Has ‘rock snot’ been here all along? – Experts respond

The freshwater algae known as "rock snot" or "didymo." Credit: Brad Taylor
The freshwater algae known as “rock snot” or “didymo.” Credit: Carole-Anne Gillis

Controversial new research claims that noxious blooms of the fresh water algae didymo –aka ‘rock snot’ — are caused by changes in native algal species and suggest that current efforts to prevent the spread of the organism may be misguided.

In a new forum article published in BioScience, two North American researchers conclude that the growing problem of didymo blooms worldwide is caused by native species responding to changing environmental conditions rather than by accidental introductions of invasive species by fishermen or the emergence of a new genetic strain.

They state: “Nuisance blooms have been reported in rivers worldwide and have been hastily attributed to introductions. However, evidence indicates that blooms are probably not caused by introductions but, rather, by environmental conditions that promote excessive stalk production by this historically rare species.”

The authors cite the historical presence of didymo cells in American and European waterways, as shown in fossil records and sediment cores, as evidence that didymo has been hiding in lakes and rivers for decades and possibly longer – although has not bloomed.

They suggest rock snot blooms have become more common because of climate change and other human-caused environmental changes that are decreasing phosphorus to levels that promote the formation of didymo blooms in many remote, otherwise pristine rivers worldwide.

The Science Media Centre rounded up the following New Zealand expert comments on the research.

Dr Cathy Kilroy, Freshwater Ecologist, NIWA, comments:

“A response to environmental change is a highly likely explanation for the expansion of Didymosphenia geminata (didymo) blooms in the Northern Hemisphere. Our research in New Zealand demonstrated that that didymo blooms are caused by low dissolved phosphorus concentrations. The Taylor and Bothwell paper refers to large-scale processes that can (and do) lead to declining phosphorus in North American streams.

“However, there is no evidence that those processes apply to New Zealand streams. There has been no sign of a general decline in phosphorus in rivers, even in undeveloped catchments. The South Island rivers currently affected by didymo had very low phosphorus long before the discovery of the first didymo blooms in 2004. The subsequent rapid spread of didymo in the South Island indicated transport by humans.

“I believe that didymo was introduced to New Zealand. My view is that the expansion of blooms in the Northern Hemisphere starting in the late 1980s meant that didymo became much more common; ever increasing international travel meant that it was inevitable that cells would eventually find their way to New Zealand. Once the cells were here, the many low-phosphorus waterways in South Island provided perfect habitat for blooms.

“I agree with Taylor and Bothwell that high phosphorus explains why no didymo blooms have occurred in any North Island river. Failure to detect even a single didymo cell – despite continued surveillance – is more puzzling. It seems likely that a combination of environmental factors, such as temperature and water chemistry, prevent the long-term survival of didymo in North Island waters.”

Dr Marc Shallenberg, Research Fellow, Zoology, University of Otago, comments:

“In 2004, an alga previously unreported in New Zealand first bloomed to nuisance proportions in the Lower Waiau and Mararoa Rivers in Southland. It then rapidly spread and colonised many other rivers of the South Island. In some of these rivers it also bloomed to greater or lesser extents. In the mid-2000s, blooms of the same organisms, Didymosphenia geminata (didymo) were also reported in some rivers in the Northern Hemisphere, where it is part of the native diatom flora. Since didymo hadn’t been previously reported from New Zealand waters, it was assumed that didymo invaded New Zealand and rapidly proliferated, as many other invasive exotic organisms have done.

“In this article, the authors present evidence from various locations around the world that didymo blooms don’t only occur as a result of recent invasions, but that blooms also occur in regions where it has been present for a long time. Unfortunately, the article doesn’t shed light on whether didymo had been in New Zealand prior to 2004 and bloomed because environmental conditions suddenly became more suitable, as the authors argue was the case in the North American and European locations where didymo bloomed. To make this case convincingly in the New Zealand context, sediment core studies would need to be undertaken, examining diatom fossils in lakes sediments and demonstrating the presence of didymo in well-dated, historical sediment strata. The authors review such studies from the Northern Hemisphere, which have identified didymo cells in old lake sediments. While we have undertaken a number of detailed sediment core studies in New Zealand, didymo has never been reported from our lake sediments. However, as the authors point out, the absence of didymo from the cores examined to date is not necessarily unassailable evidence of the historical absence of didymo from New Zealand waters.

“One of the authors, Max Bothwell, has done a lot of work on didymo in New Zealand and is familiar with the studies that have been carried out here. The article cites much of the excellent work that has been carried out in New Zealand on the didymo problem and it is good to see this work at the forefront of scientific thinking about the distribution and dynamics of didymo.

“The take home message of the article seems to be that environmental conditions (and possibly recent changes in these conditions) play an important role in blooms of didymo and in the proliferation of species. This is a sensible message and won’t ruffle feathers with scientists. However, the authors have written their paper in a way that appears to suggest that didymo blooms only occur in response to changes to environmental conditions and not at all to recent invasions.

“While their attempt to downplay the importance of invasion biology is provocative, it is not convincing. Unfortunately, the all-or-nothing style of argument employed by the authors of this article provokes more than provides comprehensive answers to the complex problem of the widespread proliferation of didymo and other nuisance species. ”

Prof Brendan Hicks, Centre for Biodiversity & Ecology Research, University of Waikato, comments:

“Didymo in NZ is widely accepted to be the result of a new introduction rather than climate change causing blooms for an organism that has been here for many year. Evidence from the pattern of spread, now well documented, supports introduction to a Southland river and then spread to adjacent rivers in the first few years followed by spread to wider areas of the South Island over the past 6 years. The situation in Chile appears to be similar to NZ. This is different to the situation in the US, Canada, and Europe where didymo has been for many years, and areas where it is a native. There changing climate might have had an influence on the emergence of bloom-level biomass increases.”

Dr Phil Novis, Phycologist, Landcare Research, comments:

“The authors raise many sensible points, such as their commentary on response spending on Didymo around the world, its historical presence and bloom formation in several continents, the ecology of its bloom formation, the limitations of current molecular evidence, and the potentially unacceptable nature of post-hoc control measures.

“However, their assertion that Didymo is probably not a recent introduction to New Zealand is problematic.  In my view, this is vulnerable to two potential errors: treating the dispersal of Didymo like that of any other microbe, and treating New Zealand, with its isolation and human history, as a continental country.

“It is true that microbial biogeography – the study of distribution patterns of microbial species – is a minefield, because it is impossible to prove conclusively that a microscopic species is not present in a given country.  However, Didymo is unusual.  It is a very large, easily recognised diatom, and paradoxically (given its nominally invasive status) it is known to be very sensitive to drying.  This sensitivity precludes the long-distance dispersal that would have been required to transfer the species to New Zealand in pre-human times.

“The authors concede that Didymo has been known for some time in areas where it does not bloom.  If it was not overlooked in those areas, why so in New Zealand, when widespread and extensive sampling of river periphyton has taken place here for decades?

“In response to their commentary on the apparent length of time for Didymo to arrive in New Zealand, I suggest that it was (again paradoxically) a reasonably difficult thing to achieve, thanks to its drying sensitivity: an exceptional event.  Once here, it was able to rapidly transfer between adjacent catchments, probably using a variety of vectors.  Finally, their argument relies on South Island rivers having only recently attained the low levels of phosphorous needed to support Didymo blooms.  Others would be more qualified to comment on that, but I think it’s unlikely.”

Prof Craig Cary, Professor of Environmental Biotechnology, University of Waikato, comments:

“There is no indication that what they are thinking happened in Canada is the same here in New Zealand.  To my knowledge there is no historical record of any type that suggests didymo was in New Zealand prior to 2004. In fact the authors of the article do not include New Zealand in their analysis of any fossil records.

“Of course, the fossil record is not a genetic record. The fossil record only tells you that some genotype of didymo was present, it doesn’t tell you anything about whether the current organisms have been introduced or not. A “blooming” genotype could have been introduced recently and is the primary genotype currently causing the problem. Organisms change. All it would take is a very small genetic change in an organism, maybe something around a growth regulation or even nutrient uptake, and an organism can move from being a benign background organism to something that is pervasive and invasive – a bloomer.  We just don’t know enough about didymo genetics.

“What I worry about that policy makers could see this as being a normal situation, in that we should know whether in organism is present in the environment before we start protecting our borders against a possible incursion.

“I think we must very very careful about that. In my opinion, here in New Zealand it was amazing how fast MAF biosecurity responded to didymo. I think that they certainly appear to have slowed down didymo’s movement up to the North Island. And have stopped it – as far as we don’t have it here in the North Island. There are many reasons why that might be the case but for sure the didymo campaign has been very successful and I know is currently being modelled in other countries around the world.

“Through that response we have learned a lot about didymo. Through the policies that were put in place for didymo we have also strengthened our borders and surveillance capabilities against possible invasions of other organisms. I’d rather we err much more on the side of caution and put policies in place that are stronger than is necessary than to have something really harmful come into the system.

“There is nothing wrong with the paper, it is just that it can’t delve into the genetics, as the genetics are completely unknown. The weight of their observations in the fossil record are a strong as the possibility of a different genotype coming into the system and being a bloomer – spreading just like it has been suggested. I think that there is equal weight to that – there is no indication that’s not the case.”

Dr Susie Wood, Senior Scientist – Microalgae and Algal Biotechnology, Cawthron Institute, comments:

“This is a compelling article and I agree with the proposition that not all problematic species are necessarily new introductions. We recently demonstrated this for a toxic bloom forming cyanobacteria (blue-green algae) in the Waikato lakes. Using molecular techniques we were able to show that recent blooms of this species were most likely due to changes in water quality, and that this species had been in New Zealand for a long time.”

“Research by multiple agencies across New Zealand suggests that this is not the case with didymo in New Zealand. As the authors state – there is no fossil or confirmed historical records of didymo in New Zealand. Whilst the recent blooms in the Northern Hemisphere might be a sign of environmental change rather than a new introduction, I still believe that the evidence from Zealand, including the way in which it spread across South Island rivers,  suggests that it was a recent introduction. Attempts and on-going efforts to prevent its spread, particularly to the North Island,  are warranted.”

“The research undertaken by author Max Bothwell (author of this article) and New Zealand researcher Dr Cathy Kilroy (NIWA) that have unravelled the story of bloom formation and the links to low phosphorus are ground-breaking and have explained the paradox of why didymo blooms in some of our most pristine rivers.  Despite the suggestion in this article, I am not convinced that the only reason that didymo is not in the North Island is that the levels of phosphorus are generally higher in these rivers. In my opinion this is too simplistic. Our recent laboratory-based research on didymo suggests that levels of other compounds – including nitrogen, magnesium and calcium are really important in determining whether a cell attaches to a surface. If a cell does not attach, it will not produce the long stalk that accounts the bulk of the biomass in a bloom.

“There is still a lot to learn about how didymo functions, for example, how water chemistry and other micro-organisms effect its attachment, growth and reproduction. New Zealanders have led the way internationally in didymo research and I hope this continues. Reducing and/or preventing the impact of didymo and other introduced freshwater pests should remain a top priority for New Zealand.”