MICROORGANISMS (Chapter 11)

Lead Author:  Connie Lovejoy

SUMMARY

Microbes, defined here as Bacteria, Archaea and singlecelled Eukaryota (protists) are ubiquitous and diverse members of all biological communities. In marine and many freshwater systems, photosynthetic microbes form the base of the food chain supporting higher trophic levels. Among the photosynthetic species are biologically diverse small flagellates that also graze on bacteria and other protists and hence are functional heterotrophs at times. Strictly phagotrophic protists are also diverse and contribute to the complexity of microbial food webs, with a multitude of trophic interactions. The fate of Arctic primary production emerges from the assembly of the entire microbial community. Heterotrophic bacteria break down fixed organic carbon and recycle nutrients, while other bacteria and Archaea with diverse metabolic capacities are active in the remineralization of carbon, nitrogen and other elements. There is a lack of longterm comprehensive baseline data on microbial biodiversity in terrestrial, freshwater and marine systems that largely impedes understanding ecosystem structure and resilience over both local and regional scales.

Because of their small size and often large populations, microbes in principle may have global distributions as they are transported by moving masses of air and water. Microbial communities are strongly selected for by their immediate environment, and successful global transport will be influenced by the ability of organisms to remain viable during transport between favorable environments. Species are more likely to have geographically restricted distributions if they lack a dormant stage or are specialists, and if their preferred environments are rare and distant from each other. Local communities may also resist invasions in the absence of physical or ecological perturbations that would give invading species or ecotypes an advantage. In the absence of ice cover, increased light availability and increased water column stratification can influence microbial community structure, affecting both phototrophic and heterotrophic species. Both the duration of the productive season and the species composition of protists have implications for higher food webs, thus ecosystem services by microbes are of direct relevance to wildlife and fisheries biologists as well as local communities. In addition, microbial community interactions largely determine the efficacy of the biological carbon pump.

The diversity of heterotrophic protists and other microbes is largely unknown, since for the most part they cannot be identified morphologically. Hence, sound historical records are lacking. New tools are being used to identify these heterotrophs from their DNA and RNA collected from the environment. Even among larger species of phytoplankton, cryptic species have been identified from DNA sequences. The small sub-unit ribosomal RNA (SS rRNA) gene is the most common target for gene surveys and enables the identification of microbes at the level of genus and even species and ecotype for some groups. In addition, genomic and transcriptomic signatures of microbes from the Arctic will provide valuable insight into the resilience and capacity of Arctic ecosystems. The recent rapid advances in sequencing technology will enable the expansion of microbial surveys, facilitating the integration of microbial biodiversity data into coupled biogeochemical climate models. Further, monitoring could provide the means to test whether there are linkages between climate change, environmental perturbation and the emergence of southerly species, enabling robust projections about dynamic shifts in ecosystem structure over time. For these reasons there is an urgent need to increase knowledge of microbial communities at the finest taxonomic and functional levels.

INTRODUCTION

Microbes represent the majority of biodiversity on Earth and are integral components of all ecosystems. In terms of numbers, microbes also dominate with c. one million cells per milliliter (ml) of seawater and most freshwaters. Marine sediments host an even more impressive number of bacterial cells per ml (in the order of 1 billion). Sea ice also harbors distinct microbial communities that live within brine channels and at high local concentrations (Deming 2002). Distinct communities can be found attached to the bottom of first year ice and occurring in surface melt ponds (Mundy et al. 2011). Concentrations of bacteria in Arctic soils are less than in temperate soils, but can still reach substantial numbers in key microhabitats (Yergeau et al. 2010, Wilhelm et al. 2011). Although heterotrophic protists and other microbes are the primary drivers of marine food webs and play key roles in freshwaters and soils, they are rarely included in general assessments of biodiversity (Archambault et al. 2010).

As reported in other chapters, climate and environmental change is rapidly reshaping northern ecosystems. These perturbations include the loss of summer ice, changes in the annual production cycle and changes in the depth of the most biologically active layers in both pelagic water columns and soils. Such environmental changes will have a direct effect on visible animals and plants (Falk-Petersen et al. 2009) and also have direct impact on the microbial food webs that support higher trophic levels.

In addition, as mentioned in other chapters, terrestrial and aquatic habitats merge over much of the sub-Arctic and Arctic. Distinct ponds and lakes are formed in polygons and runnels surrounding polygons within the permafrost and in the high Arctic water increasingly remain as ice over the year. Deeper lakes are also scattered throughout the Arctic, and the microbial community structure of ponds and lakes is influenced by their depth, catchment area, orientation and underlying basin geology. Freshwater microbes are poorly studied with only sporadic reports of species. Most surveys of soil microbes have focused on disturbed sites with few reports on the microbes from pristine regions (Steven et al. 2008a, Martineau et al. 2010, Niederberger et al. 2010, Wilhelm et al. 2011). In this chapter, I provide a brief summary of existing knowledge, identify key gaps and suggest strategies for monitoring microbial biodiversity.

CONCLUSIONS AND RECOMMENDATIONS

Sensitive areas and hotspots

In the terrestrial and freshwater habitats, areas identified either as sensitive or as hotspots for animals and plants should also be considered as microbiologically significant regions. In addition, unique or rare habitats such as saline springs should be protected to preserve unique biomes and specialized microbiota. In coastal and oceanic regions, areas where mammals and birds congregate should also be monitored. For example, marine productivity is related not only to the quantity of photosynthetic biomass produced but also the quality. In oceanic regions, the diversity and stability of microbial food webs dictates lipid concentrations in the zooplankton that support higher trophic levels. As longer ice free periods become the norm, microbial food chains are predicted to lengthen, and less energy will be available to the highest trophic levels in the oceans (Lovejoy 2011). Such changes will also have major impacts on benthic communities and on the carbon and nutrient cycling that occurs in the benthos (see also Josefson & Mokievsky, Chapter 8). The potential loss of multiyear ice as a habitat and changes in the duration and type of sea ice with different communities (Comeau et al. 2012b) will have consequences for biodiversity and carbon cycling. More research is needed to better estimate which com communities may be lost as a result of the loss of summer sea ice. These changes are likely to have significant effects on the diversity and functioning of Arctic ecosystems. In terrestrial based systems, increased liquid water, higher temperatures and longer growing seasons will affect all biological activity, and northward expansion of species can be expected. Although at present much of the Arctic appears poor in life, microbial communities are active and complex, and ‘non-hot spot’ regions need to be monitored as well as highly productive regions in order to anticipate new distributions and community associations. Microbes will respond to ecosystem changes much sooner than higher plants and invertebrates and are thus sensitive indicators of directional changes.

Key knowledge gaps and recommendations

Ecosystem assessments and the role of complex interacting factors, which may influence ecological patterns, can only be explored through long time series of biological collections and surveys at local to regional scales. The only open-ocean long-term observatory in the Arctic is HAUSGARTEN, coordinated by the Alfred Wegener Institute for Polar and Marine Research (Soltwedel et al. 2005; see also Josefson & Mokievsky, Chapter 8). The Arctic is vastly under-sampled and heterotrophic protists, Bacteria and Archaea play a critical role in ecosystem support. Currently, there are only a small handful of researchers interested in microbial biodiversity and how it directly relates to ocean ecosystem function. There is a need to foster greater interest in microbial ecology among Arctic researchers. Microbial communities must be included in any Arctic monitoring effort aimed at understanding biodiversity and ecosystem function.