Toxin Producing Algae Across U.S. Landscapes—Are They Gaining a Foothold? Completed
There are still many unknowns related to the occurrence and potential range of various types of algae in inland waters. To fill some of these gaps, scientists at the U.S. Geological Survey (USGS) recently published a review and synthesis of toxic algae in inland waters of the conterminous United States.
Many people are concerned with excessive algae in waterways of the United States. Naturally occurring toxins from some algae can affect the health of aquatic and terrestrial organisms, including humans by causing acute and chronic illnesses. The first reports of toxic algae in the scientific literature were recorded centuries ago. However, more recent harmful algal bloom (HAB) events, including a 2001 Prymnesium parvum (golden alga) bloom that killed about 2 million fish, have raised concerns for environmental and human health, as well as economic effects, across the United States and throughout the world.
In the review, USGS scientists highlighted gaps in current knowledge and proposed potential future research directions. For instance, the observation that two cyanobacterial toxins (cyanotoxins), microcystin and cylindrospermopsin, were found at higher rates in brackish lakes than freshwater lakes, suggest that efforts to assess the risk of cyanotoxin exposure should account for lake salinity. In addition, future studies on HAB biogeography should consider the ionic composition of inland waters separately from salinity. Other factors such as nutrient and agrochemical inputs, changes in land cover types over watersheds, and seasonal variables (day length and light intensity) should also be treated as potentially important factors regulating HAB incidence and toxin production. An emerging concern is the potential for inland HABs and their toxins to gain a foothold in US estuaries.
The USGS review and synthesis covers three planktonic (floating) algal groups: cyanobacteria, haptophytes, and euglenoids, which have strains capable of producing toxins in inland waters. This review provides information on the toxic members of these groups, including an analysis of data collected from 1,090 lakes in the 2017 National Lakes Assessment (U.S. Environmental Protection Agency, 2017).
The review addresses the topics of HAB ecology (with emphasis on physicochemical factors), algal toxins, and the potential for future range expansion of HAB-causing organisms. For all taxa, the authors present a comprehensive account of bloom habitats and their known associations with land cover and use from the perspective of water quality, including novel analyses of the potential effect of salinity on inland cyanobacteria.
The scientists report that blooms of the toxic cyanobacteria (also called blue green algae) Aphanizomenon, Microcystis, and Planktothrix are present at higher frequency in slightly saline, or oligohaline water bodies than freshwater lakes. Dolichospermum (cyanobacteria) show no significant salinity preferences, but its relative bloom rates are numerically higher in oligohaline waters. The relative detection rates of two cyanotoxins, microcystins and cylindrospermopsin, are also significantly higher in saline water bodies than in freshwater lakes. These observations indicate that lake salinity is important to consider when assessing the risk of HAB occurrence and cyanotoxin exposure.
Although blooms of the toxic haptophyte P. parvum have been documented exclusively in brackish inland waters, there is increasing evidence that this species may commonly occur in low-salinity freshwaters—perhaps explaining a case where a singular discharge of saline wastewater into a freshwater stream without a history of P. parvum blooms led to the acute formation of a toxic bloom. Potentially toxic Euglena spp. have the same relative rates of occurrence in freshwater and oligohaline lakes, indicating that, like cyanobacteria, the inland distribution of this taxon is not restricted to freshwater lakes.
The authors concluded that studies on HAB biogeography should consider the ionic composition of inland waters separately from salinity. Major ions such as calcium, magnesium, and sulfate have all been associated with the growth and toxicity of HAB-causing species. Sulfate presents a particularly interesting suite of possible effect scenarios in which this anion may directly or indirectly inhibit or stimulate (or both) harmful algal growth.
Most harmful algae, especially those of widespread distribution, are “hardy” species that tolerate not only weather and climate extremes but also exposure to a variety of anthropogenic pollutants, including agricultural herbicides. Moreover, some commonly used herbicides such as glyphosate can stimulate the growth of HAB species (for example, the cyanobacteria Microcystis aeruginosa, the haptophyte P. parvum) at low, environmentally relevant concentrations. Thus, agrochemical inputs into surface waters, beyond the well-established effect of nutrients, could increase the risk of HABs.
In addition, loss of wetlands and riparian buffers may contribute to HAB occurrence because of the loss of multiple services that these lands provide to aquatic habitats, including the removal of nutrients and herbicides. A better understanding of HAB trends and their drivers, from the individual lake to the global scale, may be gained by paying increased attention to land uses other than fertilizer application and land cover types other than agriculture and development.
Like most phytoplankton, cyanobacteria and Euglena normally grow between the spring and fall when day length and light intensity are relatively high compared to the rest of the year. Prymnesium parvum is unique in that its typical growth period in the United States is the reverse (from the fall to spring). The growth season of P. parvum may not be the result of environmental preferences (of day length and light intensity) but of growth opportunities at a time when most other taxa are dormant.
The toxin classes covered in the review include neurotoxins (affecting the central nervous system), hepatotoxins (affecting the liver), and ichthyotoxins (harming fish and other aquatic organisms). However, some toxins may have multiple modes of action, and blooms may contain organisms that produce multiple toxins. The neurotoxins, anatoxin and saxitoxin, are produced by cyanobacteria and have a wide global distribution, but many of the literature examples indicate production from benthic taxa. Microcystin and cylindrospermopsin are hepatotoxins produced by cyanobacteria. Microcystin is widely distributed but seems to be concentrated in the Central USA. The distribution of cylindrospermopsin is concentrated primarily in Mid-Atlantic States, Midwestern States, Texas, and Florida, but detections seem to have expanded from tropical and subtropical areas into temperate (mild temperature) zones. Cold water blooms of some cyanobacteria may be of increasing importance, and research has indicated that some toxins, such as saxitoxin, may be associated with lower water temperatures. Information on the geographic range of the ichthyotoxin euglenophycin, produced by Euglena, is limited, but the toxin has been detected in fish culture ponds in the South Central and Southeastern USA. Prymnesins, ichthyotoxins produced by P. parvum, have been detected in sites from a wide geographic range, primarily in the temperate and subtropical zones.
There is evidence indicating that inland cyanobacterial HAB species may be gaining a foothold in U.S. estuaries. The basis for this apparent seaward expansion is uncertain; however, inland strains isolated from saline coastal waters elsewhere have acquired genes whose function may be to allow their growth in brackish waters, indicating that adaptation may be a means for freshwater strains to overcome instream salinity barriers. Dispersal of brackish inland strains into estuaries via natural or anthropogenic vectors could also result in the seaward expansion of toxic events caused by historically “inland” taxa. Genetic and comparative physiological studies of strains from inland and estuarine environments would help evaluate the feasibility of these scenarios.
References Cited
U.S. Environmental Protection Agency, 2017, National Lakes Assessment—A collaborative survey of the Nation’s lakes: Washington, D.C., U.S. Environmental Protection Agency, accessed August 15, 2022, at https://www.epa.gov/national-aquatic-resource-surveys/nla.
There are still many unknowns related to the occurrence and potential range of various types of algae in inland waters. To fill some of these gaps, scientists at the U.S. Geological Survey (USGS) recently published a review and synthesis of toxic algae in inland waters of the conterminous United States.
Many people are concerned with excessive algae in waterways of the United States. Naturally occurring toxins from some algae can affect the health of aquatic and terrestrial organisms, including humans by causing acute and chronic illnesses. The first reports of toxic algae in the scientific literature were recorded centuries ago. However, more recent harmful algal bloom (HAB) events, including a 2001 Prymnesium parvum (golden alga) bloom that killed about 2 million fish, have raised concerns for environmental and human health, as well as economic effects, across the United States and throughout the world.
In the review, USGS scientists highlighted gaps in current knowledge and proposed potential future research directions. For instance, the observation that two cyanobacterial toxins (cyanotoxins), microcystin and cylindrospermopsin, were found at higher rates in brackish lakes than freshwater lakes, suggest that efforts to assess the risk of cyanotoxin exposure should account for lake salinity. In addition, future studies on HAB biogeography should consider the ionic composition of inland waters separately from salinity. Other factors such as nutrient and agrochemical inputs, changes in land cover types over watersheds, and seasonal variables (day length and light intensity) should also be treated as potentially important factors regulating HAB incidence and toxin production. An emerging concern is the potential for inland HABs and their toxins to gain a foothold in US estuaries.
The USGS review and synthesis covers three planktonic (floating) algal groups: cyanobacteria, haptophytes, and euglenoids, which have strains capable of producing toxins in inland waters. This review provides information on the toxic members of these groups, including an analysis of data collected from 1,090 lakes in the 2017 National Lakes Assessment (U.S. Environmental Protection Agency, 2017).
The review addresses the topics of HAB ecology (with emphasis on physicochemical factors), algal toxins, and the potential for future range expansion of HAB-causing organisms. For all taxa, the authors present a comprehensive account of bloom habitats and their known associations with land cover and use from the perspective of water quality, including novel analyses of the potential effect of salinity on inland cyanobacteria.
The scientists report that blooms of the toxic cyanobacteria (also called blue green algae) Aphanizomenon, Microcystis, and Planktothrix are present at higher frequency in slightly saline, or oligohaline water bodies than freshwater lakes. Dolichospermum (cyanobacteria) show no significant salinity preferences, but its relative bloom rates are numerically higher in oligohaline waters. The relative detection rates of two cyanotoxins, microcystins and cylindrospermopsin, are also significantly higher in saline water bodies than in freshwater lakes. These observations indicate that lake salinity is important to consider when assessing the risk of HAB occurrence and cyanotoxin exposure.
Although blooms of the toxic haptophyte P. parvum have been documented exclusively in brackish inland waters, there is increasing evidence that this species may commonly occur in low-salinity freshwaters—perhaps explaining a case where a singular discharge of saline wastewater into a freshwater stream without a history of P. parvum blooms led to the acute formation of a toxic bloom. Potentially toxic Euglena spp. have the same relative rates of occurrence in freshwater and oligohaline lakes, indicating that, like cyanobacteria, the inland distribution of this taxon is not restricted to freshwater lakes.
The authors concluded that studies on HAB biogeography should consider the ionic composition of inland waters separately from salinity. Major ions such as calcium, magnesium, and sulfate have all been associated with the growth and toxicity of HAB-causing species. Sulfate presents a particularly interesting suite of possible effect scenarios in which this anion may directly or indirectly inhibit or stimulate (or both) harmful algal growth.
Most harmful algae, especially those of widespread distribution, are “hardy” species that tolerate not only weather and climate extremes but also exposure to a variety of anthropogenic pollutants, including agricultural herbicides. Moreover, some commonly used herbicides such as glyphosate can stimulate the growth of HAB species (for example, the cyanobacteria Microcystis aeruginosa, the haptophyte P. parvum) at low, environmentally relevant concentrations. Thus, agrochemical inputs into surface waters, beyond the well-established effect of nutrients, could increase the risk of HABs.
In addition, loss of wetlands and riparian buffers may contribute to HAB occurrence because of the loss of multiple services that these lands provide to aquatic habitats, including the removal of nutrients and herbicides. A better understanding of HAB trends and their drivers, from the individual lake to the global scale, may be gained by paying increased attention to land uses other than fertilizer application and land cover types other than agriculture and development.
Like most phytoplankton, cyanobacteria and Euglena normally grow between the spring and fall when day length and light intensity are relatively high compared to the rest of the year. Prymnesium parvum is unique in that its typical growth period in the United States is the reverse (from the fall to spring). The growth season of P. parvum may not be the result of environmental preferences (of day length and light intensity) but of growth opportunities at a time when most other taxa are dormant.
The toxin classes covered in the review include neurotoxins (affecting the central nervous system), hepatotoxins (affecting the liver), and ichthyotoxins (harming fish and other aquatic organisms). However, some toxins may have multiple modes of action, and blooms may contain organisms that produce multiple toxins. The neurotoxins, anatoxin and saxitoxin, are produced by cyanobacteria and have a wide global distribution, but many of the literature examples indicate production from benthic taxa. Microcystin and cylindrospermopsin are hepatotoxins produced by cyanobacteria. Microcystin is widely distributed but seems to be concentrated in the Central USA. The distribution of cylindrospermopsin is concentrated primarily in Mid-Atlantic States, Midwestern States, Texas, and Florida, but detections seem to have expanded from tropical and subtropical areas into temperate (mild temperature) zones. Cold water blooms of some cyanobacteria may be of increasing importance, and research has indicated that some toxins, such as saxitoxin, may be associated with lower water temperatures. Information on the geographic range of the ichthyotoxin euglenophycin, produced by Euglena, is limited, but the toxin has been detected in fish culture ponds in the South Central and Southeastern USA. Prymnesins, ichthyotoxins produced by P. parvum, have been detected in sites from a wide geographic range, primarily in the temperate and subtropical zones.
There is evidence indicating that inland cyanobacterial HAB species may be gaining a foothold in U.S. estuaries. The basis for this apparent seaward expansion is uncertain; however, inland strains isolated from saline coastal waters elsewhere have acquired genes whose function may be to allow their growth in brackish waters, indicating that adaptation may be a means for freshwater strains to overcome instream salinity barriers. Dispersal of brackish inland strains into estuaries via natural or anthropogenic vectors could also result in the seaward expansion of toxic events caused by historically “inland” taxa. Genetic and comparative physiological studies of strains from inland and estuarine environments would help evaluate the feasibility of these scenarios.
References Cited
U.S. Environmental Protection Agency, 2017, National Lakes Assessment—A collaborative survey of the Nation’s lakes: Washington, D.C., U.S. Environmental Protection Agency, accessed August 15, 2022, at https://www.epa.gov/national-aquatic-resource-surveys/nla.