Stimulation of nitrogen-fixing cyanobacteria in a Baltic Sea plankton community by land-derived organic matter or iron addition

Abstract

In the Baltic Sea, floating blooms of nitrogen-fixing cyanobacteria occur yearly during late summer. These blooms can sometimes be limited by iron. Due to extensive foresting around the Baltic Sea, iron is entering the Baltic Sea partly bound to dissolved organic material (DOM) via rivers. An experiment was performed in 300 l laboratory mesocosms to test the hypothesis that riverine highmolecular weight dissolved organic matter (HMWDOM), extracted by tangential flow filtration >1000 Da, stimulates the biomass of nitrogen-fixing cyanobacteria, by increasing the availability of iron. The addition of iron/EDTA and of DOM resulted in 5 to 10 times higher biomass of nitrogenfixing cyanobacteria. Accordingly, higher primary production and particulate nitrogen concentration at the end of the experiment were observed in those treatments compared to the control. The removal of mesozooplankton grazers did not have a significant effect on the microphytoplankton biomass and species composition. Nodularia spumigena biomass was highest in the treatments receiving DOM, but addition of iron alone had no significant effect on this. N. spumigena was less positively affected by iron addition than Anabaena cf. inaequalis, suggesting that N. spumigena is a better competitor for iron. Separate microcosms comparing additions of iron, manganese and cobalt showed that iron was limiting for cyanobacterial biomass development. The results strongly suggest that iron bound to DOM can contribute to the iron demands of nitrogen-fixing cyanobacteria in the Baltic Sea.

Dissolved iron (II) in the Baltic Sea surface water and implications for cyanobacterial bloom development

“…has been described to produce the siderophore schizokinen in response to iron stress (Clarke et al, 1987) as well as the cyanotoxin microcystein that also has siderophore type characteristics (Kaebernick and Neilan, 2006;Utkilen and Gjolme, 1995;Humble et al, 1997). Further, Stolte et al (2006) observed that cyanotoxin production by Nodularia spumigena increases during incubations with combined additions of DOM and Fe. The toxin production of Nodularia spumigena could be a response to increase metal bioavailability in media with strong DOM-metal complexation, where the dissolved iron concentrations are high, but the actual concentration of the bioavailable inorganic iron species (Fe') is low, which also is the case here (Fig.…”

“…remains unclear, but given the shift in the conditional stability constants paralleling the cyanobacterial bloom development during the summer it seems plausible that at least a fraction can be attributed to cyanobacterial toxin and thus ligand production, next to bloom byproducts (TEP, surfactants). Stolte et al (2006) conclude that iron concentrations in the Baltic Sea are sufficient for cyanobacterial growth, but that species composition could be affected by species specific iron acquisition strategies. Our observations of abundance and successive growth of Aphanizomenon sp., Nodularia spumigena, and Anabaena sp.…”

“…Alternatively, phosphorus release from up-welled deep waters may stimulate cyanobacteria growth (Stal et al, 2003;Kononen et al, 1996). Iron as a potentially limiting nutrient for cyanobacterial bloom development and nitrogen fixation in the Baltic Sea has been suggested (Stal et al, 1999;Stolte et al, 2006). N 2 fixing cyanobacteria have a 2.5-11 fold higher iron demand than other phytoplankton (Kustka et al, 2003a(Kustka et al, , 2003b(Kustka et al, , 2002Sanudo-Wilhelmy et al, 2001) and thus cyanobacterial bloom development may to some extent be controlled by iron bioavailability.…”

Environmental controls on the nitrogen-fixing cyanobacterium Nodularia spumigena in a temperate lagoon system in SE Australia

“…N 2 -fixing cyanobacteria require high concentrations of Fe and Mo for nitrogenase synthesis, and it has been hypothesised that these elements may limit cyanobacterial growth, particularly in estuaries (Howarth et al 1988). Dissolved organic matter is another factor that has been shown to stimulate the growth of N 2 -fixing cyanobacteria, including Nodularia (Howarth et al 1988, Paerl 1990, Stolte et al 2006. Grazing may therefore stimulate the growth of Nodularia by liberating dissolved organic matter (Moller 2005) and/or metal co-factors from the phytoplankton that they consume.…”

Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone

“…Alternatively, up-welled phosphorus from deep waters may stimulate growth of cyanobacteria (Kononen et al, 1996;Stal et al, 2003). Iron as a potentially limiting nutrient for cyanobacterial bloom development and nitrogen fixation in the Baltic Sea has been suggested (Stal et al, 1999;Stolte et al, 2006). Nitrogen-fixing cyanobacteria have a Fe demand that is 4-6 times higher than other phytoplankton (Kustka et al, 2002;Sanudo-Wilhelmy et al, 2001) and the development of cyanobacterial blooms may to some degree be regulated by iron bioavailability, as previously suggested for phytoplankton in other coastal regions (Özturk et al, 2002;Hutchins and Bruland, 1998).…”