O! MY HOMETOWN

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I was born in Qingdao, a coastal city known for its mountains, sea, and clear blue skies. In my childhood memories, the seawater was crystal clear, sunlight sparkled on the sand, and the tidal zone was scattered with colorful shells and small crabs.

However, since 2008, things have quietly begun to change. Every summer, vast expanses of green algae would appear along the coastline, covering the sea surface like a blanket. These green patches are known as Ulva prolifera, the primary culprit behind the recurring “green tides” that now occur in Qingdao each year from July to August.

The once-crowded First Bathing Beach became covered in green sludge, emitting a pungent odor. As a child, I didn't understand where it came from. Today, I want to be part of the solution.

Lifecycle

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Through field investigations, literature analysis, and expert interviews conducted by our Human Practices team, we identified two key reasons for the recurrence of green tides in Qingdao:

1. The seaweed farming rafts in northern Jiangsu

In spring, large quantities of Ulva prolifera spores remain in the shallow sea areas of northern Jiangsu. These spores require a solid substrate for early-stage development, and the structures of Porphyra aquaculture rafts provide ideal surfaces for attachment.

Under eutrophic conditions, these spores rapidly proliferate into filamentous thalli. Once the biomass reaches a critical level, their buoyant structure enables them to float to the surface and continue expanding through photosynthesis.

Ultimately, dense mats of floating Ulva form in the southern Yellow Sea and are carried northward by prevailing winds and currents, accumulating along the coasts of Qingdao and Jiaozhou Bay.

2. Seasonal factors: July-August is the “outbreak window”

Several environmental factors converge during this period:

1. Optimal water temperature (20-25°C): ideal for Ulva growth.^[1]
2. High nutrient levels: excess nitrogen and phosphorus from the Yangtze River and Yellow Sea promote eutrophication.^[2]
3. Prevailing southerly winds and warm currents: help transport algal masses northward.^[2]
4. Frequent typhoons or cyclones: enhance surface turbulence, further dispersing floating thalli.^[2]
5. High light inhibits Ulva prolifera growth by causing photodamage and stress. Moderate light supports its bloom formation.^[3]

Hence, Qingdao is not the origin of the bloom, but rather a convergent zone, where oceanographic conditions result in the accumulation and visible outbreak of green tides.

Impacts

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Massive proliferation of macroalgae can have serious negative effects on coastal environments and human society. In the Yellow Sea, large-scale green tides caused by Ulva prolifera have occurred annually off the coasts of Jiangsu and Shandong provinces for 18 consecutive years (2007-2025). These blooms typically originate as small floating patches in the coastal waters of Jiangsu between mid-April and early May. Driven by monsoon winds and ocean currents, they gradually drift northward along the southern Yellow Sea, accumulating near the Shandong Peninsula during June and July before slowly dissipating.^[4]

A notable example is the 2008 outbreak, which coincided with the Olympic Sailing Regatta in Qingdao. The massive arrival of floating algae created severe logistical and environmental challenges for the host city. Direct economic losses from the bloom were estimated to exceed 1.3 billion RMB.

1. Ecological disruption: Massive macroalgal blooms significantly alter coastal ecological dynamics by impeding air-sea gas exchange and inducing hypoxic conditions, which adversely affect aquaculture species such as Apostichopus japonicus (sea cucumber) and bivalves.^[3]

2. Water quality degradation: The decomposition of accumulated algal biomass during bloom senescence releases malodorous gases (e.g., hydrogen sulfide, ammonia) and leads to a marked decline in water quality.^[3]

3. Biogeochemical impacts: Prolonged and recurrent bloom events can induce substantial shifts in marine biogeochemical processes, including nutrient cycling, oxygen dynamics, and carbon fluxes, thereby exerting long-term stress on ecosystem functioning.^[3]

4. Tourism and fisheries losses: The visual and olfactory pollution impacts tourism and disrupts coastal aquaculture and fisheries.^[3]

O! Ongoing Solutions

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I. Physical Control

1. Manual removal & containment nets: Floating nets and manual collection are widely used to contain and remove biomass. While intuitive and effective short-term, these methods are labor-intensive, expensive, and unable to target early-stage spores.
2. Mechanical swirl isolation: Devices generate vortex flows to gather algae into confined areas for centralized cleanup. However, effectiveness depends heavily on sea conditions.^[3]

II. Chemical Control

1. Algaecides: Salts, ozone, or oxidizing agents can quickly lyse algal cells. While fast-acting, they pose risks to non-target marine organisms and are not suitable for large-scale deployment.
2. Coagulants/precipitants: Use of polymers or metal ions (e.g., polyacrylamide, Fe³⁺) accelerates algal sedimentation. However, these chemicals may require post-treatment and alter water chemistry.

III. Biological Control

1. Algal-degrading microbes or enzymes: Pre-treatment with selected bacterial strains or enzymes breaks down cell walls and reduces buoyancy, accelerating sedimentation. One study reported a 14-fold increase in sinking speed after pre-degradation, but such processes may consume dissolved oxygen and induce local hypoxia.
2. Allelopathic competition: Certain macroalgae (e.g., Sargassum horneri) secrete compounds (e.g., fatty acids, phenols, terpenoids) that inhibit Ulva growth. These could be used to introduce ecological checks and balances.
3. Resource-oriented utilization: Harvested Ulva biomass can be processed into organic fertilizers, biochar, or carbon sequestration materials, offering both ecological and economic value. However, industrial-scale implementation requires technological and market support.

To date, no single method provides a fully effective, low-cost, environmentally safe, and scalable solution. Experts emphasize the importance of early-stage intervention and advocate for the integration of multiple complementary strategies to achieve sustainable and comprehensive control of Ulva prolifera green tides.^[3]

O! Super Carpet

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To address the challenges mentioned above, O! Super Carpet presents a promising strategy involving four interdependent modules: Survival, Light Suicide, Biofilm Enhancement, and Arginine-Inducible Sedimentation Module.

Survival Module

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In the Survival module, we introduced the salt-tolerant genes, gspM and echM^[5] to arm our biofilm with the necessary adaptability against inhospitable conditions. They make it easy for E. coli to maintain normal physiological activities under high-salinity condition (750mM NaCl), which is more harder than the normal sea salinity (600mM NaCl).

Light suicide module

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L.U.C.I.A. (Light Unlocks Cytotoxic Inducible Adaptor)

In spired by that, we designed a blue light-inducible genetic self-destruction circuit to prevent the spread of engineered bacteria in the marine environment:

No light

When light (about 465 nm light irradiation) is absent, the tetR repressor binds tightly to the operator preventing transcription by RNA polymerase.

With light

When cells are under about 465 nm light irradiation, the LOVdeg tag promotes the degradation of the tetR repressor protein. This allows transcription by RNA polymerase.

This initiates the downstream expression of the mazF toxin gene, triggering programmed cell death.

Programmable biofilm

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The Ag-Nb system, a synthetic bacterial cell-cell adhesion toolbox, enables controlled multicellular self-assembly via specific nanobody-antigen interactions. It regulates adhesion specificity and mediates diverse morphologies and patterns compatible with cell growth.
In light of this property, we developed a strategy for bacterial functional partitioning in biofilms, seeking to achieve a biofilm that is both multifunctional and programmable. Its two specific functions are described in detail below.

We have also designed a model to assist researchers in selecting biofilm structures to evaluate the performance of different biofilm structures.

Biofilm Enhancement

The construction of a polycistronic construct harboring the Ag43 and OmpA genes to enhance Escherichia coli biofilms relies fundamentally on the functional complementarity of these two proteins. Specifically, Ag43, a self-recognizing surface adhesin, promotes intercellular aggregation via its α-domain to increase bacterial density; it also exhibits cross-species biofilm-enhancing activity and the ability to enhance surface colonization through glycosylation, thereby serving as the primary foundation for biofilm formation^[7][8][9]. In contrast, OmpA—a highly conserved outer membrane protein—facilitates the translocation of extracellular matrix (ECM) components to maintain biofilm structural stability^[10]. It further regulates surface colonization capacity by repressing cellulose production via the CpxRA two-component system and aids horizontal gene transfer to enhance biofilm stress tolerance, ultimately sustaining biofilm maturation and stability on the hydrophobic surface(like our MBBR made of PLA)^[11].

Arginine-inducible Sedimentation

Beyond the passive adhesion of Ulva spores relying on hardware operational modes and the intrinsic physicochemical properties of biofilms, a more effective approach is required to promote the spontaneous settlement of spores onto our biofilms. This necessity arises because actively settling spores release the contents of their adhesive vesicles, enabling them to anchor more firmly to the biofilm matrix.

It has been observed that arginine-rich oligopeptides exhibit strong interactions with Ulva spores^[12], inducing multiple forms of spore settlement. Inspired by this finding, we aim to fuse such small-molecule oligopeptides with the common E. coli extracellular platform CsgA. This strategic fusion is intended to endow the biofilm with the capability to actively induce spore settlement, thereby enhancing the efficiency and stability of spore adhesion through a biologically mediated mechanism.

References

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[1] Zhang, X. et al. (2022). A review of physical, chemical, and biological green tide prevention methods in the Southern Yellow Sea. Marine Pollution Bulletin, 180, p.113772. doi: 10.1016/j.marpolbul.2022.113772.↩︎

[2] Kang, P.J. and Nam, K.W. (2016). Effects of temperature and irradiance on growth of Ulva prolifera (Chlorophyta).  Korean Journal of Fisheries and Aquatic Sciences, 49(6), pp.845-848. doi:10.5657/KFAS.2016.0845.↩︎

[3] Zhang, Y. et al. (2019). Ulva prolifera green-tide outbreaks and their environmental impact in the Yellow Sea, China. National Science Review, 6(4), pp.825-838. doi: 10.1093/nsr/nwz026. ↩︎

[4] Zhao, X. et al. (2019). Cooperation between photosynthetic and antioxidant systems: an important factor in the adaptation of Ulva prolifera to abiotic factors on the sea surface. Frontiers in Plant Science, 10, Article 648. doi: 10.3389/fpls.2019.00648.↩︎

[5]Kapardar,K. et al. (2010) ‘Identification and characterization of genes conferring salt tolerance to Escherichia coli from pond water metagenome’, Bioresource Technology, 101(11), pp. 3917–3924. doi: 10.1016/j.biortech.2010.01.017.↩︎

[6]Nathan Tagueet al. (2024) ‘Light-inducible protein degradation in E. coli with the LOVdeg tag’,  eLife, 12. doi:10.7554/eLife.87303.↩︎

[7]Klemm, P., Hjerrild, L., Gjermansen, M. & Schembri, M.A. (2004) 'Structure-function analysis of the self-recognizing Antigen 43 autotransporter protein from Escherichia coli', Molecular Biology, 51(1), pp. 283-296. doi: 10.1046/j.1365-2958.2003.03833.x.↩︎

[8]Sherlock, O., Dobrindt, U., Jensen, J.B., Vejborg, R.M. & Klemm, P. (2006) 'Glycosylation of the Self-Recognizing Escherichia coli Ag43 Autotransporter Protein', Journal of bacteriology, 188(5), pp. 1798-1807. doi:10.1128/JB.188.5.1798-1807.2006.↩︎

[9] Reidl, S. et al. (2009) 'Impact of O-glycosylation on the molecular and cellular adhesion properties of the Escherichia coli autotransporter protein Ag43', International Journal of Medical Microbiology, 299(6), pp. 389-401. doi: 10.1016/j.ijmm.2009.01.001. ↩︎

[10] Orme, R., Douglas, C.W.I., Rimmer, S. and Webb, M. (2006) 'Proteomic analysis of Escherichia coli biofilms reveals the overexpression of the outer membrane protein OmpA', Proteomics, 6, pp. 4269–4277. doi: 10.1002/pmic.200600193.↩︎

[11] Ma, Q. and Wood, T.K. (2009) ‘OmpA influences Escherichia coli biofilm formation by repressing cellulose production through the CpxRA two-component system’, Environmental Microbiology, 11(10), pp. 2735-2746–2746. doi:10.1111/j.1462-2920.2009.02000.x.↩︎

[12] Ederth, T. et al. (2009) ‘Interactions of Zoospores of Ulva linza with Arginine-Rich Oligopeptide Monolayers’. Available at: https://search.ebscohost.com/login.aspx?direct=true&db=edsoai&AN=edsoai.on1234364910&site=eds-live&scope=site (Accessed: 5 October 2025). ↩︎