Algae raise the pH of surrounding water during daylight hours, sometimes dramatically. Through photosynthesis, algae and cyanobacteria pull dissolved carbon dioxide out of the water, shifting the chemistry toward more alkaline conditions. In a calm pond choked with a summer bloom, daytime pH can climb well above 9, while in open-ocean kelp forests the effect is subtler but still measurable. The size and direction of the shift depend on the type of algae, how dense the growth is, the time of day, and the chemistry of the water itself, and the downstream effects on water quality range from nutrient release and fish stress to challenges for drinking-water treatment.
How Photosynthesis Pushes pH Upward
The core mechanism is straightforward. During photosynthesis, algae need carbon to build sugars. They get it by absorbing dissolved COâ‚‚ and, in many species, bicarbonate ions from the water around them. Removing COâ‚‚ from water has the same chemical effect as removing an acid: the balance tips toward alkalinity, and pH rises. Research on microalgae growth has confirmed that increasing light intensity accelerates this process, because greater photosynthetic activity pulls more hydrogen ions into the cell and removes more dissolved carbon from the surrounding medium.1PubMed. Role of culture solution pH in balancing CO(2) input and light intensity for maximising microalgae growth rate The green alga Scenedesmus, for instance, has been shown to operate two separate carbon-concentrating systems: one that pulls in COâ‚‚ across a broad pH range and one that actively takes up bicarbonate at higher pH values up to about 11.2Plant Physiology. Two Systems for Concentrating CO2 and Bicarbonate during Photosynthesis by Scenedesmus That means algae don’t simply run out of fuel as pH climbs. Many species have evolved tools to keep extracting carbon even when conditions are already quite alkaline.
At night the cycle reverses. Algae respire just like animals, consuming oxygen and releasing COâ‚‚ back into the water. That lowers pH. The result is a seesaw: pH peaks in the late afternoon when photosynthesis has been running all day, then drops through the night as respiration dominates. The swing can be surprisingly large.
Day-Night pH Swings
In coastal waters dominated by kelp forests, researchers have found that the daily pH swing driven by photosynthesis and respiration already exceeds the pH drop projected for the open ocean by the end of this century under ocean acidification scenarios.3PubMed Central. Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification In other words, organisms living among dense seaweed already experience bigger pH swings every 24 hours than what climate models predict for open-water species over the coming decades.
Field measurements of green-tide algal mats in the Yellow Sea recorded diel pH fluctuations of roughly 0.22 units in the water just above the mat, with corresponding swings in dissolved COâ‚‚ of over 200 microatmospheres per day.4Science of The Total Environment. Diel metabolism of Yellow Sea green tide algae alters bacterial community composition under in situ seawater acidification of coastal areas Those numbers may sound modest, but pH is a logarithmic scale, and even tenths of a unit translate into meaningful changes in the concentration of hydrogen ions. The same study found that these metabolic swings also reshaped the bacterial communities living near the algae, shifting which microbes thrived during the day versus at night.
In a subarctic fjord in Greenland, macroalgae-dominated habitats showed pH variability of 0.2 to 0.3 units across different scales, from within a single cubic meter of kelp canopy to along the length of the fjord and between seasons. In vegetated tidal pools, the diel pH range exceeded 1.5 units, and the thin boundary layer right at the surface of macrophyte fronds swung by up to 0.8 units.5Biogeosciences. Macroalgae contribute to nested mosaics of pH variability in a subarctic fjord The researchers suggested that these productive coastal zones could serve as high-pH refuges in an increasingly acidified Arctic Ocean.
The Tiny pH Bubble Around a Single Cell
You don’t need a massive bloom to see pH effects. Every photosynthesizing algal cell creates its own micro-zone of elevated pH in the water immediately surrounding it, called the phycosphere. Measurements using nano-scale pH probes have captured this in striking detail. A single cell of the green alga Chlamydomonas concordia, only about five micrometers across, raised the pH at its surface to 8.27 when the surrounding seawater sat at 8.01.6The ISME Journal. Phycosphere pH of unicellular nano- and micro- phytoplankton cells and consequences for iron speciation Moving the probe away from the cell surface showed a smooth gradient back down to bulk seawater pH within a few cell-lengths’ distance.
Larger cells create bigger pH envelopes. The diatom Coscinodiscus radiatus (about 50 micrometers in diameter) elevated its phycosphere pH by roughly 0.4 units above bulk seawater.7PubMed Central. Phycosphere pH of unicellular nano- and micro- phytoplankton cells and consequences for iron speciation A broader review of phycosphere measurements found that freshwater algae tend to produce even larger pH shifts than marine species: on average, freshwater algae elevated phycosphere pH by about 1.28 units in the light, compared to around 0.32 units for marine algae.8PubMed Central. Unravelling Metal Speciation in the Microenvironment Surrounding Phytoplankton Cells to Improve Predictions of Metal Bioavailability The difference is partly because seawater’s carbonate buffering system resists pH changes more effectively than most freshwater does.
These microscopic pH shifts matter because they change the chemistry of trace metals like iron in the immediate vicinity of the cell. Higher pH can alter which chemical forms of iron are available, and iron availability in turn affects whether algae can keep growing. So each cell’s photosynthesis modifies its own nutrient landscape on a scale invisible to the naked eye.
When Blooms Raise pH Across Entire Water Bodies
Scale up from individual cells to a dense bloom and the pH effects become dramatic. Thick cyanobacterial surface blooms can push pH above 9 or even 10 in shallow, poorly buffered freshwater. Microelectrode measurements within cyanobacterial blooms have shown that high surface pH promotes the chemical uptake of atmospheric COâ‚‚ into the water, a phenomenon known as chemical enhancement.9Limnology and Oceanography. Photoinhibition and the availability of inorganic carbon restrict photosynthesis by surface blooms of cyanobacteria In effect, the bloom raises pH so high that it creates a steep concentration gradient for COâ‚‚ between the atmosphere and the water surface, accelerating gas transfer. Even so, the bloom eventually becomes carbon-limited as photosynthesis outpaces the rate at which COâ‚‚ can dissolve back in.
Laboratory work with the common bloom-forming cyanobacterium Microcystis aeruginosa confirmed that the alga significantly raises the pH of its growth medium, and that this pH increase actually benefits further algal growth, creating a positive feedback loop.10Water Science and Engineering. Optimal dosing time of acid algaecide for restraining algal growth That feedback is one reason blooms can intensify so quickly: once a bloom raises the water’s pH, conditions become even more favorable for additional growth, at least until other resources run out.
Which Species Win at High pH
A longstanding assumption held that cyanobacteria outcompete green algae at low COâ‚‚ and high pH, while eukaryotic algae (like greens and diatoms) do better when COâ‚‚ is plentiful. Recent experimental work has challenged that neat division. Researchers found that some green algae are highly effective competitors at low COâ‚‚ levels, while certain cyanobacterial strains actually struggled under those conditions and became stronger competitors only when COâ‚‚ was elevated.11Journal of Experimental Botany. Competition between cyanobacteria and green algae at low versus elevated CO2: who will win, and why? The diversity of carbon-concentrating mechanisms across species means there is no simple rule about which type of alga “owns” the high-pH niche. This matters for predicting what a bloom will look like in a given lake or reservoir, and for understanding why different water bodies develop different dominant species even under similar nutrient loads.
What High pH Does to Nutrients and Sediments
Algae-driven pH increases don’t just change the water column. They also unlock nutrients trapped in bottom sediments, which can fuel further algal growth. When photosynthesis raises pH in the water above the sediment, hydroxide ions begin to compete with phosphate ions for binding sites on sediment particles. The result is that phosphorus that had been locked onto iron and aluminum compounds in the mud gets released back into the water.12ScienceDirect. Mechanisms driving phosphorus release during algal blooms based on hourly changes in iron and phosphorus concentrations in sediments This is another positive feedback loop: the bloom raises pH, high pH releases phosphorus from sediment, and more phosphorus feeds more algal growth.
This internal loading of phosphorus from sediments is a major reason why some lakes remain eutrophic even after external nutrient inputs from agriculture or wastewater have been reduced. The bloom has essentially trained the sediments to keep supplying phosphorus on a daily cycle. Breaking this feedback often requires interventions beyond simply cutting nutrient runoff, such as sediment capping or chemical treatment to bind phosphorus more permanently.
Consequences for Fish and Other Aquatic Life
Most freshwater fish thrive in a pH range of roughly 6.5 to 9. When algal blooms push pH well above 9, the water itself becomes stressful. High pH increases the proportion of ammonia in its toxic un-ionized form, meaning that even modest ammonia concentrations become dangerous. Gill function can be impaired, and the stress makes fish more vulnerable to disease. In severe cases, pH spikes combine with the oxygen crash that follows bloom die-off to cause mass fish kills.
The day-night pH swing adds another layer of stress. Aquatic organisms experience a rising alkaline environment through the afternoon, then a plunge toward acidic conditions overnight, with oxygen levels falling at the same time. For sensitive species or early life stages like eggs and larvae, these rapid fluctuations can be more harmful than a steady pH at either extreme, because the organism never has time to acclimate.
Challenges for Drinking Water Treatment
When algae bloom in a reservoir used for drinking water, the pH increase creates a chemical headache for treatment plants. Most coagulation processes, which clump particles together for removal, work best within a specific pH window. When bloom-driven photosynthesis pushes raw water pH above about 8.2, the aluminum-based coagulants commonly used in treatment plants start to form the wrong precipitates and lose effectiveness. Research on diatom blooms in drinking-water sources found that adjusting pH back down to the 7.5 to 8.0 range improved coagulant-only removal of algal cells by over 38 percent, directly addressing the bottleneck that alkaline conditions had created.13PubMed. Emergency response technologies for Aulacoseira granulata blooms in drinking water resource: Pre-oxidation-Enhanced coagulation and mechanisms
Beyond coagulation problems, high-pH water can also carry taste and odor compounds produced by the bloom itself, and it complicates disinfection chemistry. Treatment operators dealing with bloom-affected source water often need to dose acid to bring pH back into the effective range before their normal processes can work, adding cost and complexity.
Algae and pH in Aquaculture
Fish and shrimp farmers often cultivate algae intentionally, because a moderate algal community provides food for larvae and helps stabilize water chemistry. But that stabilization works both ways. In experiments with small recirculating aquaculture systems, tanks containing algae required roughly three times as much hydrochloric acid to lower pH from 7 to 4 compared to tanks without algae, demonstrating how effectively algal photosynthesis resists pH decreases.14PLOS ONE. Resistance and resilience of small-scale recirculating aquaculture systems (RAS) with or without algae to pH perturbation From the farmer’s perspective, that buffering effect is usually beneficial. It prevents the sharp overnight pH drops that stress fish. But during peak growing season, algal overgrowth can push daytime pH dangerously high, so pond managers often aerate aggressively in the afternoon to strip excess dissolved oxygen and COâ‚‚ dynamics back toward equilibrium, or add carbon sources to counter the alkaline drift.
Kelp Forests as Local Buffers Against Ocean Acidification
On coastlines, macroalgae like kelp play a role that has attracted growing interest from ocean-acidification researchers. Because kelp photosynthesis raises local pH during the day, intact kelp forests create pockets of water that are less acidic than the surrounding open ocean. Field measurements inside a Ecklonia radiata kelp forest in Australia found that daily maximum pH inside the forest was 0.11 units higher than in adjacent barren habitat, and the daily pH range was also 0.11 units wider, reflecting the stronger photosynthesis-respiration cycle within the canopy.15Scientific Reports. Assessing the role of natural kelp forests in modifying seawater chemistry Researchers have argued that these forests could serve as local refugia for shellfish and other calcifying organisms sensitive to acidification, at least in sheltered bays where water exchange is slow enough for the pH signal to persist.
The caveat is that the effect is strongest during the day and weakest at night, and it depends on the forest staying healthy. Kelp forests around the world are under pressure from warming waters and overgrazing by sea urchins. If the kelp disappears, so does its pH-buffering service, leaving coastal organisms more exposed to background acidification trends.
Using Algae’s pH Effect on Purpose
Engineers and biotechnologists have started harnessing the pH-raising behavior of algae rather than fighting it. One approach involves growing microalgae at extremely high pH, above 10, to capture COâ‚‚ directly from the atmosphere. At such alkaline conditions, COâ‚‚ dissolves into water far more readily because it reacts with hydroxide ions, meaning open-air algae ponds can absorb atmospheric carbon without needing piped-in gas. Research on microbial communities from alkaline soda lakes has shown that biofilms grown at pH up to 10 and high alkalinity can sustain biomass productivity above one kilogram per cubic meter per day for months at a time.16PubMed Central. Robust, high-productivity phototrophic carbon capture at high pH and alkalinity using natural microbial communities
A related strategy pairs algal biomass production with permanent carbon storage through mineral precipitation. Researchers have grown the marine microalga Nannochloropsis oceanica at lab and pilot scale, timed to raise pH after an initial growth phase. The elevated pH then triggered calcium carbonate to precipitate out of the culture medium, locking carbon into mineral form alongside the harvestable biomass.17PubMed Central. Nannochloropsis oceanica IMET1 and its bacterial symbionts for carbon capture, utilization, and storage: biomass and calcium carbonate production under high pH and high alkalinity An added benefit of running cultures at extreme pH is that very few contaminating organisms can survive above pH 10, which reduces the need for sterilization and makes outdoor cultivation more practical.18PubMed Central. Hunting for Extremophiles: A Systematic Screening of Freshwater Microalgae for Tolerance to High-pH and High-Alkalinity Cultivation
Timing Algae Control Around the pH Cycle
Understanding the daily pH curve created by algae opens a window for more effective control. The acid algaecide research on Microcystis aeruginosa found that applying an acid-based treatment during the bloom’s stable growth phase, when pH reaches its daily peak, had the strongest suppressive effect on further algal growth.10Water Science and Engineering. Optimal dosing time of acid algaecide for restraining algal growth The logic is that the algae have already pushed conditions to their alkaline extreme, so an acid dose at that moment creates the largest possible chemical shock. Applying the same treatment at night, when respiration has already lowered pH, wastes much of the dose on chemistry rather than biology.
For lake managers and water utility operators, this means monitoring pH throughout the day rather than relying on a single morning grab sample. A morning reading might suggest the water is within normal bounds, while an afternoon measurement could reveal pH above 9. Automated continuous pH logging gives a much more accurate picture of what aquatic life and treatment infrastructure are actually experiencing.
Algae and Soil pH
The influence of photosynthetic organisms on pH is not limited to water. Biological soil crusts, which include cyanobacteria, green algae, lichens, and mosses, colonize bare ground in arid and semi-arid landscapes around the world. These crusts alter soil chemistry at fine scales. A study of crusts along a grazing gradient found that while cyanobacteria-dominated crusts kept soil pH relatively similar to bare ground, lichen-dominated crusts showed measurable pH decreases along with shifts in mineral nutrient concentrations like sodium, iron, and zinc.19Soil Biology and Biochemistry. Biological soil crusts greatly contribute to small-scale soil heterogeneity along a grazing gradient The picture on land is more complicated than in water: organisms in soil crusts produce organic acids, trap moisture, and interact with mineral weathering in ways that can push pH either up or down depending on the community composition and the underlying soil type.
Cyanobacteria, the oldest oxygen-producing photosynthesizers on Earth with a lineage stretching back roughly 3.5 billion years, have been shaping atmospheric and water chemistry since long before plants existed.20PubMed Central. Climate change: a catalyst for global expansion of harmful cyanobacterial blooms Their capacity to raise pH is not some incidental byproduct of modern eutrophication. It is an ancient metabolic consequence of pulling carbon from water to build organic matter, one that has been influencing Earth’s chemistry for billions of years and will continue to do so wherever sunlight reaches water with nutrients in it.