Algae
Biology, Diversity, Growth Requirements and Management in Water Systems
Abstract. This document reviews the biology and ecological diversity of algae, their classification by photosynthetic pigment, cellular water-regulating structures (vacuoles), environmental growth requirements, and the conditions under which algal growth becomes problematic in engineered and recreational water systems. Statements are supported by peer-reviewed and authoritative sources, listed with links in the References section. Species counts are drawn from the AlgaeBase-derived literature.
Contents
Vacuoles in algae — which types have them
Cyanotoxins and the recreational threshold
Control and treatment: environmentally friendly methods
Preventive / nutrient-based (source control)
Overview
Algae are photosynthetic organisms — ranging from microscopic single cells (microalgae) to large multicellular seaweeds (macroalgae) — whose ancestors have existed for billions of years. Photosynthetic life is extremely ancient: microfossil and geochemical evidence places oxygenic photosynthesis and early cyanobacteria in the Archean, on the order of 3–3.8 billion years ago.
Estimates of algal diversity vary widely with definition and method. Older estimates suggested figures upward of 300,000+ species, while a widely cited conservative analysis using AlgaeBase put the total near 72,500 described-or-nameable species, of which only a fraction have been studied in detail. More recent AlgaeBase tallies (Nov 2023) document ~50,600 living species across four kingdoms, 14 phyla and 63 classes. By any measure, algae represent extraordinary biological diversity and contribute roughly 40–50% of global photosynthesis (CO₂ assimilation and oxygen production).
While algae are valuable as a potential biofuel feedstock and as an oxygen-producing, carbon-sequestering resource, uncontrolled growth in industrial and recreational water systems is a serious problem — causing fouling, blockages, toxin release, and water-quality deterioration.
Classification by pigment
Algae are commonly grouped by their dominant photosynthetic pigments and the resulting colour. This is a practical scheme; genetically the algae are polyphyletic (they arose in several independent evolutionary lineages), so “colour” groups do not all share a single common ancestor. The table below preserves the common colour grouping and adds the verified scientific class, the vacuole status, and notes.
Common name | Class / group | Has vacuole? | Notes (verified) |
Green algae | Chlorophyceae | Yes — contractile (freshwater) | ~6,850 living species; dominant in freshwater. Motile freshwater forms (e.g. Chlamydomonas) use contractile vacuoles for osmoregulation. |
Brown algae | Phaeophyceae | Yes — physodes / large vacuoles | Macroalgae (kelps/seaweeds); giant kelp reaches ~30–45 m. Marine; store phenolics in vacuole-like physodes. |
Red algae | Rhodophyceae | Yes — central vacuole (non-contractile) | ~7,276 living species; mostly marine. Some dinoflagellate (not rhodophyte) blooms cause ‘red tides’. |
Blue-green algae | Cyanobacteria | No true vacuole — has gas vesicles | Prokaryotic (no membrane-bound organelles). Microcystis produces microcystin toxins; gas vesicles give buoyancy. |
Black algae | Various (often cyanobacterial) | Varies by organism | A practical/aesthetic term, not a taxonomic class; dark pigmented, strongly adherent biofilms on surfaces (e.g. pool grout). |
Yellow-green algae | Xanthophyceae (Tribophyceae) | Yes — vacuoles present | Common in soils and freshwater; part of the heterokont (stramenopile) lineage. |
Note on “red tide”: true red algae (Rhodophyceae) are mostly benign marine seaweeds; the toxic, water-discolouring “red tide” events are usually caused by dinoflagellates (e.g. Karenia, Alexandrium), not by Rhodophyceae. The colour name and the bloom phenomenon are frequently conflated in non-technical usage.
Vacuoles in algae — which types have them
A common student question is which algae possess a vacuole. The key distinction is between eukaryotic algae (which have membrane-bound organelles, including vacuoles) and cyanobacteria (which are prokaryotes and therefore have no true, membrane-bound vacuole at all). Among eukaryotic algae, the functionally important structure is the contractile vacuole, used for osmoregulation.
Freshwater flagellated green algae are the classic vacuole-bearing example: the model green alga Chlamydomonas reinhardtii uses contractile vacuoles to expel the excess water that continually enters the cell in hypotonic freshwater. The vacuole fills (diastole) and then contracts to expel water (systole); water entry is facilitated by the aquaporin MIP1. Marine algae, living in roughly isotonic seawater, generally lack contractile vacuoles (though some can be induced under osmotic stress).
In contrast, cyanobacteria such as Microcystis have no true vacuole; instead they contain gas vesicles — gas-filled protein compartments (often loosely called “gas vacuoles”) that provide buoyancy, letting colonies float to the surface for light and atmospheric CO₂ and form the characteristic surface scums of a bloom.
Vacuole type | Which algae | Function |
Contractile vacuole | Freshwater flagellated green algae (e.g. Chlamydomonas, Euglena, Mesostigma) | Osmoregulation — periodically expels excess water that enters the cell in hypotonic freshwater (fill = diastole, expel = systole). |
Central / storage vacuole | Many eukaryotic algae — red, brown, yellow-green, and non-motile green algae | Turgor, storage of ions/metabolites, and pH/volume regulation. Brown algae store phenolics in physodes. |
Gas vesicle (NOT a true vacuole) | Cyanobacteria (e.g. Microcystis, Dolichospermum) | Buoyancy control — gas-filled protein compartments let cells float to optimal light/CO₂ and form surface scums. Prokaryotes have no membrane-bound vacuole. |
Growth requirements
Algal growth is governed by the availability of light, inorganic carbon, nutrients (especially nitrogen and phosphorus), and suitable temperature. The essential inputs are:
- Light — photosynthesis is the primary energy source.
- Carbon — CO₂ dissolved in the water or drawn from the atmosphere (also bicarbonate for many species).
- Nitrogen — as nitrate, ammonium, or urea.
- Minerals — magnesium, potassium, calcium, and phosphate (phosphorus is very often the limiting nutrient that triggers blooms).
- Trace elements — iron, zinc, copper, and others (aluminium is common in the environment though not an essential nutrient).
Temperature: many freshwater algae grow across roughly 15–35 °C, with cyanobacterial blooms in particular favoured by warm, calm, nutrient-rich (eutrophic) water. Through photosynthesis, algae consume CO₂ (carbon sequestration) and release oxygen, which makes them ecologically important primary producers — but this same vigorous metabolic activity is exactly what makes them problematic in closed or semi-closed water systems, where biomass, oxygen swings, and pH fluctuations accumulate.
Problem environments
Uncontrolled algal (and cyanobacterial) growth is a recurring operational and health problem in the following settings:
- Ornamental and fish ponds — toxin risk (cyanotoxins) and large diurnal pH and dissolved-oxygen fluctuations that stress or kill fish.
- Swimming pools and natural bathing ponds — aesthetic problems, slippery surfaces, and — where cyanobacteria dominate — health advisories.
- Cooling-water circuits — industrial installations and cooling towers, where biofilms and algal mats reduce heat-transfer efficiency and promote biofouling/biocorrosion.
- Rainwater collection and recirculation systems — filter blockage and contamination.
- Open reservoirs for agriculture and horticulture — biomass and biofilm cause dripper/emitter blockage in irrigation lines.
Cyanotoxins and the recreational threshold
Blue-green algae (cyanobacteria) are the main health concern. Bloom-forming Microcystis produces microcystins — potent liver toxins (hepatotoxins). The World Health Organization’s recreational-water guidance defines a high-probability-of-adverse-effects band at roughly 20 µg/L microcystin (broadly corresponding to about 100,000 cyanobacterial cells/mL, or visible scum), which is why bathing is typically prohibited at or above about 20 µg/L. For comparison, the WHO provisional drinking-water guideline for microcystin-LR is far stricter at 1 µg/L. Exact action levels vary by country and agency (commonly in the 4–25 µg/L range for recreational water).
Control and treatment: environmentally friendly methods
Because conventional chemical algicides (notably copper sulphate) can accumulate in sediment, harm non-target organisms, and release toxins when they lyse cyanobacterial cells, there is strong interest in more environmentally friendly control. Modern practice favours an integrated, multi-barrier approach that combines prevention (nutrient control) with biological and physical methods, reserving chemicals as a last resort. The most effective long-term lever is almost always limiting the nutrients — especially phosphorus — that drive growth in the first place.
Preventive / nutrient-based (source control)
Reducing the phosphorus supply starves blooms at the source. Eco-friendly options include phosphorus-binding materials such as lanthanum-modified bentonite clay, which locks phosphate into the sediment, and constructed wetlands or vegetated buffer strips that intercept nutrient runoff before it reaches the water body. Shading, mixing/destratification, and maintaining healthy submerged macrophytes (which compete with algae for nutrients and light, partly through allelopathy) also suppress growth without biocides.
Biological treatment
Biological control uses living organisms or natural products to suppress algae, offering long-term, low-residue benefits:
- Biomanipulation — adjusting the food web to favour algae grazers. Classic approaches increase large zooplankton (e.g. Daphnia) grazing pressure by managing planktivorous fish, or stock filter-feeding fish; filter-feeding fish have been used to control Microcystis blooms in hypereutrophic lakes over decades of application.
- Algicidal micro-organisms — bacteria (e.g. Myxobacteria, Bacillus and other strains), actinomycetes, fungi, and cyanophage viruses that lyse or inhibit cyanobacteria. They can be highly specific and leave little secondary pollution, though field reliability and biosafety still need careful validation.
- Barley (and other) straw — as straw decomposes aerobically it releases phenolic and other allelopathic compounds that are algistatic (inhibit growth rather than killing outright). Field studies report large reductions in algal biomass and a shift away from cyanobacteria toward diatoms; in one reservoir trial ~25 g/m³ of barley straw cut algal cell counts by about 90% within ~12 days. Results are species-dependent and not universal.
- Constructed wetlands & biofilters — engineered plant/microbe systems that remove nutrients and algal cells biologically, useful for recirculation and irrigation reservoirs.
Physical treatment
Physical methods add no chemicals and are attractive for ponds, reservoirs, and closed circuits. Two increasingly used technologies are ultrasound and nanobubbles.
Ultrasound. Ultrasonic transducers emit sound above ~20 kHz into the water. The proposed mechanism is that pressure waves collapse the gas vesicles that give cyanobacteria their buoyancy; cells then sink out of the well-lit surface layer and lose their competitive advantage, and higher-intensity treatment can also disrupt cell walls and photosynthetic membranes. Because only gas-vacuolate cyanobacteria (e.g. Microcystis) are strongly affected, ultrasound is relatively selective. Laboratory studies show clear inhibition and removal rates as high as ~85%, continuous “dosing” is needed. Importantly, ultrasound is best viewed as one component of an integrated program rather than a stand-alone cure. We can help to make it work.
Nanobubbles. Nanobubbles are gas bubbles smaller than a human hair that, unlike ordinary bubbles, stay suspended for long periods and implode within the water column rather than bursting at the surface. Oxygen nanobubbles re-oxygenate deep water and disrupt the low-oxygen, nutrient-releasing conditions that favour blooms; ozone nanobubbles (NBOT) go further, releasing ozone that oxidises algal cells and breaks down cyanotoxins throughout the water column. Reported results are strong in controlled settings — mesocosm ozone-nanobubble doses cut chlorophyll-a and phycocyanin by ~98–99% and microcystins by up to ~92%, and a 24-hour field treatment at one Florida site reduced algal cells by 95–100% with cyanotoxins no longer detectable. Effectiveness in large open water is dose- and coverage-dependent: trials on very high-biomass blooms showed limited reach, underlining that treated-volume fraction and ozone dose matter. Oxygen-only nanobubbles improve dissolved oxygen but, on their own, may not reduce cyanobacteria.
Choosing a method
No single method is universally best. The environmentally sound strategy is to prevent blooms by controlling nutrients, use biological methods for sustainable long-term suppression, and apply physical methods (ultrasound, nanobubbles, mixing) for targeted, chemical-free intervention — monitoring throughout and turning to chemical algicides only when rapid control is essential and other options have failed. For closed and semi-closed engineered systems (cooling circuits, rainwater recirculation, irrigation reservoirs), physical methods combined with nutrient/biofilm management are usually the most practical fit.
References
All sources retrieved July 2026. Some journal links may require institutional access for full text; abstracts and the cited figures are publicly visible.
[1] Guiry, M. D. (2012). How many species of algae are there? Journal of Phycology 48(5):1057–1063. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1529-8817.2012.01222.x
[2] Guiry, M. D. (2024). How many species of algae are there? A reprise. Four kingdoms, 14 phyla, 63 classes and still growing. Journal of Phycology. https://www.researchgate.net/publication/377573882_How_many_species_of_algae_are_there_A_reprise_Four_kingdoms_14_phyla_63_classes_and_still_growing
[3] AlgaeBase — global algal species database (Guiry & Guiry). https://www.algaebase.org
[4] Encyclopedia of Life — Algae (diversity and age overview). https://eol.org/docs/discover/algae
[5] Andersen, R. A. (1992). Diversity of eukaryotic algae. Biodiversity and Conservation 1:267–292. https://link.springer.com/article/10.1007/BF00693765
[6] Barsanti & Gualtieri, ‘Algae’ overview (Current Biology / ScienceDirect): ~72,500 described species; ~half of global primary production. https://www.sciencedirect.com/science/article/pii/S0960982214006046
[7] Komsic-Buchmann, K. et al. (2014). The contractile vacuole as a key regulator of cellular water flow in Chlamydomonas reinhardtii. Eukaryotic Cell 13(11):1421–1430. https://pubmed.ncbi.nlm.nih.gov/25217463/
[8] Contractile vacuole — overview (osmoregulation; predominance in freshwater unicellular algae such as Chlamydomonas). https://en.wikipedia.org/wiki/Contractile_vacuole
[9] Hausmann, K. & Patterson, D. J. (1984). Contractile Vacuole Complexes in Algae. In: Compartments in Algal Cells and Their Interaction, Springer. https://link.springer.com/chapter/10.1007/978-3-642-69686-2_15
[10] Xu, S. et al. (2019). Extracellular polysaccharide synthesis in a bloom-forming strain of Microcystis aeruginosa: implications for colonization and buoyancy. Scientific Reports 9:1251 (gas vesicles and buoyancy). https://www.nature.com/articles/s41598-018-37398-6
[11] Thomas, R. H. & Walsby, A. E. (1985). Buoyancy regulation in a strain of Microcystis. Journal of General Microbiology 131:799–809. https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-131-4-799
[12] WHO (2020). Cyanobacterial toxins: microcystins — background document for drinking-water and recreational guidance. https://cdn.who.int/media/docs/default-source/wash-documents/wash-chemicals/microcystins-background-201223.pdf
[13] U.S. EPA (2019). Recommended Human Health Recreational Ambient Water Quality Criteria for microcystins and cylindrospermopsin. https://www.epa.gov/sites/default/files/2019-05/documents/hh-rec-criteria-habs-document-2019.pdf
[14] WHO recreational action levels for cyanobacteria/microcystin (high-probability band ≈ 20 µg/L ≈ 100,000 cells/mL); summarized in Nevada DEP risk-threshold rationale (2024). https://ndep.nv.gov/uploads/water-wqm-docs/APPENDIX_B_Risk_Thresholds_Rationale.pdf
[15] Manoa/Hawaii ‘Exploring Our Fluid Earth’ — algal diversity (>350,000 species figure; ancestry). https://manoa.hawaii.edu/exploringourfluidearth/biological/aquatic-plants-and-algae/evidence-common-ancestry-and-diversity
[16] Anas, A. R. J. et al. (2024). Advancements in Biological Strategies for Controlling Harmful Algal Blooms (HABs). Water 16(2):224 (biomanipulation, algicidal microorganisms, barley straw). https://doi.org/10.3390/w16020224
[17] Sigee, D. C. et al. (2001). Biological control of cyanobacteria: principles and possibilities. Hydrobiologia 459:219–232. https://link.springer.com/article/10.1023/A:1017097502124
[18] Islami, H. R. & Filizadeh, Y. (2011). Use of barley straw to control nuisance freshwater algae. Journal AWWA 103(6). https://awwa.onlinelibrary.wiley.com/doi/abs/10.1002/j.1551-8833.2011.tb11458.x
[19] Rajasekhar, P. et al. (2012). A review of the use of sonication to control cyanobacterial blooms. Water Research (mechanisms: gas-vesicle collapse, cell damage). https://www.sciencedirect.com/science/article/abs/pii/S1568988311000710
[20] Bohrerova, Z. et al. (2023). Cyanobacteria mitigation using low power ultrasound for gas vesicle collapse. AWWA Water Science e1346 (field study; limited effect at real device pressures). https://awwa.onlinelibrary.wiley.com/doi/full/10.1002/aws2.1346
[21] Xu, Z. et al. (2024). Effectiveness of ozone nanobubble treatments on high biomass cyanobacterial blooms: a mesocosm experiment and field trial. Journal of Environmental Management. https://www.sciencedirect.com/science/article/pii/S0301479724033929
[22] NOAA NCCOS (2018). Validates Nanobubble Technology for Remediation of Harmful Freshwater Algal Blooms (ozone/oxygen nanobubble mechanism and field results). https://coastalscience.noaa.gov/news/nccos-validates-nanobubble-technology-for-remediation-of-harmful-freshwater-algal-blooms/