To get everyone on the same page, the “microbiome” is the combination of bacteria, viruses and fungi that (usually) live in harmony in the gut and every other cavity of animals. The “gut-brain-microbiome axis” has become appreciated only during the last 10-15 years — as an incredible demonstration of trans-kingdom symbiosis, with cooperation between gut microorganisms and their animal hosts to regulate development and function of the immune, metabolic and nervous systems by means of dynamic bidirectional communication.
Ocean-dwelling multicellular-animal organisms also live in close association with their microbiomes which provide their hosts with key functions — including nutrient supply, defense mechanisms, and even additional metabolic pathways — thus representing an integral component of “the holobiont” (i.e., the host organism and various microorganisms such as bacteria, viruses, and fungi that live within and on the host organism), which is able to influence host physiology and increase adaptation to environmental conditions (gene-environment interactions…!! ).
Microbiome-host interactions are far more widespread than previously thought (i.e., all animals having “a cavity of any sort” — have a microbiome; this even includes sponge and jellyfish) and can notably influence the auto-ecology of marine organisms, playing a role in health of the whole ecosystem. The host-associated microbiota changes from species to species — also in relation to a variety of environmental (i.e., geographic location, seasonal variations, and nutrient availability) and biological factors (i.e., metabolic state, feeding strategy, and host phylogeny). However, the presence of a stable core (i.e., any set of microbial taxa characteristic of a specific host or environment) that has been reported for several holobionts, suggests that some microbes and related genomic or functional features are essential for the well-being of the host species (a great example of symbiosis).
Some specific bacterial taxa can be vertically transmitted and persist across life stages and generations, favoring mutual adaptation. Mechanisms of transmission of microbiome components to their hosts may include horizontal acquisition, which is generally based on a selection of beneficial bacterial members from the environment anew by each host generation. [Vertical transmission (VT) and horizontal transmission (HT) are two modes of transmission that can occur during evolution. VT occurs when genes or parasites are passed down through generations, usually from parent to offspring, e.g., plant virus DNA is vertically transmitted when they pass from a parent plant to its offspring. HT occurs when genes or parasites are passed between individuals in the same generation, usually independently of reproduction, e.g., plant virus DNA is horizontally transmitted when they pass between plants].
Microbiomes can also have a role in the host-microbe adaptation to extreme environmental conditions and in evolutionary processes. In this regard, information on marine host-microbiota phylosymbiosis (microbial community relationships that reiterate the phylogeny of their host) as an eco-evolutionary pattern — in which the ecological relatedness of host-associated microbial communities parallels the phylogeny of related host species — is contrasting. In some organisms, phylosymbiosis has been documented even to different extents in specific portions of their body, whereas in others, it seems not to have a primary role — suggesting the lack of strong affinity between bacterial members and their specific host lineage.
The processes driving microbial associations and influencing the adaptations of marine organisms to environmental conditions are still open questions. Investigations in extreme ecosystems can certainly contribute to explore the co-evolutionary processes and ecological interactions of the microbiota-host associations. The most isolated continent on Earth, Antarctica, can provide additional information into the adaptation of marine life to extreme conditions. This geological isolation, along with the stability of extreme environmental conditions, for more than 34 million years has produced a range of unique adaptations to low temperatures with a highly diverse fauna composed of about 17,000 marine invertebrate species and with the highest proportions of endemic species of the world oceans [see attached article].
In polar ecosystems, marine poikilotherm metazoans (i.e., in which the internal temperature is in equilibrium with the environmental temperature) must adapt to extreme cold conditions using different biological and physiological mechanisms (e.g., the slowdown in embryonic development and growth rate and increase in oxygen consumption to maintain homeostasis, to prevent internal liquid freezing, and to induce antioxidant defense systems).
Previous studies also hypothesized the presence of thermal hysteresis proteins in the hemolymph of ectothermic invertebrates, such as Antarctic nemerteans (ribbon worms live in the Southern Ocean; they are flat, long, and smooth, and can range in length from a few millimeters to 30 meters. They are unsegmented, have no differentiated head, and are often brightly colored) and terrestrial invertebrates, but no general conclusions could be provided. At the same time, microbes own functional traits for adaptation to low temperatures and, when associated with Antarctic benthic (living on bottom of ocean, lake or river) invertebrates, play key roles in several metabolic processes. However, information on the mechanisms adopted by marine invertebrates to cope with extreme cold and freezing conditions is limited and controversial, and the role of their microbiota in these mechanisms is even neglected.
In the attached study, authors investigated the microbiome of three dominant endemic species of Antarctic polychaetes or bristle worms (Leitoscoloplos geminus, Aphelochaeta palmeri, and Aglaophamus trissophyllus) that play a key role in the benthic life of extreme-cold water. The identity, origin, and proteome of the microbiota associated with these Antarctic invertebrates, as well as the factors influencing its structure, including phylosymbiosis, were investigated to provide insights into the role of the microbiome in marine invertebrate adaptation to the extreme conditions of Antarctic ecosystems.
The close phylosymbiosis between Meiothermus and Anoxybacillus and these Antarctic polychaetes shows a connection with their hosts that started long ago in the evolutionary past to support holobiont adaptation to the Antarctic Ocean. Authors suggest that the wide suite of bacterial cryoprotective proteins found in Antarctic polychaetes may be useful for the development of nature-based biotechnological applications.
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