Algae Diversity & Dispersal: Implications For Water Catchment Management And Conservation
Published on 29 March 2010 in Climate, water and energy , Ecosystems and biodiversity
Introduction
It is taken for granted that many species of birds, mammals and flowering plants occur only in particular parts of the world, sometimes only in a particular mountain range or archipelago. In some cases this is because the species are very fussy in their ecological requirements, needing special conditions that are found nowhere else, but more often the geographical range is determined not only by ecology but also by the capacity of animals and plants to disperse to and colonise new places. For example, the mammals of Australia have evolved largely in isolation from those of other continents, even nearby SE Asia, because the surrounding seas have prevented immigration and emigration. The importance of dispersal as a constraint is also demonstrated well when the constraint is removed by human intervention, for example during invasions by ‘alien’ species such as the grey squirrel, giant hogweed or ruddy duck. Once the original geographical barrier has been overcome through accidental or deliberate transfers, alien species often spread and increase rapidly, being preadapted to the ecological conditions in their new homes and having lost many or all of their natural enemies, such as predators and parasites. Active conservation measures are therefore necessary for larger animals and plants, both to manage and maintain species in their natural environments and to prevent species from becoming established outside their natural ranges.
Do we need to take the same care with microscopic organisms, such as the planktonic and bottom-living microscopic algae (microalgae) that comprise the base of most food chains in the world’s oceans, lakes and rivers? Is dispersal a constraint on the distribution of microscopic organisms, or do they quickly spread to every corner of the globe? Should we worry about the conservation of these organisms or will they look after themselves? Can we use the same floras and guides to identify algae or protozoa in both Australia and Europe? Can we apply similar protocols for monitoring water quality in Scotland as in, say, Germany or New Zealand?
One popular hypothesis, argued persuasively in recent years, is that most microscopic organisms are dispersed very effectively, so effectively in fact that their distributions mirror their ecologies almost exactly. This is the ‘ubiquitous dispersal hypothesis’ (UDH), which is not new (versions of it have been circulated for a century or so) but which has been revived and popularised in recent years by Prof. Bland Finlay and his co-workers. The UDH suggests that, even if there is only a very small chance that a single microscopic cell or colony will become dispersed to a new habitat, spread of the species will nevertheless be very effective and rapid because populations of microscopic organisms are unimaginably large.
For example, populations of a rare planktonic microalga, present at a concentration of only 1 cell per litre of the surface waters where it lives, would greatly exceed 10 billion (1010) in a lake like Windermere or Loch Lomond. So, even if the chance of a single cell being transported to a nearby lake, e.g. by the wind or waterfowl, is only 1 in a million per year, the species will quickly expand its range. Accordingly, the UDH predicts that, if the same ecological conditions occur in, say, North America and Europe, then the same species of microalgae and protozoa will be found in both areas. And if a new habitat is created, then it will quickly (within perhaps a few years or a few tens of years) become colonized by all the microscopic species that can potentially live there.
If the UDH is true, microscopic organisms will be the perfect choice for monitoring climate change and the quality of the environment (such as for the EU’s Water Framework Directive), because they will faithfully and quickly track any changes in ecological parameters. Not only that, the same identification aids will be usable worldwide, so that an algal flora written for temperate parts Europe will also be useful in climatically equivalent regions in the Americas, Africa, Asia or Australia. Furthermore, the conservation of a diversity of habitats will automatically ensure conservation of microbial diversity. And it will be impractical to prevent the spread of microscopic algae or protozoa, if suitable environments become available outside the current geographical range.
However, doubts have been raised about the UDH. For example, statistical analyses of the distributions of microalgae suggest that dispersal is not as effective as the UDH requires. And there is circumstantial evidence from the recent spread of shellfish poisonings of humans that the microalgae that produce the shellfish toxins were originally not ubiquitous but have been spread via the ballast water of large ocean-going cargo ships.
Key Points
- The taxonomy and identification of microscopic algae and protozoa have been based on morphology (e.g. the shape and size of cells, and patterns of internal and external detail) but it is unclear if morphology gives an adequate picture of diversity in microscopic organisms.
- Almost all existing data concerning the distributions of microscopic algae and protozoa are based on morphology-based identifications and are therefore of questionable value.
- The less effectively dispersed microscopic organisms are, the less justification there will be for using the same monitoring protocols across wide geographical ranges and the more important it will be for each country to develop its own, dedicated taxonomic products
- If microscopic algae and protozoa are not dispersed rapidly and effectively (as required for the ‘ubiquitous dispersal hypothesis’), then they too, like mammals and flowering plants, require special conservation measures if human alteration of natural distributions is to be avoided.
Research Undertaken
(1) We focused on diatoms, which contain more species than any other group of microscopic algae and perform c. 20% of global photosynthesis. The first requirement was to have rigorous methods of identification. Diatoms have traditionally been identified on the basis of morphology, particularly the shape and pattern of the cell wall, but we considered this to be inadequate and explored the use of molecular genetic methods instead.
(2) We developed ‘DNA barcode methods’ for identifying diatom species and tested them on a ‘model’ system, the freshwater diatom Sellaphora. We showed that previous classifications greatly underestimate the diversity of diatoms. Morphology is a crude and imprecise basis for identifying species, which undermines most previous data on species’ distributions and therefore also the basis of the UDH. Although we showed that some species are indeed widely distributed, as required by the UDH, we cannot be sure that these wide distributions have not been created artificially, through accidental transport of cells during introductions of fish stocks or aquatic plants. Other species seem to be more restricted in their distributions, for example, occurring in Australia but apparently not in the UK.
(left to right) Sellaphora capitata from Lake Purrumbete, Victoria, Australia; Sellaphora capitata from Blackford Pond, Edinburgh, Scotland; and a quite separate, recently discovered Sellaphora species from the Hacking River, New South Wales, Australia.
(3) We next developed sensitive genetic techniques (microsatellites) to test whether populations of the same freshwater species from different regions and continents are similar (suggesting that dispersal is highly effective) or not. We found that the populations were very different, but that there was no simple relationship between geographical separation and genetic difference.
(4) Summarising: it cannot be assumed that diatom species are ubiquitously dispersed. Dispersal does seem to be a significant constraint on distributions and most diatom species probably evolve when populations become separated and diverge from each other through natural selection and genetic drift, just as in ‘higher’ organisms such as vertebrates and flowering plants.
Policy Implications
The diversity of microscopic algae and protozoa is much more poorly known than was previously evident. We are not close to a complete catalogue of microalgal diversity in Scotland or Europe, despite relatively long and intensive research.
Molecular methods (e.g. DNA barcoding), coupled with more traditional morphological examination, give a much more secure basis for microalgal taxonomy than has ever been possible hitherto, and imaginative use of the Internet provides novel means for disseminating relevant information to users of taxonomy, e.g. those using diatoms to monitor water quality or climate change. Scottish and UK institutions are well placed to take the lead in such work.
It is dangerous, given current knowledge, to use taxonomies and ecological characterisations of species that were developed for different regions or continents. Monitoring systems should be developed locally, or adopted from elsewhere only with great caution.
Transfers of water or microalgal communities between lakes or rivers, or between marine environments (e.g. with fish stocks, on or in ships), should be minimised.
Assessments of biodiversity for conservation purposes should increasingly take into account microalgae and protozoa, rather than assuming that ‘everything is everywhere’.
Author
Professor David G. Mann, Senior Principal Research Scientist, Royal Botanic Garden Edinburgh d.mann@rbge.org.uk
Topics
Climate, water and energy , Ecosystems and biodiversity
