Picoplankton & Nanoplankton LifeForms: theoretical basis and assessment of candidate lifeforms

This document is a contribution towards the Defra project “The missing 80%. Incorporating nano- and pico- plankton into the assessment of pelagic habitats”. We build on an existing theory for planktonic lifeforms and apply it to assess 8 candidate lifeforms amongst the pico- and nano- plankton: the cyanobacteria Synechococcus and Prochlorocccus; certain coccolithophores; the cryptomonads; the PEUK (pico-eukaryotes) and the NEUK (nano-eukaryotes); and two het- erotrophic groups, the HNA (high nucleic acid) and LNA (low nucleic acid) bacteria.
A lifeform is defined operationally as the set of traits that identifies a functional group of phytoplankters, thus the set of species (observed to possess these traits) that are supposed to respond in similar ways to ecohydrodynamic conditions and which are similar in their biogeochemical and trophic interactions. Traits recognised by microscopy are used operationally to identify species and supposedly functionally adaptive traits are used to assign species to lifeforms, a procedure that needs rethinking in the case of flow cytometry used for most of the candidate lifeforms. It is suggested that the term ‘macro-trait’ be used for a functional characteristic, shared by all the species that comprise a lifeform, and adapting them to the lifeform’s ‘macro-niche’. Monophyletic macro-traits equate lifeforms with high-level taxa; polyphyletic macro-traits require attention to be paid to size (the topic of this project), possession of flagella (enabling better nutrient acquisition and associated with mixotrophy) and predator defences (cell walls, toxicity, life-cycle).