Abstract
Defining the physiological traits that characterise phytoplankton involves comparison with related organisms in benthic habitats. Comparison of survival time in darkness under natural conditions requires more information. Gas vesicles and flagella as mechanisms of upward movement relative to surrounding water, allowing periodic vertical migration, are not confined to plankton, although buoyancy changes related to compositional changes of a large central vacuole may be restricted to plankton. Benthic microalgae have the same range of photosynthetic pigments as do phytoplankton; it is not clear if there are differences in the rate of regulation and acclimation of photosynthetic machinery to variations in irradiance for phytoplankton and for microphytobenthos. There are inadequate data to determine if responses to variations in frequency or magnitude of changes in the supply of inorganic carbon, nitrogen or phosphorus differ between phytoplankton and benthic microalgae. Phagophotomixotrophy and osmophotomixotrophy, occur in both phytoplankton and benthic microalgae. Further progress in identifying physiological traits specific to phytoplankton requires more experimentation on benthic microalgae that are closely related to planktonic microalgae, with attention to whether the benthic algae examined have, as far as can be determined, never been planktonic during their evolution or are derived from planktonic ancestors.
Highlights
In considering the evolution of physiological traits of phytoplankton, it is necessary to take into account the habitat of the ancestors of the planktonic organisms and their physiological traits
In this review, we have focussed on examining what physiological traits might be ascribed to the planktonic habitat by comparison to those of cyanobacteria and algae found in other environments, especially the benthic habitat
Periodic vertical migration can occur by means of flagella motility in eukaryotes, by variations in cell density in cyanobacteria as a result of variations in gas vesicle volume and in ballast content
Summary
In considering the evolution of physiological traits of phytoplankton, it is necessary to take into account the habitat of the ancestors of the planktonic organisms and their physiological traits. Is a given trait an evolutionary result of life in the plankton, or was it inherited from adaptation to whatever environments that they occupied previously? This is not always considered, as exemplified by a quotation from Sabir et al [1] in their review of the history of the “accidental phytoplankton” Phaeodactylum tricornutum: “Despite the many observations suggesting a benthic component of this diatom’s life history, it continues to be discussed explicitly or explicitly as a model diatom in the context of planktonic environments”. In this review, we have focussed on examining what physiological traits might be ascribed to the planktonic habitat by comparison to those of cyanobacteria and algae found in other environments, especially the benthic habitat
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