Cyanobacteria: Good week, or bad week..? Part II

Continuing our look at the newsworthy world of the blue-greens.

Image: NASA Earth Observatory/ Wikipedia

DOM, a double-edged sword …

From a bad news point of view – bad for those ‘BBGs’ (bad blue-greens [URL for Part I]), but good for the rest of us – is work by Amanda Neilen et al. (2019) that looks at the effect of DOM (the acronym for dissolved organic matter as used in their article)* on cyanobacteria. In most scenarios DOM is a good thing adding nutrients back to the soil or water, after their temporary residence within the bodies of living things, and therefore has a life-promoting benefit for other organisms that can access the compounds released from its decomposition (Robert Wetzel and Gene Likens, 1991). But, in some situations that same DOM may have a life-limiting capacity, which is what Neilen et al. examined.

Using laboratory experiments they showed that DOM extracts from leaves of soil-rooted Casuarina cunninghamiana and Eucalyptus tereticornis were able to inhibit photosynthesis of the blue-green Raphidiopsis raciborskii (a freshwater, bloom-forming blue-green that produces the toxins cylindrospermopsins (CYNs) and saxitoxins (STXs) (Michele Burford et al., 2016)) when grown alone as a single species in culture. Importantly, sunlight-exposed leaf matter was more phytotoxic than non-sunlight-exposed material, implicating reactive oxygen species (ROS), at least partially, for DOM-caused cyanophytotoxicity. This work builds upon previous studies by several members of the same research team (Amanda Neilen et al., 2017) that showed that plant-derived DOM was more toxic against a blue-green alga (Raphidiopsis raciborskii but then named Cylindrospermopsis raciborskii) than against a eukaryotic green alga (Mono raph id ium sp.) – under laboratory conditions.

As with all such work many questions are raised. For example, does this finding apply in natural environments outside of the lab., i.e. ‘in the real world’? Can this knowledge be used as a control measure to eliminate HAB-forming cyanobacteria from environments where they would interfere with human uses of such water bodies? What effects does DOM from other plants have on the ecology of water bodies? Are the compounds released from DOM extract-killed blue-greens into the water body as bad as those contained and retained within the bodies of bloom-forming individuals? What effects might there be of DOM-leached compounds on other organisms in the water body that are as yet untested? In the study leaves were removed from the trees – i.e. they were “green non-senescent leaves” – and investigated; would the same results be found if the leaves were allowed to undergo ab scission and fall naturally – and then have their decomposition products tested for cyanobacterial toxicity? What might happen in nature where the blue-green grows in a mixed culture with other species of algae, etc. – might any toxicity towards the blue-green be reduced or ameliorated or increased by the presence and physiology of other organisms? Important questions to be considered before the work has any practical cyanobacterial-management relevance. But, and nevertheless, this work is an important reminder of the ecological connection between land and ocean; what happens on terrestrial habitats can impact what takes place in aquatic environments.

More good/bad/indifferent blue-green news follows in Part III [Botany One URL to add…]

* DOM is also the initialism for dead organic matter.