Environmental Science and EOS

Since its emergence in 2013, sea star wasting disease (SSWD) has quickly spread along the west coast of North America, infecting dozens of sea star species from Mexico to Alaska and upending marine ecosystems. A variety of causes of SSWD have been proposed over the past decade, but no clear cause has been isolated for what is now considered one of the largest marine epidemics. Sunflower sea stars, or Pycnopodia helianthoides, are considered one of the most vulnerable species to SSWD, with billions dying from SSWD since its emergence. Although sunflower sea stars once inhabited the entirety of the west coast of North America, they are now considered functionally extinct in much of their southern range. Over 87% of the population has been lost in the remaining northern areas, earning the species a classification of critically endangered. The large-scale decline of sunflower sea stars due to SSWD has had a cascading effect on ecosystems, in which sea urchin populations have experienced uninhibited growth in the absence of predation. This ecological imbalance has led to the mass destruction of kelp forests and the creation of “urchin barrens” (locations where a previous kelp forest was destroyed by sea urchin overgrazing), demonstrating the profound impact SSWD has on kelp ecosystems and the species that rely on them.

After a series of exposure experiments and genetic sequencing tests of sunflower sea stars infected with SSWD, scientists identified the common bacterium Vibrio pectenicida as a causative agent (a pathogen that directly leads to disease, but may occur under the influence of other environmental or physical conditions) for SSWD. These findings may have lasting impacts on attempts to stem the spread and population losses caused by SSWD, including future efforts to recover the population of sunflower sea stars. 

Over the course of three years (2021-2024), scientists conducted a total of seven exposure experiments on sunflower sea stars. Using tissue extracts, coelomic fluid injections (an essential fluid similar to blood for sea stars that circulates immune system cells), and tank water from diseased sunflower sea stars, exposed sea stars were infected with SSWD. Healthy sunflower sea stars were collected in Washington state or raised at Friday Harbor Laboratories, and were first isolated in a 2-week quarantine period to ensure that collected stars did not develop SSWD after potential exposure in the wild. All exposure methods led to transmission of SSWD, with 92% (46/50) of exposed individuals displaying symptoms of SSWD. The disease stages were progressively categorized as “arm twisting,” “arm autonomy,” and “mortality.” Stars exposed to SSWD often died between 6 days to 2 weeks post exposure, usually within a week after showing the first symptoms of the disease. 

While using diseased coelomic fluid and tissue sample injections to infect healthy sea stars, scientists also utilized control samples, in which tissues or coelomic fluid from a diseased star were first treated with heat or filtered before injection into a healthy star. All 54 individuals injected with treated samples survived, with limited indications of SSWD. Most sea stars injected with untreated tissue (24 out of 26) or coelomic fluid (16 out of 18) samples from diseased stars contracted SSWD. The dramatic decrease in disease spread after heat treatment indicated that the causative agent (pathogen) of SSWD was likely cellular.

After identifying that the cause of SSWD was likely cellular, scientists genetically sequenced diseased sea star coelomic fluid and tissues from both in-lab sea stars and sea stars at field outbreak sites. Coelomic fluid from healthy stars and stars exposed to SSWD was also collected to contrast the microbes present in sea stars at all disease stages. After RNA and DNA analysis (particularly using 16S ribosomal RNA gene amplicon datasets), the most significant microbial difference between healthy and diseased groups was identified to be the bacterium V. pectenicida (r^2 ≥ 0.90), which was found in abundance in samples from stars with SSWD and was absent in samples from healthy stars. This difference in microbial presence allowed scientists to pinpoint V. pectenicida as a likely causative agent of SSWD. Small bacterial loads of V. pectenicida were found in healthy stars, leading scientists to propose that sea stars can remain healthy with low concentrations of V. pectenicida in ideal environmental conditions. This may indicate that outbreaks occur when environmental conditions (such as increasing temperatures) compromise the star’s immune system and allow the bacterium to flourish.

After genetic sequencing identified V. pectenicida as a candidate for the causative agent of SSWD, scientists conducted a series of exposure experiments using pure V. pectenicida cultures isolated from infected stars. When injected into healthy sea stars, V. pectenicida bacterium strains FHCF-3 and FHCF-5 cultures resulted in SSWD. Healthy sea stars were then injected with high (10^5 colony forming units) and low (10^3 c.f.u.) amounts of V. pectenicida strain FHCF-3 and heat-treated controls. 13 out of 14 stars injected with living bacteria all contracted SSWD and died, while all stars injected with heat treated (dead) bacteria survived. The disease progressed faster in stars injected with a higher concentration of V. pectenicida strain FHCF-3, with mortality occurring 6-11 days post exposure. Meanwhile, the group exposed to a lower concentration of live bacteria progressed through the disease more slowly, with mortality occurring 11-16 days post exposure.

After identifying V. pectenicida as a strong possible cause of SSWD, gene sampling was also conducted at field sites across British Columbia in May and October 2023. Although no individuals sampled at the five sites exhibited signs of SSWD or had V. pectenicida in May, V. pectenicida was identified in two outbreak populations in October. Vibrio pectenicida was found in 16% of healthy stars from visually unaffected sites, 74% of visually normal stars in outbreak sites, and 86% of diseased stars in outbreak sites. The analysis of a genetic database from southeast Alaska in 2016 during an SSWD outbreak also found V. pectenicida in both diseased and normal stars in outbreak sites but not healthy sites, suggesting that V. pectenicida also played a role in past outbreaks of SSWD. Scientists hypothesized that instances of Vibrio pectenicida in apparently disease-free stars may be due to exposure to other diseased stars in the wild. 

The discovery of V. pectenicida as a contributing cause of SSWD has strong implications for future research and conservation efforts for struggling sea star populations. V. pectenicida has been found globally (ranging from Australia to Asia to Europe to the US) from 2009-2019 in a variety of marine hosts, particularly in shellfish and bivalve aquaculture. Future research can focus on the mechanism of V. pectenicida as a pathogen, further distinguishing where the bacterium can be found, and modes of transmission both between sea stars and from prey shellfish populations. Scientists proposed that warming oceans due to climate change may make stars more vulnerable to outbreaks of V. pectenicida and other pathogens that thrive in warmer environments, which would support an observed trend between SSWD and warming water temperatures. Since sea stars respond to unfavorable environmental conditions (such as warming water) with similar symptoms to SSWD, it has been difficult to classify SSWD outbreaks. The discovery of V. pectenicida as a causative agent allows researchers to identify V. pectenicida as an indicator of SSWD in sampling, supporting the expansion of sampling across different environments and sea star species. This is essential for continuing to understand SSWD and crafting a response to protect struggling sea star populations and affected ecosystems. 

References:

Mazza, Marco. “How Sunflower Stars Can Save California’s Vanishing Kelp Forests.” The Independent, Santa Barbara Independent, 21 June 2024, https://www.independent.com/2024/06/21/how-sunflower-stars-can-save-californias-vanishing-kelp-forests/ 

Prentice, M.B., Crandall, G.A., Chan, A.M. et al. “Vibrio pectenicida strain FHCF-3 is a causative agent of sea star wasting disease.” Nat Ecol Evol 9, 1739–1751 (2025). https://doi.org/10.1038/s41559-025-02797-2

Global warming is steadily transforming Earth’s oceans. Between 1901 and 2023, sea surface temperatures have increased at an average rate of 0.14℉ per decade (US EPA, 2016). This seemingly small thermal shift is enough to disrupt circulation patterns, alter nutrient availability, and restructure entire marine communities. As oceans absorb over 90% of excess atmospheric heat, they become both a buffer against and a victim of climate change (Climate Change, 2025). Among the many organisms affected by these changes, phytoplankton—the microscopic, photosynthetic organisms that drift near the ocean’s surface—serve as a critical case study. These single-celled producers are responsible for about half of Earth’s oxygen production, and they form the foundation of aquatic food webs, converting sunlight into chemical energy that sustains nearly all marine life (Hook, 2023). Therefore, understanding how phytoplankton respond to warming is essential for predicting the future of marine ecosystems.

Phytoplankton are highly sensitive to temperature fluctuations. Since their metabolic processes, growth rates, and enzymatic activities are temperature-dependent, even minor thermal changes can reshape their abundance and distribution. When waters warm beyond a species’ thermal tolerance, populations may decline or shift toward cooler regions (Barton et al., 2016). At the microscopic level, these shifts can cascade upward through the food web, reducing food availability for zooplankton, fish, and the higher-level predators that feed on them, such as sharks, whales, and seals. However,  one key question remains: can phytoplankton adapt to rising temperatures, or will their thermal limits determine the structure of future marine ecosystems?

Huertas et al. (2011) directly addressed this question through controlled laboratory experiments designed to measure the capacity of phytoplankton to evolve under warming. The researchers selected twelve species representing a range of environments—freshwater, coastal, open-ocean, and coral symbiotic systems—to test whether thermal tolerance varied among ecological types. To simulate long-term warming, they employed a “ratchet technique,” in which phytoplankton populations were gradually exposed to higher temperatures. Each population started from a single cloned cell to remove preexisting genetic variation. Then, the cell cultures were repeatedly grown and transferred into warmer conditions, forcing the populations to either adapt to the changes through genetic mutations or face extinction.

The results revealed striking differences among species. Freshwater species, such as Scenedesmus intermedius, exhibited remarkable resilience, adapting to temperatures as high as 40°C. Coastal species like Tetraselmis suecica and Dictyosphaerium chlorelloides tolerated up to 35°C, while open-ocean species such as Emiliania huxleyi and Monochrysis lutheri showed little to no capacity for adaptation. Coral symbionts (Symbiodinium species) demonstrated limited but detectable resistance, reflecting the thermal stress already observed in coral reef environments. Importantly, adaptation was not simply a case of short-term acclimation. The researchers found that resistant populations arose at different times across replicate cultures. This serves as evidence that adaptation stemmed from rare, spontaneous genetic mutations instead of physiological flexibility. Growth rates of adapted populations diverged significantly from their ancestral strains, confirming that true evolutionary change had occurred.

These findings carry major implications for understanding the ecological future of the oceans. If phytoplankton species differ so widely in their ability to adapt, warming will likely reorganize marine communities from the bottom up. Species capable of rapid genetic adaptation may dominate, while others could decline or disappear. This uneven resilience could favor smaller, faster-growing species, altering nutrient cycling and potentially weakening the ocean’s ability to sequester carbon. Because phytoplankton drive roughly half of global primary production, any restructuring of these communities could ripple through food webs, climate regulation, and fisheries.

While Huertas et al. focused on individual species in controlled conditions, Poloczanska et al. (2016) broadens this picture to the scale of global ecosystems. Their review synthesized nearly 2,000 observations of marine organisms responding to climate change, confirming that uneven adaptation is already occurring across taxa and ocean regions. On average, species distributions are shifting towards the north and south poles by about 72 kilometers per decade, and spring life-cycle events such as breeding or migration are advancing by four days per decade. Warm-water species are becoming more abundant, while cold-water species decline. Coral calcification, the process by which corals take in calcium and carbonate ions to build their exoskeletons, is weakening under combined warming and acidification stress. These patterns mirror the interspecific variability observed by Huertas et al.; some organisms adjust successfully to changing conditions, while others falter. Here, the broader conclusion is that climate change does not affect marine life uniformly—it selectively reshapes communities based on biological flexibility, dispersal ability, and evolutionary potential.

Fig 1. Global distribution of documented marine biological responses to climate change across major ocean regions (Poloczanska et al., 2016). Bars show the proportion of observed responses as consistent (dark blue), equivocal (light blue), or no change (yellow). Numbers indicate total observations per region; symbols identify taxa with ≥10 observations. Background colors represent regional sea-surface warming from 1950–2009 (yellow: low; orange: medium; red: high). Regions are defined by ecological structure and oceanographic features. eveal that climate-driven shifts in abundance, distribution, and phenology vary sharply across ocean basins—mirroring the uneven adaptive capacities described by Huertas et al. (2011).

Together, these studies illustrate both the mechanisms and the consequences of ocean warming. Huertas et al. provides mechanistic insight—showing that adaptation in phytoplankton depends on genetic change, and that some species are inherently more capable than others. Building off of this, Poloczanska et al. reveals how these species-level differences scale up, driving global shifts in abundance, distribution, and ecosystem structure. The two perspectives complement one another; laboratory experiments explain how adaptation might occur, while global syntheses show where and to what extent it already has.

As climate change accelerates, understanding the adaptability of foundational organisms like phytoplankton becomes increasingly urgent. Their evolutionary potential will determine not only the structure of marine ecosystems, but also the ocean’s capacity to regulate the planet’s climate. By linking experimental evidence with global ecological trends, researchers are beginning to map out a future ocean defined by winners and losers—a mosaic of adaptation, migration, and loss. The challenge ahead lies in predicting how these microscopic shifts will ripple through the web of life that depends on them.

References:

Barton, A. D., Irwin, A. J., Finkel, Z. V., & Stock, C. A. (2016). Anthropogenic climate change drives shift and shuffle in North Atlantic phytoplankton communities. Proceedings of the National Academy of Sciences, 113(11), 2964–2969. https://doi.org/10.1073/pnas.1519080113 

Climate Change: Ocean Heat Content | NOAA Climate.gov. (2025, June 26). https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content 

Hook, B. (2023, May 31). Phenomenal Phytoplankton: Scientists Uncover Cellular Process Behind Oxygen Production | Scripps Institution of Oceanography. https://scripps.ucsd.edu/news/phenomenal-phytoplankton-scientists-uncover-cellular-process-behind-oxygen-production 

Huertas, I. E., Rouco, M., López-Rodas, V., & Costas, E. (2011). Warming will affect phytoplankton differently: Evidence through a mechanistic approach. Proceedings of the Royal Society B: Biological Sciences, 278(1724), 3534–3543. https://doi.org/10.1098/rspb.2011.0160 

Poloczanska, E. S., Burrows, M. T., Brown, C. J., García Molinos, J., Halpern, B. S., Hoegh-Guldberg, O., Kappel, C. V., Moore, P. J., Richardson, A. J., Schoeman, D. S., & Sydeman, W. J. (2016). Responses of Marine Organisms to Climate Change across Oceans. Frontiers in Marine Science, 3. https://doi.org/10.3389/fmars.2016.00062 

US EPA, O. (2016, June 27). Climate Change Indicators: Sea Surface Temperature [Reports and Assessments]. https://www.epa.gov/climate-indicators/climate-change-indicators-sea-surface-temperature 

Microplastic pollution is a global issue effectively impacting all aquatic systems from the poles to tropical reefs. Current emission patterns project to around 35 – 98 metric tons of annual microplastic emission by 2030. Yet, this may only be an underestimation, as our current understanding of microplastic concentrations based on traditional sampling practices overlooks smaller debris (Lindeque et al. 2020, Borrelle et al. 2020). With this scale of rapid increase in concentrations, the implications of microplastic accumulation in marine systems have become an increasing concern. In response to this global concern, Adam Porter and his team looked towards the ocean’s floor to better understand how microplastics interact with dynamic ecosystems.

Microplastics emitted into the marine environment can adversely impact a wide range of processes from cellular metabolism to digestive functions, fertility, locomotion, and growth (Foley et al. 2018; Bour et al. 2018). Furthermore, bioaccumulation, or trophic transfer when contaminated prey is consumed by predators, magnifies microplastic burdens in organisms higher in the food chain. These above properties, in conjunction to the rapidly increasing environmental concentrations, highlight the pressing need to quantify how much microplastics marine organisms are ingesting.

Historically, our understanding of individual microplastic burdens has often assumed that levels of environmental contamination directly map onto their uptake by marine organisms. However, studies have found that this isn’t always the case. Other factors, such as feeding strategies and community composition, also impact a species’ uptake rate (Pagter et al. 2021; Bour at al. 2018). 

To bridge the mismatch of environmental concentration and individual burden, Porter et al. reviewed 412 studies on marine invertebrates from around the globe to investigate how different species traits could influence microplastic uptake. First, they gathered data from each study and assigned a geographic sector to each sampling site. Next, they evaluated each observation for a variety of variables, including feeding mode, position within the sediment, and wet weight (mass) of the individual. Then, Porter’s team used statistical tests to examine the potential influence each parameter had on plastic uptake with statistical analyses tests and visualized their findings. 

Geographically, the Pacific Northwest, Yellow Sea and Japan Trench, had the highest mean individual microplastic burden. In terms of animal class, the highest mean burden occurred in the Malacostraca class. Malacostraca encompasses common commercial species such as crabs and lobsters, which could have commercial implications on industries like lobster fishing and aquaculture. 

Of all the outlined parameters, feeding strategies had the greatest impact on microplastic uptake. Omnivores were shown to have the highest rate of uptake, followed by predators, herbivores, grazers, suspension feeders, deposit feeders, and lastly scavengers. These findings support the bioaccumulation theory, one of several hypotheses concerning microplastic uptake patterns (Wang 2014). According to the bioaccumulation theory, microplastics enter the food web through primary consumers like suspension feeders, grazers, and filter feeders. The plastic they retain in their systems will then be ingested by higher trophic levels like secondary and tertiary consumers that are omnivores,predators, and scavengers. Accordingly, the microplastic burdens would be highest in predators and omnivores, which matches the study’s findings.

In addition to the quantity of microplastics retained, feeding patterns were also found to influence the size and type of microplastics consumed were also different across groups. The most reported shape was fibers. The mean sizes of these fragments ranged from 0.2 micrometers to 17 centimeters, and herbivores in general retained the largest particles, but the precise mechanisms driving these patterns remain unclear.  

These findings precisely highlight our gap in knowledge of microplastic distribution amongst marine communities. As Porter et al. highlights, a holistic consideration of subtle processes related to feeding patterns is essential in fine tuning our understanding of how our world is changing. Thus, although the study describes general trends on a global scale, future research focusing on regional subtleties is important. Subsequently, applying these findings as policy is crucial, as many marine organisms are frequently consumed commercial species. Being major consumers of seafood, the microplastic accumulation in marine animals can directly impact humans. This is particularly concerning in context of our status as the apex predator, and therefore the final stop in the chain of bioaccumulation. As the microplastic burden in marine organisms is rising at an alarming pace, the need for action is more urgent than ever.

Works Cited

Borrelle, S. B., Ringma, J., Law, K. L., Monnahan, C. C., Lebreton, L., McGivern, A., Murphy, E., Jambeck, J., Leonard, G. H., Hilleary, M. A., Eriksen, M., Possingham, H. P., De Frond, H., Gerber, L. R., Polidoro, B., Tahir, A., Bernard, M., Mallos, N., Barnes, M., & Rochman, C. M. (2020). Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science, 369(6510), 1515–1518. https://doi.org/10.1126/science.aba3656 

Bour, Agathe, Carlo Giacomo Avio, Stefania Gorbi, Francesco Regoli, and Ketil Hylland. “Presence of Microplastics in Benthic and Epibenthic Organisms: Influence of Habitat, Feeding Mode and Trophic Level.” Environmental Pollution (Barking, Essex: 1987) 243, no. Pt B (December 2018): 1217–25. https://doi.org/10.1016/j.envpol.2018.09.115.   

Foley, Carolyn J., Zachary S. Feiner, Timothy D. Malinich, and Tomas O. Höök. “A Meta-Analysis of the Effects of Exposure to Microplastics on Fish and Aquatic Invertebrates.” The Science of the Total Environment 631–632 (August 1, 2018): 550–59. https://doi.org/10.1016/j.scitotenv.2018.03.046.   

Lindeque, P. K., Cole, M., Coppock, R. L., Lewis, C. N., Miller, R. Z., Watts, A. J. R., Wilson- McNeal, A., Wright, S. L., & Galloway, T. S. (2020). Arewe underestimating microplastic abundance in the marine environment? A comparison of microplastic capture with nets of different mesh-size. Environmental Pollution, 265, 114721. https://doi.org/10.1016/j.envpol.2020.114721

Pagter, Elena, Róisín Nash, João Frias, and Fiona Kavanagh. “Assessing Microplastic Distribution within Infaunal Benthic Communities in a Coastal Embayment.” Science of The Total Environment 791 (October 15, 2021): 148278. https://doi.org/10.1016/j.scitotenv.2021.148278. 

​​Porter, A., Godbold, J. A., Lewis, C. N., Savage, G., Solan, M., & Galloway, T. S. (2023). Microplastic burden in marine benthic invertebrates depends on species traits and feeding ecology within biogeographical provinces. Nature Communications, 14(1), 8023. https://doi.org/10.1038/s41467-023-43788-w 

Wang, W. -X. “Chapter 4 – Bioaccumulation and Biomonitoring.” In Marine Ecotoxicology, edited by Julián Blasco, Peter M. Chapman, Olivia Campana, and Miriam Hampel, 99–119. Academic Press, 2016. https://doi.org/10.1016/B978-0-12-803371-5.00004-7.

As more CO2 enters the ocean, the water’s pH and temperature change in processes called ocean warming and acidification. Both processes pose a risk to marine microbes, as they are unaccustomed to their new, more acidic environment. Several marine species depend on the microbes that dwell in the ocean, and if the change in pH negatively impacts the oceanic microbiome, there would be negative implications for a large number of organisms.

Microbes are essential to the development of many species in the world’s oceans. They are able to activate genes, sculpt the bodies of multicellular organisms, and provide vital life information to juvenile species (Yong 2016). But these abilities may be disrupted if the ocean’s change in pH negatively affects the microbiomes both in the water and living within ocean creatures.

Dr. Elliot Scanes and his colleagues at the University of Technology Sydney evaluated the effects of ocean acidification on the Sydney rock oysters’ (S. glomerata) ability to transfer its microbiome down to its offspring during reproduction. Oysters reproduce via broadcast spawning, a process in which sedentary organisms release all of their eggs and sperm into the surrounding water in hopes that a portion of the gonads will be fertilized (Bondar, 2018). Because these broadcasted embryos are now exposing their microbiomes to warmer, more acidic environments than the microbiomes of previous generations have been accustomed to, the microbes living within these embryos are not well adapted to the new conditions. This poorly equipped microbiome is causing fewer and fewer embryos to develop properly. An oyster’s microbiome is a necessary part of its body, and without it, a juvenile oyster may not be able to develop and function as effectively (Scanes et al. 2023).

Scanes set out to examine whether exposure to ocean warming and acidification during both broadcast spawning and early reproduction would alter an oyster’s microbiome strength.

The lab team acclimated these oysters to the lab tanks and then harvested their eggs and sperm, later fertilizing them (Figure 1.) (Scanes et al. 2023). Half of the oyster embryos were raised in tanks with a normal pH, and the other half were raised in tanks with decreased pH to mimic ocean acidification. The team conditioned both sets of S. glomerata for reproduction, then used eggs and sperm from each set to breed the next generation of oysters. The next generation was divided into four groups: first, the oyster embryos collected in tanks with a normal pH were split into two groups, with one group being raised in another tank with a normal pH and the other being raised in a tank with a low pH that mimics ocean acidification. Then, the oyster embryos collected in tanks with a low pH were also split into two groups, with one group being raised in another tank with a low pH

and the other being raised in a tank with a normal pH.

The embryos produced from these four sets of oysters informed Scanes et al. of the physiological differences that occur between oyster microbiomes that are exposed to ocean acidification at different steps in the reproductive process. The team found significant alteration of the microbiome in the parent oysters exposed to ocean acidification and concluded that when oyster parents were exposed, more oyster embryo microbiomes were prepared for the new conditions, and so the more protected oyster embryos survived (Scanes et al. 2023). This information is of much consequence because it provides a baseline for studying other microbe–sea creature relationships in the future. The marine microbiome plays a critical role in the development and wellbeing of animals like the Hawaiian bobtail squid and the Hydroides elegans, otherwise known as the “squiggly worm,” who depend on them for gene activation and information on safe places to live, respectively (Yong 2016). Now that there is evidence that the changing conditions of ocean water harms microbes, and therefore harms the creatures that depend on them, as well as evidence that exposure to these conditions protects the microbes in future generations, scientists are better informed about how to protect marine species moving forward.

Literature Cited

Bondar C. Wild Moms. 2018.

Messina Z et al. Partial Pressure of Carbon Dioxide. National Library of Medicine. 2022.

Scanes E et al. Transgenerational transfer of the microbiome is altered by ocean acidification in oyster larvae. Aquaculture. 2023.

Yong E. Body Builders. I Contain Multitudes: The Microbes Within Us and a Grander View of Life. 2016. 49-59.