This article by Matissek and Luttge goes in some depth into the definition of "Gaia" as a planetary holobiont. It is rich in ideas and concept, although I think it is not really as deep as it could be. On the other hand, we still have lots of things to learn!!
The strict definition of holobiont is that it is a host organism (plant or animal) in interaction with all associated microorganisms as an entity for selection in evolution. This definition can be generalized when not only microorganisms internal of or endogenous to a host organism are concerned but any regular organisms which strongly interact between each other also externally. Such latter associations may altogether function as co-evolutionary entities at the community level, as can be concluded from their evolutionary history. A thorough inspection of habitats and ecosystems shows that such kinds of mutualistic associations prevail. Therefore, we can scale up examples of endogenous symbioses to ecosystems, biomes and the entire biosphere as holobionts, where the planet holobiont is Gaia.
James Lovelock defines Gaia as the biosphere being a self-regulating entity that ensures the planet’s capacity for harboring life by controlling the chemical and physical environment in conducive ways. Thus, the question is of whether Gaia manages a global equilibrium which sustains life on Earth. Possibly this holds for life as such but not for any specific forms of life as illustrated by several extinction waves of organisms during geological history. Such events were typically followed by the emergence of innovative new forms as arising from adaptive radiation into abandoned ecological niches. Will man be subject of the next extinction wave? Such a possibility raises ethical imperatives for man to sustain the biosphere sensu Gaia on which he not only depends, but also is part of. Holobiont research must aim at gaining advanced understanding, both in mechanistic and holistic terms, by extending the holobiont concept across the spatio-temporal scales of ecological organization. This implies to delineate the humble position of man in the biosphere and to explore conditions for man’s sustained survival on Earth.
https://www.leopoldina.org/fileadmin/redaktion/Probekapitel_NAL391.pdf
