Environmental variability is an intrinsic characteristic of nature. Variability in factors such as temperature, UV, salinity, and nutrient availability can influence structural and functional properties of marine microbial organisms. This influence has profound implications for biochemical cycles and the ecosystem services provided by the oceans. In this review we discuss some of the most relevant mechanisms underpinning adaptive strategies of microbial organisms in variable and dynamic oceans. We assess the extent to which the magnitude and rate of environmental change influence plastic phenotypic adjustments and evolutionary trajectories of microbial populations. This understanding is fundamental for developing better predictions regarding microbial dynamics at ecological and evolutionary time-scales and in response to climate change.

Extreme thermal conditions on rocky shores are challenging to the survival of intertidal ectotherms, especially during emersion periods. Yet, many species are highly successful in these environments in part due to their ability to regulate intrinsic mechanisms associated with physiological stress and their metabolic demands. More recently, there has been a growing awareness that other extrinsic mechanisms, such as animal-associated microbial communities, can also influence the tolerance and survival of ectotherms under stressful conditions. The extent to which intrinsic and extrinsic mechanisms are functionally linked as part of the overall adaptive response of intertidal animals to temperature change and stress is, however, poorly understood. Here, we examined the dynamics and potential interactions of intrinsic and extrinsic mechanisms in the ecology of the tropical high shore rock oyster, Isognomon nucleus. We found that oysters modulate their internal biochemistry (PUFA-oxidised metabolites including 5-F2t-IsoP, 10-F4t-NeuroP, 13-F4t-NeuroP, and 16-F1t-PhytoP) as part of their adaptive regulation to cope with physiological stress during periods of extreme temperatures when emersed. While we detected extrinsic microbiome changes in alpha diversity, the overall taxonomic and functional structure of the microbiome showed temporal stability with no association with the host biochemical profiles. Our finding here suggests that the microbiome taxonomic and functional structure is maintained by a stable host control (not associated to the host biochemistry) and/or that the microbiome (independent of the host) is resilient to the temperature fluctuations and extremes. This microbiome stability is likely to contribute to the oyster, I. nucleus thermal tolerance, in addition to the intrinsic biochemical adjustment, to survive in the thermally challenging intertidal environment.