Microcystin production by Nostoc in Greenlandic lakes

Benthic primary producers are recognized for their important role in
contributing to ecosystem productivity and nutrient cycling in lake and
stream ecosystems, particularly in polar environments. In Arctic lakes,
benthic producers often comprise mats or colonies of cyanobacteria capable
of producing cyanotoxins. However, the extent to which benthic communities
contribute cyanotoxins in polar regions remains poorly described. We
evaluated the potential for benthic colonies of the cyanobacterium Nostoc
pruniforme from lakes in Kangerlussuaq, Greenland, to contribute
microcystins (MCs) to lake water using three approaches. First, we
dissected field-collected Nostoc colonies and measured MCs within multiple
layers of fresh colony tissue. Second, we conducted a laboratory
experiment to evaluate the temporal dynamics of MC release by incubated,
intact colonies. Finally, we quantified whether MC concentrations in water
and sediment samples in the field were higher in and above dense bands of
benthic Nostoc as compared to bare sediment. Field-collected Nostoc
colonies contained MCs throughout the colony tissue, suggesting that
damage to colonies from grazers or physical disturbance could facilitate
the release of toxins into the water. Undamaged Nostoc colonies incubated
in high-nutrient conditions in the laboratory leaked MCs into the
surrounding water at a steady mass-specific rate over the course of seven
days. MC concentrations in water and sediment from two Greenlandic lakes
were highly variable, but slightly higher in lake water immediately above
dense bands of Nostoc than in water immediately above bare sediments,
suggesting that benthic Nostoc colonies contribute cyanotoxins to lake
water and that MCs vary at very fine, 1-2 m spatial scales. Benthic
cyanobacteria may be important in releasing MCs into aquatic ecosystems,
especially in systems where benthic producers dominate, such as polar
environments.