Methodological/ethical considerations
We controlled for environmental contamination by rinsing nodules and
including leaf controls, which presumably were exposed to the same
sources of environmental contamination. However, post-harvest plants
have significantly different microbiomes than actively growing plants
both in taxonomic diversity and function. This “post-mortem”
microbiome was found to be a significant portion of the microbes
identified on herbarium specimens (Alternaria alternata comprised
up to 7% of the total reads, Bieker et al. 2020). However, authors
found that non-bacterial microbes were the largest contributors of the
leaf post-mortem microbiome (Bieker et al. 2020). Potentially, some of
the microbes that we cultured are constituents of Medicago’spost-mortem microbiome. We did isolate genera that are known
constituents of the Medicago nodule microbiome includingMicromonospora and Paenibaccillus (Martínez-Hidalgo et al.
2014, Lai et al. 2015, Hansen et al. 2020). Further work should
compare historic and contemporary nodule microbiomes of Medicago
lupulina to determine occurrence of taxa found in our collection in
contemporary populations.
General ethical issues that occur when using herbarium specimens are
also relevant for type of work. We took a highly conservative sampling
approach because our approach requires destructively sampling specimens.
We chose a common species that is abundant in herbaria across the
country. It would be impractical and unethical to work with rare
specimens that occur sporadically in herbaria. Fortunately, with the
digitization of collections all over the country and world access to
suitable specimens this kind of work is increasingly possible. Finally,
it is important to acknowledge that herbaria, like any biological
collection, benefitted from the period of overt colonialism spanning the
15th to mid-20th centuries. The
majority of plant specimens are held in the global north, despite being
collected globally, a pattern that has created an inverse relationship
between global biodiversity and where collections are housed (Park et
al. 2023). This trend in collections highlights not only a trend for the
colonizing countries to exploit foreign resources, but also wrests the
control of knowledge and biological resources away from countries of
origin, depriving resident explorers/scientists/researchers from local
resources and discoveries.
CONCLUSIONS :
Herbarium collections contain specimens that represent taxa from a wide
range of locations collected over a timeframe that is well beyond that
of modern research. Our work demonstrates that not only are herbarium
specimens a source of genetic/genomic/metagenomic information about
plant species and their associated microorganisms, but they are also the
source of historic microbes. While microbial diversity has been examined
extensively, microbial function is only now being explored. A source of
data for understanding microbiome function are growing culture
collections. Enhancing culture collections through herbarium collections
will not only expand the source of culture collection material but might
help identify novel functions that are relevant to pressing problems
such as climate change (Rudgers et al. 2020). Additionally, herbarium
microbial culture collections will allow us to test hypotheses about
plant-microbe interactions over larger temporal and spatial scales. For
example, previous work has suggested that increasing soil nitrogen
availability could cause the erosion of interactions between nitrogen
fixing microbial bacteria and plant hosts, Although some support exists
for this idea (Weese et al. 2015) , herbarium cultures could further the
temporal window to test these ideas.