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Article

Warmer Oceans Will Increase Abundance of Human Pathogens on Seaweeds

by
Sidney Wilson
1,2 and
Mahasweta Saha
1,*
1
Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK
2
School of Biological and Marine Sciences, Faculty of Science and Engineering, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
*
Author to whom correspondence should be addressed.
Phycology 2025, 5(3), 38; https://doi.org/10.3390/phycology5030038
Submission received: 16 April 2025 / Revised: 28 May 2025 / Accepted: 4 June 2025 / Published: 14 August 2025
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Abstract

Anthropogenic warming of the world’s oceans is not just an environmental crisis, but may result in a significant threat to human health. The combination of a warming ocean and increased human activity in coastal waters sets the stage for increased pathogenic Vibrio–human interaction. Warming patterns due to climate change have already been related to the emergence of Vibrio outbreaks in temperate and cold regions. Seafoods, including seaweeds, are uniquely poised to contribute to global food and nutrition security. In recent years there has been a resurgence of interest in seaweeds due to their many uses, high nutritional value, and ability to provide ecosystem services such as habitat provision, carbon and nutrient uptake, and coastal protection. However, some seaweed species can be a reservoir for harbouring pathogenic Vibrio, and illnesses like gastroenteritis have recently been associated with foods prepared with seaweeds. In this study, we investigated the impact of elevated water temperatures on abundances of the major human pathogens Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio vulnificus/cholerae on seaweed and in coastal waters. Three seaweed species, Fucus serratus, Palmaria palmata, and Ulva spp., were exposed to temperature treatments (16 °C and 20 °C) to assess the effects of mean-temperature rise on Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio vulnificus/cholerae colonisation. Colony-forming units (CFUs) on seaweed surfaces and in surrounding water were counted. F. serratus and P. palmata showed significantly higher Vibrio abundances at higher temperatures compared with Ulva spp.; however, temperature did not significantly affect abundances of tested Vibrio species in surrounding waters. These results indicate that certain seaweed species may serve as major hotspots for human pathogenic bacteria in warmer conditions, with implications for human health.

1. Introduction

Waterborne diseases are a global burden causing several million deaths and uncounted cases of sickness every year [1]. These infectious illnesses are emerging or resurging due to factors such as coastal development, climate change, and extreme events such as marine heatwaves, all of which alter the environment for favourable growth and survival of microbial pathogens. Over the past 20 years, global sea surface temperature (SST) has increased by about 0.4 °C. By the end of the century, it is projected to rise further by anywhere between 0.9 and 5.4 °C [2]. This warming trend has led to various issues, including decreased biodiversity and variability, and ecosystem loss [3]. Another major consequence of higher SST is the growth of marine pathogens. This is because some harmful bacteria, such as those belonging to the genus Vibrio, prefer the warm and brackish waters typically found in coastal areas and estuaries [4]. As water temperatures continue to rise, these species are likely to become more abundant [5]. Projections show that under the most unfavourable scenario a global increase in 38,000 km of coastline areas suitable for Vibrio could occur by 2100 [6]. This spatial expansion in the disease burden for Vibrio infections is predicted to particularly affect higher latitudes, indicating an increased future risk in temperate regions. In coastal communities, this raises public health concerns because these areas often serve as a main water source and are heavily involved in the seafood industry and recreational activities [7]. Contaminated seafood and water have caused millions of individuals worldwide to suffer from illnesses like typhoid, cholera, and harmful infections (e.g., Salmonella and E. coli) because of pathogenic bacteria [8].
Seaweeds form some of the largest bodies of marine vegetation in the world, providing the valuable ecological services of carbon sequestration, nutrient recycling, and coastal habitat protection [9]. Seaweed, as a versatile resource, has served assorted purposes throughout history, including food, medicine, cosmetics, biofuel, and agriculture [10]. As a result of its varied uses, the seaweed aquaculture industry has experienced substantial growth in the past 25 years, with annual harvesting production reaching around 35 million tonnes [11].
Seaweed holobionts, which are ecological units comprised of a host plant and its microbiota [12], are influenced by both abiotic and biotic factors in the marine environment, determining the dominant microbial species that colonise the algal surface [13,14]. This surface layer of the seaweed, referred to as the eco-chemosphere, is a region abundant in chemicals and nutrients released by seaweeds [15]. When environmental stressors occur (e.g., temperature fluctuations, changes in nutrient availability, water deoxygenation, acidification [16], seaweeds can release copious amounts of amino acids and sugars into its eco-chemosphere and surrounding water column as a response [17,18]. Bacteria in the marine environment, including pathogenic Vibrio, can then utilise the released materials for their advantage.
Vibrio spp. are Gram-negative chemoorganotrophic bacteria that can be found on many coastal seaweed species [19,20]. For survival and growth, Vibrio utilises a multitude of compounds from seaweed, such as galactose, fucose, and mannose, through its ability to penetrate the epithelial membrane of its seaweed host [18,21]. Notably, Vibrio species are responsible for the highest amount of disease from the marine environment [22], with the most commonly attributed pathogenic species being Vibrio cholerae, Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio vulnificus [19,23]. Infections can occur through various routes, including via cuts, open wounds, the gastrointestinal tract, or inhalation, typically following exposure to contaminated water or consumption of raw or undercooked seafood [19]. Individuals at higher risk include those with chronic health conditions, particularly liver disease, diabetes, or a weakened immune system [24]. Vibrio infections can lead to life-threatening complications such as septicaemia, necrotising fasciitis, wound infections, gastrointestinal issues, and even death [25]. In northern Europe, Vibrio infections have increased in recent years, largely due to an increased presence of Vibrio in coastal waters [26,27].
Vibrio abundance in the water is highly associated with seasonal temperature changes. Vibrio experiences exponential increases in abundance during warmer months and declines in colder months [6,22,23,28,29]. When there are changes in environmental factors such as temperature or salinity, Vibrio demonstrates a remarkable ability to resist and even thrive under stress [30] (i.e., entering a Viable but Non-culturable State (VBNC) [5,31]. Vibrio optimally grows in water temperatures of above 15 °C and salinities of below 25 ppt [4,29,32,33]. Abundances of Vibrio are notably influenced by temperature and salinity [6] making these two environmental variables the most important for Vibrio studies [4,34], with other environmental factors, such as dissolved oxygen, nitrogen, pH, suspended solids, and turbidity, exerting only marginal effects [35,36].
The global increase in sea surface temperature (SST) has led to stress in a multitude of aquatic organisms and ecosystems [37]. In seaweed, under environmental stress, the thallus degrades, prompting a release of nutrients into the environment [21]. This nutrient release has the potential to create an avenue for the attraction and adherence of opportunistic bacteria [38,39]. It is therefore plausible that Vibrio, an opportunistic pathogen, may thrive better in these new and changing conditions [40].
Globally, seaweeds have been identified as reservoirs for Vibrio species, with major occurrences being reported throughout nations in Asia where seaweed aquaculture is extensive [28,41,42]. However, Vibrio has also been isolated in coastal regions across Europe [43,44,45], North America [46,47,48], and Africa [49,50]. There have also been documented cases of Vibrio-related illnesses associated with the consumption and use of seaweed [51]. However, we do not know how the health of seaweed-dominated coastal communities, or public health more generally, might be affected by ocean warming. Additionally, safety concerns could arise in regions that import large quantities of seaweed due to postharvest contamination [52]. Currently, there are no studies linking future climate predictions of SST with abundances of pathogenic Vibrios on seaweed surfaces and what this might mean in terms of resulting public health risks. This knowledge gap is critical given the rise in seaweed farming and its role in global food production. Understanding these dynamics is essential not only for assessing the future of public health, but also for guiding farming practices and informing marine ecosystem measures. Therefore, we investigated how ocean warming will alter the associations of pathogenic Vibrio species (Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio vulnificus/cholerae) with three different seaweed species and their surrounding water columns. Additionally, we examined whether abundances of Vibrio will be significantly higher in certain seaweed species in response to warmer oceans.

2. Materials and Methods

2.1. Sample Collection

Three species of seaweed, Fucus serratus (brown seaweed, hereafter F. serratus), Palmaria palmata, (red seaweed, hereafter P. palmata), and Ulva spp. (green seaweed, hereafter Ulva spp.) were collected from Plymouth Sound (50°21′50.4″ N 4°08′45.5″ W) on 3 June 2024 during low tide. Five biological replicates were taken from each species of seaweed. All samples were transferred to the laboratory within 30 min of collection.

2.2. Surface Area Analysis

To relate surface area to wet weight, 10 individual samples from each seaweed species (F. serratus, P. palmata, and Ulva spp.) were spin-dried for 1 min and weighed to determine their fresh weights. Photographs of all samples were then taken and run through Image J (1.54i) software to calculate surface area. The total surface area of each sample was then normalised by dividing the Image J-calculated surface area by the wet weight (cm2 g−1). These calculations were performed on separate samples (collected in May 2024) a few weeks prior to the main experiment, and the samples used for surface area calculations were not included in the main experiment. The mean surface areas per species were as follows: 32.112 cm2 g−1 (SD ± 1.932) for F. serratus, 60.212 cm2 g−1 (SD ± 3.659) for P. palmata, and 175.904 cm2 g−1 (SD ± 26.154) for Ulva spp. These averages were used for further analysis in the main experiment.

2.3. Temperature Experiment

The temperature experiment was conducted in a controlled-temperature (CT) room from 3 June to 17 June 2024. Amounts of approximately 10 g of spin-dried fresh weight from each seaweed species were placed in aquaria containing 800 mL of ambient seawater (salinity ~34 PSU) collected from Plymouth Sound. The tanks were maintained under continuous aeration and a 12:12 h light–dark cycle, with a light intensity of 18.35 µmol/m2/s. Each seaweed species was represented by five biological replicates, each of which were housed in a separate tank (n = 5), resulting in a total of five individuals per species across five replicate tanks.
A field temperature of 14 °C (0.5 metre depth) was measured at the time of collection on 3 June 2024. Two subsequent temperature treatments (+2 °C (time point, T1) and +6 °C (timepoint, T2)) were conducted based on RCP 8.5. Each temperature treatment lasted for 7 days. On days 0–7, the seaweed samples were kept in water at 16 °C; on days 7–14, the seaweed samples were kept in water at 20 °C. The water in the aquaria was replaced three times throughout the experiment (on days 4, 7, and 10) using freshly collected field seawater from Plymouth Sound.
On day 7, the +2 °C treatment was ended and 3 g (fresh weight) seaweed samples from each aquarium were collected for CFU counts along with water samples from the aquaria tanks. Following the T1 sampling, the temperature of the aquaria was increased to +6 °C and treatment was continued for 7 more days. The sampling process described below was followed for T2 samples, but with 4 g amounts of each species.

2.4. Abundances of Vibrio vulnificus/cholerae, Vibrio parahaemolyticus, and Vibrio alginolyticus

To test for abundances of Vibrio cholerae/vulnificus, Vibrio parahaemolyticus, and Vibrio alginolyticus in response to temperature stress, 3 g (fresh weight after 1 min spin drying) of the thallus from each replicate of each seaweed species was obtained at the T1 time point. The individuals were each placed in a sterile falcon tube which was then filled with 10 mL of filtered sterile seawater (FSSW). The seaweeds were gently rinsed with FSSW to remove loosely attached bacteria. Another 10 mL of FSSW was then added, with further addition of sterile glass beads. The tube was then vortexed for 3 min at 2500 rpm to create a microbial soup of 10 mL capacity. The same process was followed for T2 samples but with 4 g of each species.
The microbial soup from each seaweed replicate (10 mL), along with 400 mL aquaria water (for T1) or 200 mL aquaria water (for T2) was filtered through a 0.22 μm gridded membrane filter. The replication level was 5 for each type of seaweed and for each tank-water sample. The filter paper from each replicate was placed on a CHROMagar Vibrio (CHROMagar, Paris, France) plate and incubated at 37 °C for a total of 72 h. Colony counts were conducted manually and recorded at 24, 48, and 72 h as colony-forming units (CFUs). According to the manufacturer protocol, blue colonies were counted as presumptive Vibrio cholerae/Vibrio vulnificus, mauve colonies as presumptive Vibrio parahaemolyticus, and cream colonies as presumptive Vibrio alginolyticus. For data analysis, only the 72 h bacterial colony counts were used.

2.5. Statistical Analysis

Statistical analyses were performed using Python (3.12.0) in Visual Studio Code (1.93). Data were tested for normality using the Shapiro–Wilk test. Following this, a two-way mixed ANOVA was conducted at a 95% level of significance to examine interactions between factors. Post hoc analysis was performed using Tukey’s HSD test to assess differences among temperature treatments and species tested.

3. Results

The Shapiro–Wilk test confirmed that all variables were normally distributed, validating assumptions of normality for further analysis.
The two-way ANOVA revealed that both species and temperature significantly influenced Vibrio CFU counts on seaweed surfaces (Figure 1i), but not in aquaria water (Figure 1ii). In seaweed, species accounted for 75.08% of the variation (p < 0.0002), temperature for 58.09% (p < 0.0015), and interaction between species and temperature for 47.70% (p < 0.021). Post hoc Tukey’s tests indicated that Vibrio CFU counts for F. serratus and P. palmata were not significantly different (p > 0.51), but both had significantly higher levels of growth of pathogenic Vibrio, while Ulva spp. had notably lower CFU counts than both species (p < 0.0001, p < 0.0013). Conversely, in the aquaria water, species explained only 17.83% of the variation (p > 0.31), temperature 16.43% (p > 0.15), and interaction less than 1% (p > 0.99).

4. Discussion

This study investigated for the first time how ocean warming will affect abundances of the human pathogens Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp. on seaweed surfaces. Our findings demonstrate that increasing water temperatures, as projected by RCP 8.5, will significantly increase the total abundances of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus on Fucus serratus and Palmaria palmata surfaces. This has important future implications, as water temperatures are expected to rise significantly around the globe throughout the remainder of this century. Based on the findings of this study, we may expect that warmer oceans will increase abundances of the tested Vibrio species on certain seaweed surfaces, potentially posing risks to individuals who rely on seaweeds for food, medicine, cosmetics, etc., or who are involved in the seaweed aquaculture industry, by increasing the chance of infection with Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, or Vibrio alginolyticus [53]. The findings also have implications for recreational users, given that recreational use of the ocean has increased in recent years around the UK, having been promoted by various initiatives such as rock pooling events and structural improvements to designated bathing areas. This increase in public engagement with the marine environment will inevitably increase public exposure to Vibrio risk through contact with seaweed and its surrounding water, particularly during warmer months and marine heatwaves.
Abundances of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp. also differed significantly among the seaweed species tested. Fucus serratus and Palmaria palmata had significantly higher rates of growth of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp., compared with Ulva spp., when water temperatures rose to 20 °C, indicating that not all seaweed species support growth of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp. in the same manner with increasing water temperatures. This is likely due to physiological or biochemical differences among seaweed species, such as variations in surface structure or nutrient release, or differences in surface chemistry [14,35], which may create more favourable conditions for colonisation of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp. on certain species. These differences might also be attributed to distinct microbiomes associated with each seaweed species, as prior research has indicated that distinct species host unique bacterial communities [54,55].
Higher abundances of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus spp. on Fucus serratus, and Palmaria palmata species could lead to regions dominated by these seaweeds having a higher potential for Vibrio-related illnesses [21,56], compared with areas where Ulva species are more prevalent [57]. The abundances of the tested Vibrio species did not vary significantly between the Ulva species at either temperature tested. However, abundances were significantly lower on Ulva, compared with Fucus serratus and Palmaria palmata. A recent study showed that actinobacteria dominate the microbial communities of Ulva species such as Ulva ohnoi [58] and Ulva rigida [59]. Actinobacteria are well-known producers of antimicrobial compounds. For instance, bioactive metabolites produced by Streptomyces spp. have been shown to modulate microbiota, stimulate host immunity, and offer protective effects against Vibrio parahaemolyticus infections [60]. We therefore hypothesise that the reduced abundances of pathogenic Vibrio species on Ulva may be due to the anti-Vibrio activity of actinobacteria present on the surfaces of the tested Ulva. Further work is needed to experimentally test this hypothesis.
Seaweeds offer a sustainable source of nutrients, being rich in vitamins, minerals, proteins, and fibres. These make seaweeds an attractive resource not only for direct consumption but also as components in animal feed, as fertilisers, and as biostimulants for agriculture. Additionally, seaweed-derived compounds are now used in pharmaceuticals, cosmetics, and bio-packaging. Thus, there has been a recent resurgence of interest in seaweed cultivation; this has included cultivation of kelps and Palmaria species [61], with Fucus species also being farmed in regions such as the Baltic Sea [62]. Our results suggest that careful consideration should be given to species chosen for cultivation under the influence of ongoing climate change. Based on our findings, Ulva appears to be a better choice for farming due to its lower potential for harbouring pathogenic Vibrio colonies even under warmer water conditions.
Interestingly, while an increase in temperature significantly increased abundances of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus on seaweed surfaces, no such effect was observed in their respective aquaria waters. This result contrasts with findings of previous field studies which have reported increased Vibrio levels in warmer water columns in the open ocean [6,22,29]. Several factors might explain this discrepancy. First, the experimental setup in this study was conducted in a controlled mesocosm environment, where salinity (~35 ppt, relevant to the seaweed collection site) was above the ideal 25 ppt required for Vibrio proliferation [34]. Second, in natural ecosystems, a wider range of biotic and abiotic variables, such as nutrient fluctuations, competition, and non-climate-related stressors (such as invasive species, habitat destruction, and water pollution) may influence bacterial abundance more profoundly [63]. Third, the findings might be due to Vibrio’s opportunistic nature. When seaweed is stressed by elevated temperatures and releases more nutrients [64], Vibrio bacteria may be more likely to remain attached to the seaweed surface to access those nutrients, rather than existing freely in the water column. This trend has been observed in other seaweed-dominated ecosystems [48] as well as in seagrass meadows [65]. It is crucial to conduct future investigations into specific changes in Vibrio abundance in these seaweed-dominated ecosystems to gain a broader understanding of these patterns.

5. Conclusions

In conclusion, our findings show for the first time that abundances of Vibrio parahaemolyticus, Vibrio vulnificus/cholerae, and Vibrio alginolyticus will increase on certain seaweed species in response to a mean temperature rise, particularly in areas where susceptible seaweed species, such as Fucus serratus and Palmaria palmata, thrive. As sea surface temperatures continue to rise, these seaweed-dominated environments may serve as reservoirs for Vibrio, enhancing the risk of infections among coastal populations and those who consume seafoods or use seaweeds from these regions. This susceptibility to Vibrio colonisation has significant implications for marine ecosystems and aquaculture, emphasising the need for targeted monitoring and management strategies. Future research should focus on the long-term ecological impacts of climate change on Vibrio, the interactions between seaweed and Vibrio, and the resulting impacts on human health.

Author Contributions

S.W. conducted the experiments, analysed the data, and wrote the paper. M.S. conceptualised the idea, designed the project, contributed to funding, and edited the manuscript to the final version. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a research grant to MS from the Royal Society (RG\R1\241469) and British Phycological Society (Nr BPS 311). MS was also supported by the Simons Foundation Collaborative Project on Computational Biogeochemical Modeling of Marine Ecosystems (CBIOMES) through grant 549947 to Shubha Sathyendranath. The authors also thank University of Plymouth for partial funding to SW.

Institutional Review Board Statement

Not applicable

Informed Consent Statement

Not applicable

Data Availability Statement

Data are available at PANGAEA Data Publisher: https://doi.pangaea.de/10.1594/PANGAEA.984222.

Acknowledgments

The authors thank Plymouth Marine Laboratory and the Department of Science and Engineering, University of Plymouth. S.W. acknowledges Plymouth Marine Laboratory for hosting this project. S.W. thanks Dr. Sarah Bass from the University of Plymouth for guidance. This research was funded by the University of Plymouth. M.S. thanks the British Phycological Society and the Royal Society for partially funding the work. M.S. thanks the Simons Foundation Collaborative Project on Computational Biogeochemical Modeling of Marine Ecosystems (CBIOMES) through grant 549947 to Shubha Sathyendranath.

Conflicts of Interest

The authors declare no conflict of interest.

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Phycology 05 00038 g001
Figure 1. Mean Vibrio species CFU counts (n = 5 for each seaweed species for each temperature) with standard error (SEM) bars after 7-day water-temperature treatments. CFU counts were measured at two temperatures, 16 °C and 20 °C, for (i) seaweed surfaces (two-way ANOVA, Tukey’s HSD, p < 0.05) and (ii) aquaria water (two-way ANOVA, Tukey’s HSD, p < 0.05). Capital Latin letters indicate seaweeds that had a significantly different abundance of Vibrio colonies on their surface, whereas Greek letters indicate water columns that had a significantly different abundance of Vibrio colonies.
Figure 1. Mean Vibrio species CFU counts (n = 5 for each seaweed species for each temperature) with standard error (SEM) bars after 7-day water-temperature treatments. CFU counts were measured at two temperatures, 16 °C and 20 °C, for (i) seaweed surfaces (two-way ANOVA, Tukey’s HSD, p < 0.05) and (ii) aquaria water (two-way ANOVA, Tukey’s HSD, p < 0.05). Capital Latin letters indicate seaweeds that had a significantly different abundance of Vibrio colonies on their surface, whereas Greek letters indicate water columns that had a significantly different abundance of Vibrio colonies.
Phycology 05 00038 g001
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Wilson, S.; Saha, M. Warmer Oceans Will Increase Abundance of Human Pathogens on Seaweeds. Phycology 2025, 5, 38. https://doi.org/10.3390/phycology5030038

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Wilson S, Saha M. Warmer Oceans Will Increase Abundance of Human Pathogens on Seaweeds. Phycology. 2025; 5(3):38. https://doi.org/10.3390/phycology5030038

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Wilson, Sidney, and Mahasweta Saha. 2025. "Warmer Oceans Will Increase Abundance of Human Pathogens on Seaweeds" Phycology 5, no. 3: 38. https://doi.org/10.3390/phycology5030038

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Wilson, S., & Saha, M. (2025). Warmer Oceans Will Increase Abundance of Human Pathogens on Seaweeds. Phycology, 5(3), 38. https://doi.org/10.3390/phycology5030038

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