgae in Response to Varying Nutrient Levels

The dinoflagellate Noctiluca scintillans is a globally distributed bloom-forming species. Previous studies have shown that the primary reason for the frequent occurrence of N. scintillans blooms may be the proliferation of microalgae due to eutrophication, which provides a sufficient source of food. Meanwhile, N. scintillans may release nutrients into the environment, thus affecting the population dynamics of microalgae. Thus, to investigate the interaction between N. scintillans and co-occurring microalgae, this study examined the population dynamics of N. scintillans and their interaction with three representative microalgae species in response to varying nutrient levels. The findings indicate that the growth of N. scintillans is slow when co-cultured with diatom Skeletonema costatum. Moreover, a high density and rapid growth rate of S. costatum may have an inhibitory effect on the growth of N. scintillans. Conversely, the population abundance of N. scintillans increased with the rise in the population density and nutritional level of Heterocapsa steinii (dinoflagellate) and Heterosigma akashiwo (raphidophyceae). Notably, N. scintillans can discharge specific nutrients into the aquatic environment, which can subsequently be assimilated and exploited by H. steinii. Thus, the interaction between the species and population dynamics of plankton, as well as changes in nutrient levels within the ecosystem, played a significant role in influencing the growth and population dynamics of N. scintillans. The mutualistic association between N. scintillans and microalgae may establish a transient closed loop, thereby fostering the sustained proliferation and subsequent expansion of N. scintillans.

Plankton population dynamics and methylmercury bioaccumulation in the pelagic food web of mine-impacted surface water reservoirs

Thermal stratification of reservoirs can lead to anaerobic conditions that facilitate the microbial conversion of mercury (Hg) to neurotoxic and bioaccumulative methylmercury (MeHg). But MeHg production is just the first step in a complex set of processes that affect MeHg in fish. Of particular relevance is uptake into suspended particulate matter (SPM) and zooplankton at the base of the pelagic food web. We assessed plankton dynamics and Hg uptake into the pelagic food web of four Hg-impaired California water reservoirs. Combining water chemistry, plankton taxonomy, and stable carbon (C) and nitrogen (N) isotope values of SPM and zooplankton samples, we investigated differences among the reservoirs that may contribute to differing patterns in MeHg bioaccumulation. Methylmercury accumulated in SPM during the spring and summer seasons. Percent MeHg (MeHg/Hg*100%) in SPM was negatively associated with δ15N values, suggesting that “fresh” algal biomass could support the production and bioaccumulation of MeHg. Zooplankton δ13C values were correlated with SPM δ13C values in the epilimnion, suggesting that zooplankton primarily feed in surface waters. However, zooplankton MeHg was poorly associated with MeHg in SPM. Our results demonstrate seasonal patterns in biological MeHg uptake and how multiple data sources can help constrain the drivers of MeHg bioaccumulation.

Algorithm for monitoring the plankton population dynamics based on satellite sensing data

The scientific work describes the algorithms for processing the multispectral water coastal imagery from satellite sensing data with the aim of identifying the phytoplankton population of a spotted structure: determining the contour, distributing color gradation and as a result - determining the concentration of phytoplankton distribution inside the zones and mass centers. Such characteristics let determine the speed of changing contours spots and their concentration, the mass center shift as a consequence of the water masses movement and the processes of phytoplankton growing and dying. All these may be done on the base of the processed image series of the same water area over different time (different dates). The combination of LBP and neural network methods are observed as algorithms for image processing and the results of computer experiments are presented.

Impact of Port and harbour activities on plankton distribution and dynamics: A multivariate approach

Numerous researches have been conducted to study the impact of Port activities on water quality. However, investigation on the response of the inhabiting biota to Port related extrinsic factors is extremely limited. Using Canonical Correspondence Analysis, comprehensive assessment of plankton population dynamics and distribution corresponding to the spatial heterogeneity in physico-chemical characteristics of harbour and adjacent coastal water is conducted. The results revealed resource competition for nitrate, predator mediated co-existence of species favoured by active switching and Port influenced environmental filtering due to suspended solids, and oil and grease as the predominant factor to modulate the plankton community structure and succession in harbour. The higher heterogeneity in the measured environmental variables at coast revealed dispersion of nutrient and other pollutants from harbour into the adjacent coastal water during ebb tide. Our results indicate reengineering of Port infrastructures and efficient management policies including water quality standards to facilitate proper water circulation and minimize pollution with a long term goal to reconcile economic interest with ecosystem conservation.

Storm-Induced Predator-Prey Decoupling Promotes Springtime Accumulation of North Atlantic Phytoplankton

During NAAMES May 2016 field campaign, we examined the response of plankton population dynamics to rapid re-stratification of surface waters following a deep mixing event in the subpolar North Atlantic. Over the 4-day occupation of a station, we measured phytoplankton growth and grazer/virus-induced mortality rates in dilution assays conducted across a range of light intensities. Rates were estimated from changes in chlorophyll a and the abundance of three phytoplankton groups identified by flow cytometry. Initially, biological and physical water column properties were homogeneous down to >200 m, followed by rapid shoaling of the mixed layer to 20-30 m. Initial in situ chlorophyll a concentration was 0.4 µg L-1, and phytoplankton biomass accumulated at an average rate of 0.4 d-1 over the next two days. When mixed layer depth was maximal, there were no mortality losses and phytoplankton growth rates increased with increased light intensity. After shoaling, grazing rates increased, but never matched the magnitude of phytoplankton growth rates. When the mixed layer was shallowest, growth rates exceeded >1 d-1 at all non-dark light intensities. Chlorophyll a based grazing rates were consistent across light levels (~0.3 d-1) and were highest on Synechococcus (0.3-0.6 d-1) and lowest on pico-eukaryotes (~0.2 d-1). The delay with which grazing resumed resulted in growth exceeding losses and consumption of an average of 30% of primary production. Virus-induced mortality rates were minimal across all mixing profiles and light intensities. Overall, both chlorophyll a and group-specific phytoplankton accumulation rates predicted from the shipboard experiments matched those in situ, suggesting that incubation conditions faithfully captured the growth and loss processes governing in situ population dynamics. The observations made here indicate that transient deepening of the mixed layer followed by rapid stratification provided conditions under which phytoplankton escape protistan grazing and accumulate biomass. During the physically dynamic springtime in the North Atlantic, frequent ephemeral fluctuations in the depth of mixing may represent an important mechanism governing the formation and the magnitude of the North Atlantic spring bloom.

Physical flow effects can dictate plankton population dynamics.

Oceanic flows do not necessarily mix planktonic species. Differences in individual organisms’ physical and hydrodynamic properties can cause changes in drift normal to the mean flow, leading to segregation between species. This physically driven heterogeneity may have important consequences at the scale of population dynamics. Here, we describe how one form of physical forcing, circulating flows with different inertia effects between phytoplankton and zooplankton, can dramatically alter excitable plankton bloom dynamics. This may impact our understanding of the initiation and development of harmful algal blooms (HABs), which have significant negative ecological and socio-economic consequences. We study this system in detail, providing spatio-temporal dynamics for particular scenarios and summarizing large-scale behaviour via spatially averaged bifurcation diagrams. The key message is that, across a large range of parameter values, fluid flow can induce plankton blooms and mean-field population dynamics that are distinct from those predicted for well-mixed systems. The implications for oceanic population dynamic studies are manifest: we argue that the formation of HABs will depend strongly on the physical and biological state of the ecosystem, and that local increases in zooplankton heterogeneity are likely to precede phytoplankton blooms.

Transcriptomic Response to Feeding and Starvation in a Herbivorous Dinoflagellate

Grazing by heterotrophic protists influences plankton population dynamics, community composition, and the flux of carbon through marine planktonic food webs. To gain insight into the molecular underpinnings of grazing in dinoflagellates, a group of important heterotrophic protists, we used an RNA-seq approach to investigate the transcriptomic response of Oxyrrhis marina under fed and starved treatments with three different phytoplankton prey (Isochrysis galbana and two strains of Heterosigma akashiwo). In response to fed and starved conditions, 1,575 transcripts were significantly differentially expressed in O. marina. Fed O. marina cells upregulated transcripts involved in the synthesis of essential fatty acids and storage carbohydrates suggesting that the predator was food satiated and excess glucose was being stored as an energy reserve. Transcripts encoding voltage-gated ion channels were also upregulated during grazing, and they are known to be involved in the detection of mechanical stimuli and the regulation of swimming behavior in several eukaryotic protists. Fed O. marina cells upregulated kinases, which can dictate cell shape changes and may be associated with phagocytosis. During starvation, upregulated O. marina transcripts included those involved in the degradation of energy-storage molecules like glucan 1,4 alpha-glycosidase and those involved in antioxidant activities and autophagy, like acid ceramidase that are associated with the digestion of polar lipids present in cell membranes. Starved O. marina also upregulated transcripts with high similarity to proton pumping proteorodopsins suggesting that this heterotrophic protist may supplement its energy requirement during starvation with a light harvesting mechanism. Although herbivorous grazing is a pivotal transformation in the C cycle, logistical constraints limit our investigations of environmental and biological drivers. The molecular signals identified here provide new insights into the metabolic regulation of feeding and starvation in marine heterotrophic protists and can fuel hypothesis-driven research into predators' metabolic response to prey availability.

Herbivorous protist growth and grazing rates at in situ and artificially elevated temperatures during an Arctic phytoplankton spring bloom.

To assess protistan grazing impact and temperature sensitivity on plankton population dynamics, we measured bulk and species-specific phytoplankton growth and herbivorous protist grazing rates in Disko Bay, West Greenland in April-May 2011. Rate estimates were made at three different temperatures in situ (0 °C), +3 °C and +6 °C over ambient. In situ Chlorophyll a (Chl a) doubled during the observation period to ∼12 µg Chl a L−1, with 60–97% of Chl a in the >20 µm size-fraction dominated by the diatom genus Chaetoceros. Herbivorous dinoflagellates comprised 60–80% of microplankton grazer biomass. At in situ temperatures, phytoplankton growth or grazing by herbivorous predators <200 µm was not measurable until 11 days after observations commenced. Thereafter, phytoplankton growth was on average 0.25 d−1. Phytoplankton mortality due to herbivorous grazing was only measured on three occasions but the magnitude was substantial, up to 0.58 d−1. Grazing of this magnitude removed ∼100% of primary production. In short-term temperature-shift incubation experiments, phytoplankton growth rate increased significantly (20%) at elevated temperatures. In contrast, herbivorous protist grazing and species-specific growth rates decreased significantly (50%) at +6 °C. This differential response in phytoplankton and herbivores to temperature increases resulted in a decrease of primary production removed with increasing temperature. Phaeocystis spp. abundance was negatively correlated with bulk grazing rate. Growth and grazing rates were variable but showed no evidence of an inherent, low temperature limitation. Herbivorous protist growth rates in this study and in a literature review were comparable to rates from temperate waters. Thus, an inherent physiological inhibition of protistan growth or grazing rates in polar waters is not supported by the data. The large variability between lack of grazing and high rates of primary production removal observed here and confirmed in the literature for polar waters implies larger amplitude fluctuations in phytoplankton biomass than slower, steady grazing losses of primary production.

Complex dynamics of a nutrient-plankton system with nonlinear phytoplankton morality and allelopathy

Understanding the plankton dynamics can help us take effective measures to settle the critical issue on how to keep plankton ecosystem balance. In this paper, a nutrient-phytoplankton-zooplankton (NPZ) model is formulated to understand the mechanism of plankton dynamics. To account for the harmful effect of the phytoplankton allelopathy, a prototype for a non-monotone response function is used to model zooplankton grazing, and nonlinear phytoplankton mortality is also included in the NPZ model. Using the model, we will focus on understanding how the phytoplankton allelopathy and nonlinear phytoplankton mortality affect the plankton population dynamics. We first examine the existence of multiple equilibria and provide a detailed classification for the equilibria, then stability and local bifurcation analysis are also studied. Sufficient conditions for Hopf bifurcation and zero-Hopf bifurcation are given respectively. Numerical simulations are finally conducted to confirm and extend the analytic results. Both theoretical and numerical findings imply that the phytoplankton allelopathy and nonlinear phytoplankton mortality may lead to a rich variety of complex dynamics of the nutrient-plankton system. The results of this study suggest that the effects of the phytoplankton allelopathy and nonlinear phytoplankton mortality should receive more attention to understand the plankton dynamics.

Unexpected prevalence of parasite 18S rDNA sequences in winter among Antarctic marine protists

Parasites are not typically considered to be important components of polar marine ecosystems. It was therefore surprising when 18S rDNA surveys of protists in the West Antarctic Peninsula in winter revealed high abundances of parasite sequences. Parasite sequences made up, on average, over half (52%) of sequence reads in samples from deep water in winter. Winter surface water and sediment samples contained relatively fewer, but still strikingly high, parasite sequence reads (13 and 9%, respectively), while surface water samples in summer contained fewer parasite sequences (1.8%). A total of 1028 distinct parasite Operational Taxonomic Units were observed in winter, with the largest abundances and diversities within Syndiniales groups I and II, including Amoebophrya. Less abundant parasite sequence groups included Apicomplexa, Blastodinium, Chytriodinium, Cryptocaryon, Paradinium, Perkinsidae, Pirsonia and Ichthyophonae. Parasite sequence distributions suggested interactions with known hosts, such as diatom parasites which were mainly in the sediments, where resting spores of Chaetoceros spp. diatoms were abundant. Syndiniales sequences were correlated with radiolarian sequences, suggesting parasite–host interactions. The abundant proportions of parasite sequences indicate a potentially important role for parasites in the Antarctic marine ecosystem, with implications for plankton population dynamics, the role of the microbial loop, carbon flows and ecosystem responses to ongoing anthropogenic climate change.

Warming accelerates termination of a phytoplankton spring bloom by fungal parasites

Climate change is expected to favour infectious diseases across ecosystems worldwide. In freshwater and marine environments, parasites play a crucial role in controlling plankton population dynamics. Infection of phytoplankton populations will cause a transfer of carbon and nutrients into parasites, which may change the type of food available for higher trophic levels. Some phytoplankton species are inedible to zooplankton, and the termination of their population by parasites may liberate otherwise unavailable carbon and nutrients. Phytoplankton spring blooms often consist of large diatoms inedible for zooplankton, but the zoospores of their fungal parasites may serve as a food source for this higher trophic level. Here, we investigated the impact of warming on the fungal infection of a natural phytoplankton spring bloom and followed the response of a zooplankton community. Experiments were performed in ca. 1000 L indoor mesocosms exposed to a controlled seasonal temperature cycle and a warm (+4 °C) treatment in the period from March to June 2014. The spring bloom was dominated by the diatom Synedra. At the peak of infection over 40% of the Synedra population was infected by a fungal parasite (i.e. a chytrid) in both treatments. Warming did not affect the onset of the Synedra bloom, but accelerated its termination. Peak population density of Synedra tended to be lower in the warm treatments. Furthermore, Synedra carbon: phosphorus stoichiometry increased during the bloom, particularly in the control treatments. This indicates enhanced phosphorus limitation in the control treatments, which may have constrained chytrid development. Timing of the rotifer Keratella advanced in the warm treatments and closely followed chytrid infections. The chytrids' zoospores may thus have served as an alternative food source to Keratella. Our study thus emphasizes the importance of incorporating not only nutrient limitation and grazing, but also parasitism in understanding the response of plankton communities towards global warming.

Marine ecosystem health assessments in Korean coastal waters

Coastal pollution has long been recognized as a serious problem. Unfortunately, no simple solutions exist for sustaining a healthy ecosystem in coastal zones. Until now, approaches have only focused on affected areas and associated pollutants without much attention being paid to more preventative measures. A more comprehensive framework is appropriate for sustaining coastal marine health. Based on adequate policy making, a foundation for research could be established, and coastal environmental conditions could then be evaluated through regular assessments under new guidelines. The assessment results would provide instructive data for future decisions by policy makers concerning the restoration and maintenance of coastal health. For the purpose of developing technical tools for assessing coastal ecosystem health conditions and providing practical ecological information, a project is currently being undertaken to develop the Marine Ecosystem Health Index (MEHI) based on organizing, simplifying, and generalizing the holistic data of marine ecosystems. During the last three years, 20102012, coastal ecosystem components were monitored in Jinhae Bay and Gwangyang Bay which are known to be highly polluted and which are special management areas in the South Sea of Korea. As part of the ongoing work to increase technical development, assessment indicators dealing with the relationships between chemical pressures and biological responses were selected and a monitoring system for indicator measurements was also established. Indicator criteria for determining healthy or unhealthy conditions in the ecosystem monitored were established based on the results analyzed from past data as well as the conditions monitored in this project. Finally, a numerical expression tool applying the selected indicators and the criteria was generated for the integration of bay ecosystem health assessment. Water environments were studied in terms of eutrophication and sediment environments with regard to toxic organic compounds, and heavy metals were also monitored to understand the environmental conditions produced by chemical pollutants. Community structures and population dynamics of plankton and benthos were investigated at the same time, and these were intensely analyzed to understand the relationships with chemical factors. Each of these four subdivisions of water quality, sediment quality, plankton, and benthos was repectively expressed and integrated into a numerical expression to produce a health index or score. Finally, we were able to recognize which areas of Jinhae Bay and Gwangyang Bay are relatively healthy based on this assessment result and were able to prioritize actions to restore the health of the bay ecosystems. In a forthcoming step, the four indices will be integrated into a holistic health index of marine ecosystem assessments. The assessment results will be provided through an online service in the near future and will be shared with all concerned. The project is a pioneering effort to assess the health of marine ecosystems in Korea and is continuously making efforts to ensure technical superiority with regard to application in other special management areas and to provide technical knowhow with regard to marine ecosystem health assessment based on the work carried out in Jinhae Bay and Gwangyang Bay (Picture 1).

Chaotic dynamics in a simple dynamical green ocean plankton model

The exchange of important greenhouse gases between the ocean and atmosphere is influenced by the dynamics of near-surface plankton ecosystems. Marine plankton ecosystems are modified by climate change creating a feedback mechanism that could have significant implications for predicting future climates. The collapse or extinction of a plankton population may push the climate system across a tipping point. Dynamic green ocean models (DGOMs) are currently being developed for inclusion into climate models to predict the future state of the climate. The appropriate complexity of the DGOMs used to represent plankton processes is an ongoing issue, with models tending to become more complex, with more complicated dynamics, and an increasing propensity for chaos.We consider a relatively simple (four-population) DGOM of phytoplankton, zooplankton, bacteria and zooflagellates where the interacting plankton populations are connected by a single limiting nutrient. Chaotic solutions are possible in this 4-dimensional model for plankton population dynamics, as well as in a reduced 3-dimensional model, as we vary two of the key mortality parameters. Our results show that chaos is robust to the variation of parameters as well as to the presence of environmental noise, where the attractor of the more complex system is more robust than the attractor of its simplified equivalent. We find robust chaotic dynamics in low trophic order ecological models, suggesting that chaotic dynamics might be ubiquitous in the more complex models, but this is rarely observed in DGOM simulations. The physical equations of DGOMs are well understood and are constrained by conservation principles, but the ecological equations are not well understood, and generally have no explicitly conserved quantities. This work, in the context of the paucity of the empirical and theoretical bases upon which DGOMs are constructed, raises the interesting question of whether DGOMs better represent reality if they include or exclude chaotic dynamics, but also points to a need for a comprehensive approach to model development and testing. We contend that the DGOM community can learn important lessons from the analysis of simple models, and should consider a spectrum of methods from the analysis of simple(r) zero-dimensional models of population interactions through to numerical simulations of fully coupled biogeophysical models.

Differing assemblage composition and dynamics in T4‐like myophages of two neighbouring sub‐alpine lakes

Summary Bacteriophages play an important role in plankton population dynamics and biogeochemical cycling, but their community dynamics and diversity are still poorly known, especially in fresh waters. We conducted a 1‐year investigation of the T4‐like bacteriophages in the surface waters of two Western European peri‐alpine lakes (lakes Annecy and Bourget), using fingerprinting and cloning‐sequencing approaches targeting the major capsid protein g23 gene. Our results suggest that T4‐like bacteriophages are diverse (we found several new sequences) and differed substantially between the two lakes. Further, these phages displayed seasonal patterns with marked shifts in community composition. Examining the dynamics of some individual constituents (e.g. DGGE bands), we found that (i) the majority of these myoviruses were rare (mean relative abundance &lt;1%) and only a few were abundant (mean relative abundance &gt;5%); (ii) the dominance of the most abundant DGGE bands changed throughout the year and their dynamics were very different between the two lakes; (iii) only a few bands (e.g. &lt;20%) were detectable throughout the year and their proportion was higher in the oligomesotrophic Lake Bourget. Our results suggest a highly dynamic T4‐like myoviral assemblage. T4‐like‐viruses seem to conform to a standard model of community organisation and a common type of species abundance curve, with mainly rare organisms that occasionally may become abundant (termed a ‘seed bank’ for virus assemblages).

Stratification, diel and seasonal energy transfers in M alilangwe R eservoir in the south‐eastern lowveld of Z imbabwe

This study provides a 9-month record of Malilangwe Reservoir stratification and diel and seasonal energy transfers between February and October 2011. The reservoir was not strongly stratified during the hot-wet season (February and March), exhibiting low dissolved oxygen concentrations of <2 mg DO L−1 in the bottom layers (<6 m depth). During the cool-dry season, when both the air temperature and solar radiation input decreased, the water temperatures in the epilimnion and across the thermocline decreased, and the water column was very weakly stratified. In this state, the reservoir could easily be mixed, even by light winds, resulting in overturning, thereby giving the bottom water an opportunity to become aerated, as observed during the cool-dry season. The stability was greatest when the water level was high (during midday), while it was considerably lower during the predawn hours. The calculated annual Birgean heat budget for the reservoir was 110 MJ m−2 year−1 for February to October 2011. The diurnal Birgean heat budget was consistently associated with heat loss over a 24 hour period in all seasons, mostly between 0400 and 1000 h. Spearman's correlation suggests that the DO was significantly correlated with stability for the three study months (February r = 0.69, P < 0.05; June r = −0.76, P < 0.001; October r = 0.96, P < 0.001). The results of this study suggests that warming temperatures (warmer months or seasons) will result in high Birgean heat budgets, which could potentially trigger some important changes in plankton population dynamics in small reservoir, with severe water quality implications. The nutrient concentrations varied throughout the seasons.

Stirring effects in models of oceanic plankton populations

We present an overview and extend previous results on the effects of large scale oceanic transport processes on plankton population dynamics, considering different types of ecosystem models. We find that increasing stirring rate in an environment where the carrying capacity is non-uniformly distributed leads to an overall decrease of the effective carrying capacity of the system. This may lead to sharp regime shifts induced by stirring in systems with multiple steady states. In prey-predator type systems, stirring leads to resonant response of the population dynamics to fluctuations enhancing the spatial variability-patchiness-in a certain range of stirring rates. Oscillatory population models produce strongly heterogeneous patchy distribution of plankton blooms when the stirring is weak, while strong stirring may either synchronise the oscillatory dynamics, when the inhomogeneity is relatively weak, or suppress oscillations completely (oscillator death) by reducing the effective carrying capacity below the bifurcation point.

Biophysical interactions in the plankton: A cross‐scale review

Lay AbstractThe study of plankton ecology is by nature interdisciplinary, since both biology and physics interact to shape plankton distributions and population dynamics. Small‐scale turbulence affects how plankton feed and grow, while kilometer‐scale physical features can form large plankton patches. Recent findings also show that, in addition to physics affecting plankton dynamics, plankton may be able to influence ocean physics as well. For example, the fluid motion produced from a copepod swimming in pursuit of food can be detected by nearby organisms and might make it more vulnerable to its predators. The field of plankton biophysics has advanced rapidly in the past several decades, due in large part to technological advances that allow the study of interactions between fluid flow and plankton in their natural environment at smaller scales than ever before. Biophysical interactions in plankton ecology can occur at a wide range of scales: from the scale of individual organisms up to the scale of the world's oceans. In this article, we review biophysical interactions in plankton across a wide range of scales, focusing on the most recent research in the field. In addition, we discuss how these processes are linked with other processes that occur at smaller and larger scales. For example, plankton patches formed by ocean eddies can provide feeding grounds for other organisms, thus influencing global ocean food web dynamics. Finally, we discuss some of the potential effects of long‐term changes in global climate, which will affect ocean temperature and stratification, thus regulating the occurrence and intensity of biophysical plankton processes.

High latitude regulation of low latitude thermocline ventilation and planktic foraminifer populations across glacial–interglacial cycles

One of the earliest discoveries in palaeoceanography was the observation in 1935 that the (sub)tropical planktic foraminifer Globorotalia menardii became absent or extremely rare in the Atlantic Ocean during glacials of the late Pleistocene. Yet a mechanistic explanation for G. menardii's extraordinary biogeographic behaviour has eluded palaeoceanographers for 75 years. Here we show that modern G. menardii, along with two other species that also suffer Atlantic population collapses during glacials, track poorly ventilated waters globally in their thermocline habitats. The ventilation states of low latitude thermoclines are ‘set’, to a first order, by intermediate water masses originating at high latitudes. In the modern Atlantic this control on low latitude thermocline ventilation is exerted by relatively poorly ventilated, southern-sourced Antarctic Intermediate Water (AAIW) and sub-Antarctic Mode Water (SAMW). We suggest that the glacial Atlantic foraminifer population collapses were a consequence of a low latitude thermocline that was better ventilated during glacials than it is today, in line with geochemical evidence, and driven primarily by a well-ventilated, northern-sourced intermediate water mass. A ventilation mechanism driving the glacial population collapses is further supported by our new constraints on the precise timing of these species' Atlantic proliferation during the last deglaciation — occurring in parallel with a wholesale, bipolar reorganisation of the Atlantic's thermocline-to-abyssal overturning circulation. Our findings demonstrate that a bipolar seesaw in the formation of high latitude intermediate waters has played an important role in regulating the population dynamics of thermocline-dwelling plankton at lower latitudes.

Interactions of the algicidal bacterium Kordia algicida with diatoms: regulated protease excretion for specific algal lysis.

Interactions of planktonic bacteria with primary producers such as diatoms have great impact on plankton population dynamics. Several studies described the detrimental effect of certain bacteria on diatoms but the biochemical nature and the regulation mechanism involved in the production of the active compounds remained often elusive. Here, we investigated the interactions of the algicidal bacterium Kordia algicida with the marine diatoms Skeletonema costatum, Thalassiosira weissflogii, Phaeodactylum tricornutum, and Chaetoceros didymus. Algicidal activity was only observed towards the first three of the tested diatom species while C. didymus proved to be not susceptible. The cell free filtrate and the >30 kDa fraction of stationary K. algicida cultures is fully active, suggesting a secreted algicidal principle. The active supernatant from bacterial cultures exhibited high protease activity and inhibition experiments proved that these enzymes are involved in the observed algicidal action of the bacteria. Protease mediated interactions are not controlled by the presence of the alga but dependent on the cell density of the K. algicida culture. We show that protease release is triggered by cell free bacterial filtrates suggesting a quorum sensing dependent excretion mechanism of the algicidal protein. The K. algicida / algae interactions in the plankton are thus host specific and under the control of previously unidentified factors.