Response of the nitrogen‐fixing lichen Lobaria pulmonaria to phosphorus, molybdenum, and vanadium

Abstract

Abstract. Nitrogen-fixing lichens (cyanolichens) are an important source of nitrogen (N) in Pacific Northwest forests, but limitation of lichen growth by elements essential for N fixation is poorly understood. To investigate how nutrient limitation may affect cyanolichen growth rates, we fertilized a tripartite cyanobacterial lichen (Lobaria pulmonaria) and a green algal non-nitrogen fixing lichen (Usnea longissima) with the micronutrients molybdenum (Mo) and vanadium (V), both known cofactors for enzymes involved in N fixation, and the macronutrient phosphorus (P). We then grew treated lichens in the field for one year in western Oregon, USA. Lichen growth was very rapid for both species and did not differ across treatments, despite a previous demonstration of P-limitation in L. pulmonaria at a nearby location. To reconcile these disparate findings, we analyzed P, Mo, and V concentrations, natural abundance d 15 N isotopes, %N and change in thallus N in Lobaria pulmonaria from both growth experiments. Nitrogen levels in deposition and in lichens could not explain the large difference in growth or P limitation observed between the two studies. Instead, we provide evidence that local differences in P availability may have caused site-specific responses of Lobaria to P fertilization. In the previous experiment, Lobaria had low background levels of P, and treatment with P more than doubled growth. In contrast, Lobaria from the current experiment had much higher background P concentrations, similar to P-treated lichens in the previous experiment, consistent with the idea that ambient variation in P availability influences the degree of P limitation in cyanolichens. We conclude that insufficient P, Mo, and V did not limit the growth of either cyanolichens or chlorolichens at the site of the current experiment. Our findings point to the need to understand landscape-scale variation in P availability to cyanolichens, and its effect on spatial patterns of cyanolichen nutrient limitation and N fixation.

Biological nitrogen fixation by alternative nitrogenases in terrestrial ecosystems: a review

“…cyanolichens in Oregon, US (Horstmann et al 1982). However, a more recent study on Lobaria and Usnea cyanolichens from the same region (Oregon) reported no effect of Mo addition on BNF (Marks et al 2015b). High spatiotemporal variability in Mo limitation, which has been reported in leaf litter and cryptogams (Jean et al 2013;Rousk et al 2017;Wurzburger et al 2012), may explain some of the discrepancies between these two studies.…”

“…Unfortunately, trace metals are not routinely included in soil and litter analyses, which often focus on C and N. When trace metals are reported, Mo and especially V are rarely considered, except in studies on contaminated soils (Johnson and Hale 2004;Lawrey 1978). In addition, datasets on trace metal concentrations in terrestrial samples must be examined carefully, as sample preparation for elemental analysis must avoid metal-based equipment, such as steel Wiley mills and roller grinders, which can be a source of metal contamination (Marks et al 2015b). Mo is particularly sensitive to contamination during sample preparation due to its low environmental concentrations and its presence is numerous metal alloys.…”

Biological nitrogen fixation by alternative nitrogenases in boreal cyanolichens: importance of molybdenum availability and implications for current biological nitrogen fixation estimates

“…particles, canopy flow‐through) are the main sources of metals (e.g. V and Mo) to lichens (Darnajoux et al ., ; Marks et al ., ), we postulated that, along a vegetative succession, the amount of Mo and V, as well as the relative Mo : V ratio, might vary significantly as a result of different canopy openness and vegetation composition. A fire chronosequence might thus provide samples growing under similar climatic and geological conditions, but with contrasted Mo and V exposures.…”

How to optimize lichen relative growth rates in growth cabinets

“…In earlier laboratory experiments, relative growth rate was measured in discs cut from lichens (Scott 1956, 1960; Pearson 1970), while others dealt with whole lichen thalli (Kershaw & Millbank 1969; Pearson & Benson 1977). In recent years, growth experiments have increasingly been used to investigate the effects of environmental impacts on entire lichens in field (Hyvärinen & Crittenden 1998; Cooper & Wookey 2001; Gauslaa et al 2006, 2007; McCune & Caldwell 2009; Yemets et al 2014; Marks et al 2015) and laboratory studies (Cooper et al 2001; Bidussi et al 2013; Alam et al 2015). By integrating important processes, lichen growth is among the most robust measures of lichen performance and viability.…”