The chronic effects of oil pollution on marine phytoplankton in a subtropical bay, China

De-coupled phytoplankton growth and microzooplankton grazing in a simulated oil spill event in mesocosms

Microzooplankton (<200 μm) are essential intermediates between primary production and organisms at the higher trophic levels. Their ecological functions could be substantially affected by crude oil pollution. A natural plankton community was exposed to 10 μL L^-1 of chemically dispersed crude oil (DOil) in outdoor mesocosms for 7 days, with control (Ctrl) mesocosms set up for comparison. Dilution experiments were conducted to estimate the grazing rates of microzooplankton on the 2nd and 6th days of the pollutants exposure. Results showed 0.36-2.28 d^-1 microzooplankton grazing rates in the Ctrl mesocosms on both days but negative rates in the DOil mesocosms. A significant linear relationship between in situ phytoplankton growth and microzooplankton grazing rates was found in the Ctrl treatment but not in the DOil treatment. This suggests a de-coupling between phytoplankton growth and microzooplankton and the potential for the formation of phytoplankton blooms in seawater after an oil spill event.

Stimulatory and inhibitory effects of phenanthrene on physiological performance of Chlorella vulgaris and Skeletonema costatum

The effects of polycyclic aromatic hydrocarbons on phytoplankton have been extensively documented, but there is limited knowledge about the physiological responses of marine primary producers to phenanthrene at environmentally relevant levels. Here, we investigated the toxicity of phenanthrene (0, 1, and 5 or 10 μg L⁻¹) to the physiological performance of two cosmopolitan phytoplankton species: the green alga Chlorella vulgaris and bloom-forming diatom Skeletonema costatum. The specific growth rates of both species were remarkably inhibited at both low (1 μg L⁻¹) and high phenanthrene concentrations (5 or 10 μg L⁻¹), while their tolerance to phenanthrene differed. At the highest phenanthrene concentration (10 μg L⁻¹), the growth of C. vulgaris was inhibited by 69%, and no growth was observed for S. costatum cells. The superoxide dismutase activity of both species was enhanced at high phenanthrene concentration, and increased activity of catalase was only observed at high phenanthrene concentration in C. vulgaris. Interestingly, the low phenanthrene concentration stimulated the photosynthetic and relative electron transport rates of S. costatum, whereas hormetic effects were not found for growth. Based on our results, phenanthrene could be detrimental to these two species at a environmentally relevant level, while different tolerance levels were detected.

Cellular accumulation of crude oil compounds reduces the competitive fitness of the coral symbiont Symbiodinium glynnii

Oil spill events in the marine environment can have a deleterious impact on the affected ecosystems, such as coral reefs, with direct consequences for their socioeconomic value. The mutualistic relationship between tropical corals and their dinoflagellate symbionts (Symbiodiniaceae) provide structural and nutritional basis for a high local biodiversity in oligotrophic waters. Here, we investigated effects of crude oil water-accommodated fraction on the competitive fitness of the model zooxanthellae species Symbiodinium glynnii. Results of laboratory essays demonstrate that crude oil carbon is incorporated into the cellular biomass with a concomitant change of δ¹³C isotopic value. Carcinogenic/mutagenic polycyclic aromatic hydrocarbons were identified in the culture media and were responsible for a linear reduction in population growth of S. glynnii, presumably related to energy relocation for DNA repair. Additionally, the experiments revealed that physiological effects induced by crude oil compounds are genetically inherited by the following generations under non-contaminated growth conditions, and induce a reduction in the competitive fitness to cope with other environmental parameters, such as low salinity. We suggest that the effects of crude oil contamination represent an imparing factor for S. glynnii coping with anthropogenic drivers (e.g. warming and acidification) and interfere with the delicate symbiont-host relationship of tropical corals. This is especially relevant in the coastal areas of northeastern Brazil where an oil spill event deposited crude oil on shallow water sediments with the potential to be resuspended to the water column by physical and/or biological activity, enhancing the risk of future coral bleaching events.

Marine phytoplankton responses to oil and dispersant exposures: Knowledge gained since the Deepwater Horizon oil spill

The Deepwater Horizon oil spill of 2010 brought the ecology and health of the Gulf of Mexico to the forefront of the public's and scientific community's attention. Not only did we need a better understanding of how this oil spill impacted the Gulf of Mexico ecosystem, but we also needed to apply this knowledge to help assess impacts from perturbations in the region and guide future response actions. Phytoplankton represent the base of the food web in oceanic systems. As such, alterations of the phytoplankton community propagate to upper trophic levels. This review brings together new insights into the influence of oil and dispersant on phytoplankton. We bring together laboratory, mesocosm and field experiments, including insights into novel observations of harmful algal bloom (HAB) forming species and zooplankton as well as bacteria-phytoplankton interactions. We finish by addressing knowledge gaps and highlighting key topics for research in novel areas.

The Phytoplankton Taxon-Dependent Oil Response and Its Microbiome: Correlation but Not Causation

Phytoplankton strongly interact with their associated bacteria, both attached (PA), and free-living (FL), and bacterial community structures can be specific to phytoplankton species. Similarly, responses to environmental stressors can vary by taxon, as exemplified by observed shifts in phytoplankton community structure from diatoms to phytoflagellates after the Deepwater Horizon (DWH) oil spill. Here, we assess the extent to which associated bacteria influence the phytoplankton taxon-specific oil response by exposing xenic and axenic strains of three phytoplankton species to oil and/or dispersant. The dinoflagellates Amphidinium carterae and Peridinium sociale, and the diatom Skeletonema sp., all harbored significantly distinct bacterial communities that reflected their host oil response. Oil degrading bacteria were detected in both PA and FL communities of the oil resistant dinoflagellates, but their FL bacteria were more efficient in lipid hydrolysis, a proxy for oil degradation capability. Inversely, the growth rate and photosynthetic parameters of the diatom Skeletonema sp. was the most impacted by dispersed oil compared to the dinoflagellates, and oil-degrading bacteria were not significantly associated to its microbiome, even in the dispersed oil treatment. Moreover, the FL bacteria of Skeletonema did not show significant oil degradation. Yet, the lack of consistent significant differences in growth or photosynthetic parameters between the xenic and axenic cultures after oil exposure suggest that, physiologically, the associated bacteria do not modify the phytoplankton oil response. Instead, both oil resistance and phycosphere composition appear to be species-specific characteristics that are not causally linked. This study explores one aspect of what is undoubtedly a complex suite of interactions between phytoplankton and their associated bacteria; future analyses would benefit from studies of genes and metabolites that mediate algal-bacterial exchanges.

Consequences of a contaminant mixture of bisphenol A (BPA) and di-(2-ethylhexyl) phthalate (DEHP), two plastic-derived chemicals, on the diversity of coastal phytoplankton

To assess the impact of two plastic derived chemicals: bisphenol A (BPA) and the di-2-ethylhexyl phthalate (DEHP), on phytoplankton biomass and community structure, microcosm incubations were performed during spring and summer, with offshore and lagoon waters of a south-western Mediterranean ecosystem. Phytoplankton were exposed to an artificial mixture of BPA and DEHP and to marine water previously enriched with plastic-derivative compounds, originated from in situ water incubations of plastic debris for 30 days. After 96 h of incubation, changes were observed in phytoplankton biomass in the contaminated microcosms, with a net decrease (up to 50% of the control) in the concentration of Chlorophyll a in offshore waters. Concomitantly, plastic-derivative contamination provoked structural changes, especially for offshore waters. This suggests a relative tolerance of the lagoon communities to BPA and DEHP contamination, related to the dominance of Chaetoceros spp., which could potentially be used as a bioindicator in bioassessment studies.

PAHs sensitivity of picophytoplankton populations in the Red Sea

In this study, we investigated the in situ responses of Red Sea picophytoplankton, the dominant phytoplankton group in the oligotrophic ocean, to two toxic polycyclic aromatic hydrocarbons (PAHs), phenanthrene and pyrene. The experiments were conducted across a latitudinal gradient of the Saudi Arabian Red Sea, an area sensitive to oil pollution. We observed significant adverse effects on the growth and abundance of the picocyanobacteria Synechococcus and picoeukaryotes, at all stations sampled. Prochlorococcus, which was abundant only at one of the stations, also appeared to be affected. Pyrene was found to be more toxic to phytoplankton at all stations. In general, picoeukaryotes exhibited higher sensitivity to PAHs than Synechococcus. Populations in the highly oligotrophic Northern region of the Red Sea were more tolerant to PAHs, presumably influenced by the natural selection of more resistant strains of phytoplankton due to the prolonged exposure to PAHs. Toxicity threshold values estimated here are higher than those reported for picophytoplankton from other oligotrophic marine waters and exceed by far the natural levels of PAHs in many oceans. Our findings reveal a possible adaptation of picophytoplankton populations to oil-related contaminants, which may clearly influence their spatial distribution patterns in the Red Sea.

Rapid alterations to marine microbiota communities following an oil spill

Field data from the first several days after an oil spill is rare but crucial for our understanding of a spill's impact on marine microbiota given their short generation times. Field data collected within days of the Texas City "Y" oil spill showed that exposure to crude oil can rapidly imbalance populations of marine microbiota, which leads to the proliferation of more resistant organisms. Vibrionales bacteria were up to 48 times higher than background concentrations at the most impacted sites and populations of the dinoflagellate Prorocentrum texanum increased significantly as well. Laboratory microcosm experiments with a natural plankton community showed that P. texanum grew significantly faster under oiled conditions but monocultures of P. texanum did not. Additional laboratory experiments with natural communities from Tampa Bay, Florida showed similar results although a different species dominated, P. minimum. In both cases, tolerance to the presence of crude oil was enhanced by higher sensitivity of grazers led to a release from grazing pressure and allows Prorocentrum species to dominate after an oil spill. The results suggest careful monitoring for Vibrionales and Prorocentrum during future spills would be beneficial given the potential implications to human health.

Chronic adverse effects of oil dispersed sediments on growth, hatching, and reproduction of benthic copepods: Indirect exposure for long-term tests

Laboratory-scale sediment exposure was conducted as a preliminary study to assess the long-term effects of sediment contaminated with crude oil. For this purpose, indirect exposure using a glass filter crucible was tested and compared with direct exposure by observing several parameters (e.g., mortality, growth, reproduction, hatching, and uptake) in the benthic copepod Tigriopus japonicus. In direct exposure, short-term exposure caused significant damages to the eggs of ovigerous females, and there were difficulties in observing small oil droplets. However, indirect exposure did not induce any mortality during a 96-h exposure in adults. A 10-day exposure was also possible in an indirect exposure method and caused a decrease in reproduction and consequently a reduction in the hatching rate. In fact, the water phase collected from indirect exposure indicated significant polycyclic aromatic hydrocarbon (PAH) concentrations, although only a few components were present. The components of PAHs were similar to water-accommodated fractions (WAFs) of crude oil that are associated with the water-soluble part, but the relative portion of high-molecular-weight of PAHs was higher than WAF. In this approach, exposure tests caused reduction in the uptake rate in copepods even in the 24-h exposure. In conclusion, the biological effects of oil droplets from direct exposure were excluded by using a glass filter in indirect exposures, and several parameters could be derived in the long-term exposure. These results indicate that the indirect method could likely assess the chronic effects of oil-contaminated sediments on individual level parameters for deriving the ultimate effects on the population and community.

Study of the plasma proteome of Atlantic cod (Gadus morhua): Effect of exposure to two PAHs and their corresponding diols

Occurrence of polycyclic aromatic hydrocarbon (PAH) contamination in the marine environment represents a risk to marine life and humans. In this study, plasma samples from Atlantic cod (Gadus morhua) were analysed by shotgun mass spectrometry to investigate the plasma proteome in response to exposure to single PAHs (naphthalene or chrysene) and their corresponding metabolites (dihydrodiols). In total, 369 proteins were identified and ranked according to their relative abundance. The levels of 12 proteins were found significantly altered in PAH exposed fish and are proposed as new biomarker candidates. Eleven proteins were upregulated, primarily immunoglobulin components, and one protein was downregulated (antifreeze protein type IV.) The uniformity of the upregulated proteins suggests a triggered immune response in the exposed fish. Overall, the results provide valuable knowledge for future studies of the Atlantic cod plasma proteome and generate grounds for establishing new plasma protein biomarkers for environmental monitoring of PAH related exposure.

Immediate ecotoxicological effects of short-lived oil spills on marine biota

Marine environments are frequently exposed to oil spills as a result of transportation, oil drilling or fuel usage. Whereas large oil spills and their effects have been widely documented, more common and recurrent small spills typically escape attention. To fill this important gap in the assessment of oil-spill effects, we performed two independent supervised full sea releases of 5 m(3) of crude oil, complemented by on-board mesocosm studies and sampling of accidentally encountered slicks. Using rapid on-board biological assays, we detect high bioavailability and toxicity of dissolved and dispersed oil within 24 h after the spills, occurring fairly deep (8 m) below the slicks. Selective decline of marine plankton is observed, equally relevant for early stages of larger spills. Our results demonstrate that, contrary to common thinking, even small spills have immediate adverse biological effects and their recurrent nature is likely to affect marine ecosystem functioning.

Response of the bacterial community associated with a cosmopolitan marine diatom to crude oil shows a preference for the biodegradation of aromatic hydrocarbons

Emerging evidence shows that hydrocarbonoclastic bacteria (HCB) may be commonly found associated with phytoplankton in the ocean, but the ecology of these bacteria and how they respond to crude oil remains poorly understood. Here, we used a natural diatom-bacterial assemblage to investigate the diversity and response of HCB associated with a cosmopolitan marine diatom, Skeletonema costatum, to crude oil. Pyrosequencing analysis and qPCR revealed a dramatic transition in the diatom-associated bacterial community, defined initially by a short-lived bloom of Methylophaga (putative oil degraders) that was subsequently succeeded by distinct groups of HCB (Marinobacter, Polycyclovorans, Arenibacter, Parvibaculum, Roseobacter clade), including putative novel phyla, as well as other groups with previously unqualified oil-degrading potential. Interestingly, these oil-enriched organisms contributed to the apparent and exclusive biodegradation of substituted and non-substituted polycyclic aromatic hydrocarbons (PAHs), thereby suggesting that the HCB community associated with the diatom is tuned to specializing in the degradation of PAHs. Furthermore, the formation of marine oil snow (MOS) in oil-amended incubations was consistent with its formation during the Deepwater Horizon oil spill. This work highlights the phycosphere of phytoplankton as an underexplored biotope in the ocean where HCB may contribute importantly to the biodegradation of hydrocarbon contaminants in marine surface waters.

The positive relationship between ocean acidification and pollution

Ocean acidification and pollution coexist to exert combined effects on the functions and services of marine ecosystems. Ocean acidification can increase the biotoxicity of heavy metals by altering their speciation and bioavailability. Marine pollutants, such as heavy metals and oils, could decrease the photosynthesis rate and increase the respiration rate of marine organisms as a result of biotoxicity and eutrophication, facilitating ocean acidification to varying degrees. Here we review the complex interactions between ocean acidification and pollution in the context of linkage of multiple stressors to marine ecosystems. The synthesized information shows that pollution-affected respiration acidifies coastal oceans more than the uptake of anthropogenic carbon dioxide. Coastal regions are more vulnerable to the negative impact of ocean acidification due to large influxes of pollutants from terrestrial ecosystems. Ocean acidification and pollution facilitate each other, and thus coastal environmental protection from pollution has a large potential for mitigating acidification risk.

Interactive effects of silver nanoparticles and phosphorus on phytoplankton growth in natural waters

Increasing amounts of silver nanoparticles (AgNPs) are expected to enter the aquatic ecosystems where their effects on natural phytoplankton communities are poorly understood. We investigated the effects of AgNPs and its interactions with phosphorus (P) supply on the growth kinetics and stoichiometry of natural phytoplankton. Lake water was dosed with AgNPs (carboxy-functionalized capping agent; ∼10-nm particle size; ∼20% Ag w/w) at four different concentrations and five P concentrations and incubated in situ for 3 days. A treatment with ionic silver (AgNO3) was used as a positive control. We found that growth rates, calculated from changes in seston carbon and chlorophyll, responded significantly and interactively (p < 0.0001) to both AgNPs and P. AgNPs reduced the maximum phytoplankton growth rates by 11-85%. In the positive control, no or very little growth was observed. Inhibition of growth rates after exposure to Ag might be related to the reduction in chlorophyll and the inhibition of C and N acquisition rather than P uptake mechanisms. AgNPs, P supply and their interactions also significantly (p < 0.0001) reduced sestonic C:P and N:P ratios and increased C:N, C:Chl and cell-bound Ag stoichiometry. Our results indicate that fate and toxicity of AgNP will vary with phosphorus pollution level in aquatic ecosystems.

Dispersed oil disrupts microbial pathways in pelagic food webs

Most of the studies of microbial processes in response to the Deepwater Horizon oil spill focused on the deep water plume, and not on the surface communities. The effects of the crude oil and the application of dispersants on the coastal microbial food web in the northern Gulf of Mexico have not been well characterized even though these regions support much of the fisheries production in the Gulf. A mesocosm experiment was carried out to determine how the microbial community off the coast of Alabama may have responded to the influx of surface oil and dispersants. While the addition of glucose or oil alone resulted in an increase in the biomass of ciliates, suggesting transfer of carbon to higher trophic levels was likely; a different effect was seen in the presence of dispersant. The addition of dispersant or dispersed oil resulted in an increase in the biomass of heterotrophic prokaryotes, but a significant inhibition of ciliates, suggesting a reduction in grazing and decrease in transfer of carbon to higher trophic levels. Similar patterns were observed in two separate experiments with different starting nutrient regimes and microbial communities suggesting that the addition of dispersant and dispersed oil to the northern Gulf of Mexico waters in 2010 may have reduced the flow of carbon to higher trophic levels, leading to a decrease in the production of zooplankton and fish on the Alabama shelf.