Didymosphenia geminata
(didymo)

Didymosphenia geminata is a microscopic freshwater diatom which is widespread in Asia, Europe and North America. A pattern of expanding range and nuisance blooms has developed in North America and Europe over the past several years. The diatom was discovered in New Zealand in 2004, where it was recognized as an invasive species (Kilroy et al., 2007). Since that time, it has spread to numerous rivers and watersheds on the South Island of New Zealand. It has recently also been spreading in southern South America (Reid et al., 2012). It creates dense benthic mats that can extend for several kilometres.

D. geminata is considered to be widespread in Asia, Europe and North America. In North America, historical reports of D. geminata are primarily taxonomic or floristic (Cleve, 1896; Boyer, 1916, 1927) and often lack information on abundance. A number of records are from Alaskan sites (Manguin, 1960; Patrick and Freese, 1961; Foged, 1981). Others report D. geminata in the continental USA (Fox et al., 1967, 1969; Zingmark, 1969; Nelson et al., 1973; Patrick and Reimer, 1975; Prescott and Dillard, 1979; Stoermer, 1980; Moffat, 1994; Bahls, 2004). The genus is most diverse in Lake Baikal and surrounding rivers, and the region could tentatively be proposed as the evolutionary origin of several species within Didymosphenia. In addition to the countries listed in the Distribution Table, D. geminata is also native to Iran (H Alimohammadian, Geological Survey of Iran, Tehran, Iran, personal communication, 2008).

Nuisance blooms have been recorded in North America and Europe over the past several years, however, in the UK and Norway, D. geminata has been documented for up to 150 years (Skulberg, 1982; Ellwood and Whitton, 2007), but there has been no change in the growth in streams.

It is an invasive species in the South Island of New Zealand (Kilroy et al., 2007) and in Patagonia, South America (Reid et al., 2012; Montecino et al., 2014, 2016).

There are numerous reports of nuisance blooms of D. geminata in North America and a few in Europe. It has been proposed that a new strain of the species is now dominant (Bothwell et al., 2006), and is responsible for the invasive species type of behaviour. However, the presence of a new genetic strain has not been established.

Work in New Zealand (Kilroy et al., 2006) established that D. geminata is capable of surviving outside of water, under damp, dark conditions for more than 40 days. Cells also remained viable in the felt-soles of waders. The risk of accidental transport and introduction by anglers, boaters, and other water users is considered to be high. Decontamination of boating and angling gear is highly recommended for all waters. Because this organism may not be visible to the naked eye, humans may easily transport it without knowing of its presence.

Niche analysis based on US and Chile models indicated that there were more suitable river systems for D. geminata to spread to in Chile (Montecino et al., 2014), however, extensive repeated surveys over 5 years indicated that D. geminata may be ending its expansion in the region (Montecino et al., 2016).

Diatoms are found in nearly every freshwater and marine habitat and contribute a large percentage of the global carbon budget through photosynthesis. In both oceans and freshwaters, diatoms are one of the major groups of organisms within the plankton assemblage and also grow attached to surfaces. Diatoms store chrysolaminarin (ß1,3 linked glucan) as well as accumulating lipid within the cell. Lipids are an oil-rich source of energy, which make diatoms a valuable food for other organisms. The life history of diatoms includes both vegetative and sexual reproduction (Meyer, 1929; and reviewed in Edlund and Stoermer, 1997).

The diatom produces an extracellular stalk which may attach to rocks, plants, or any other submerged substrata. When the diatom cell divides (that is, through vegetative reproduction), stalk continues to be produced by both progeny cells, forming a dense mass of branching stalks. It is not the diatom cell itself that is responsible for the negative impacts of D. geminata, but the massive production of extracellular stalk. Extracellular polymeric substances (EPS) that make up the stalk are predominantly composed of polysaccarides with small amounts of protein (Gretz et al., 2006). They are complex, multilayered structures that are resistant to degradation. 

Colony growth can be divided into 4 stages:

1) Initial colony growth – As the diatom cells attach to a rocky (or other) substrate and begin to produce stalks, the colonies appear as small, circular clumps that range from 2 to 10 mm. The growths are visible to the naked eye, and are light tan to brown in colour. The clumps are soft and feel like a cottonwool ball when they are pulled apart. They are not slimy or slippery. This stage is common during the late winter or following periods of high flow, but can occur at any time of year, depending on the region.

2) Colony expansion – As the colonies grow, they merge together and may cover the substrate completely. Colonies attached to plant stems form rope-like strands. As several small colonies coalesce and stalk production increases, the clumps become thicker. The colonies are frequently 1-2 cm in thickness, but may be greater. The coverage on the streambed may be patchy, or the substrate across a transect may be completely covered.

3) Stalk proliferation - Under periods of low flow or with favorable growth conditions, the colonies have the opportunity to form blooms. The cells produce excessive amounts of stalk many times the length of the microscopic cells. As the stalks lengthen, they form ropy strands and are white in colour. Some reports describe the strands as being like tissue, fiberglass, toilet paper, or sheepskins.

4) Colony senescence – Although the cells may die, the stalks persist on the streambed substrate or stranded above the stream wetted zone. The stalks may persist for 2 months, or more. Stalks may change in colour as they trap fine sediment and dry. The microscopic cells may no longer be living, or present, in the aging stalk masses.

Climatic and physico-chemical variables for the range of occurrence in the US and southern Chile were compared in a study by Montecino et al.  (2014). In Chile, presence is favoured by cool air temperatures, low seasonal precipitation, low flow rate and low phosphate levels (Montecino et al., 2016). Throughout its range, low phosphate levels are associated with D. geminata blooms (James et al., 2015). 

General

• Botanical garden/zoo

Genetic fingerprinting tools for detecting the presence of D. geminata in watersheds was developed and implemented in New Zealand (Cary et al., 2007) and in Chile (Jaramilo et al., 2015). The technique includes field procedures for efficient collection of microscopic cells and identification of a species–specific DNA sequence that allows the differentiation of D. geminata from other species of diatoms

In the field, D. geminata can be easily confused with other stalk-forming diatom species. In particular, the visual appearance of D. geminata is very similar to its closest relatives, Cymbella mexicana and C.mexicana var. janischii. However, D. geminata is distinctive to the touch. When D. geminata stalks are pulled apart, there is some resistance and the stalks feel like wet cottonwool balls. Other diatom species have no such resistance, and they are slimey and slippery when the stalks are pulled apart.

At a minimum, a compound microscope with a magnification of at least 400x is needed to discern D. geminata. Note that confirmation of the identification requires higher magnification and consultation with a taxonomic key.

General cell morphology:

Cell length 80 -140 µm

Cell width, 25-43 µm

Striae number 8-10 in 10 µm

The shape of the cell resembles an ‘old-fashioned coke’ bottle. An apical porefield is present at the footpole of the valve. Cymbella mexicana and C. mexicana var. janishii are similar in size, but these species have a strongly arched dorsal margin. The shape resembles a crescent moon.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention 

Biosecurity New Zealand reviewed options for containment of D. geminata early in its discovery on the South Island. Increasing the level of public awareness was determined to be the most important action based on the following rationale: 1) Eradication or control of a microscopic alga is not likely in the short term, 2) Cells of D. geminata were likely to be present beyond the known impacted waterways, 3) Restricting user access to rivers would not address the risk of spread by birds and other animals, 4) Appropriate cleaning methods for river equipment have been developed, 5) Public awareness would be more like to gain widespread compliance than regulations and closures.

In Patagonia, South America, the prioritization of defensible conservation zones is recommended by Reid et al. (2012).

Control

Sanitary measures

Water users should scrub, disinfect and dry their equipment to prevent the transfer of diatom cells.

Physical/mechanical control

Flushing flows like those used for channel maintenance have been shown to reduce D. geminata benthic mats in the Rocky Mountains, Canada (Cullis et al., 2015).

Chemical control

Investigation of ten potential control agents (algaecides/biocides) was initiated through experimental trials (Jellyman et al., 2006). The control agents were rated based on their effectiveness on causing cell mortality and degradation of biomass. In addition, ecosystem impacts and risks, feasibility of application, cost, and duration for effective control were considered. A second phase of testing was based on the results from the first study. Trials in artificial channels showed that a chelated copper compound was the most effective in killing D. geminata cells and minimizing effects on non-target species (Clearwater et al., 2007a). Finally, a control and eradication experiment was carried out in an impacted stream (Clearwater et al., 2007b). The results indicated that treatment could be effective in reducing, but not eradicating, the population of D. geminata. A number of spring-fed creeks that are tributaries of D. geminata affected rivers were found to lack populations of D. geminata, despite their exposure to colonization (Sutherland et al., 2007). This study, to compare the survival of D. geminata on artificial substrates in rain-fed rivers vs. spring-fed creeks found poor survival of cells in spring-fed creeks. Although the factors, or combination of factors, influencing survival was not determined, this line of inquiry should be followed in future work.

As low phosphate levels are associated with bloom formation, enrichment with slow- and quick-release fertilizers was tested in a river in South Dakota, USA, and reduced D. geminata biomass locally (James et al., 2015). 

Monitoring and Surveillance

Genetic fingerprinting tools for detecting the presence and spread of D. geminata in watersheds have been used in New Zealand (Cary et al., 2007) and Chile (Jaramillo et al., 2015).

Large growths of D.geminata are of concern in stream ecosystems and there remain research needs to resolve basic and applied management questions.

1) There is a repeating pattern of the appearance of nuisance blooms in North America, beginning with Vancouver Island, British Columbia in 1989. With each successive report in Canada (Alberta, Quebec) and the USA (Colorado, Montana, Missouri, Arkansas, New Hampshire, Pennsylvania) managers are faced with the same problem of little information on the biological and environmental factors that trigger the formation of blooms. The temporal and spatial extent of mats could be mapped in association with use by human vectors. Such data could address the complex interaction of hydrologic, chemical, climatic and human roles in bloom development.

2) Preliminary work on population genetics suggests that New Zealand organisms arrived from western North America, but it is not known if a ‘nuisance form’ of D. geminata represents a new or unique strain. Molecular markers for populations of D. geminata need to be further developed to a) determine the relatedness of populations across the globe and b) establish if there is a unique genetic form responsible for invasive behavior. An effort to gather samples at recent international meetings (ASLO February 2007, SIL August 2007) was initiated by Biosecurity New Zealand and requires further support.

3) Resolution of the historical abundance of D. geminata is possible through existing archived samples and lake sediment records. Diatom samples that have been collected, processed, and archived may be available for D. geminata specific analysis. That is, slides can be examined for D. geminata abundance. In most studies, the abundance of diatoms is based on total numbers of diatom cells and is heavily biased toward the small cells that dominate most counts. Such an approach is straightforward and low in cost to accomplish. A second method to address historical abundance is to identify rivers that experience nuisance blooms and empty into lakes or reservoirs, depositing the siliceous cells into lacustrine sediments. In these sites, the surface sediments and abundance of cells can be correlated to reconstruct the historical abundance of the river drainage.

4) The abundance of D. geminata is related to trophic level impacts, that is, with greater abundance there are greater changes in algal and invertebrate communities. Several studies have documented the changes in algal and invertebrate communities, yet the quantitative and long-term implications are not known. In particular, both algal and invertebrate communities are low in diversity with high D. geminata abundance compared to those without D. geminata.

5) Despite the importance and impact of this organism, many aspects of its basic biology and life history is not known. Although sexual reproduction has been documented in the literature, it has not been observed in recent blooms. Vegetative cell division within the diatoms results in successively smaller generations of cells and a crucial part of the diatom life cycle is the restoration of maximum cell size through formation of auxospores. Investigation of the cell size distribution within a population over time would provide fundamental knowledge about the species, even if sexual stages prove difficult to observe.

14/02/08 Original text by:

Sarah Spaulding, University of Colorado, Institute of Arctic and Alpine Research, Campus Box 450560 30th Street, Boulder CO 80309, USA