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© 2018 Elsevier Ltd We report here the 98.5 Mbp haploid genome (12,924 protein coding genes) of Ulva mutabilis, a ubiquitous and iconic representative of the Ulvophyceae or green seaweeds. Ulva's rapid and abundant growth makes it a key contributor to coastal biogeochemical cycles; its role in marine sulfur cycles is particularly important because it produces high levels of dimethylsulfoniopropionate (DMSP), the main precursor of volatile dimethyl sulfide (DMS). Rapid growth makes Ulva attractive biomass feedstock but also increasingly a driver of nuisance “green tides.” Ulvophytes are key to understanding the evolution of multicellularity in the green lineage, and Ulva morphogenesis is dependent on bacterial signals, making it an important species with which to study cross-kingdom communication. Our sequenced genome informs these aspects of ulvophyte cell biology, physiology, and ecology. Gene family expansions associated with multicellularity are distinct from those of freshwater algae. Candidate genes, including some that arose following horizontal gene transfer from chromalveolates, are present for the transport and metabolism of DMSP. The Ulva genome offers, therefore, new opportunities to understand coastal and marine ecosystems and the fundamental evolution of the green lineage. De Clerck et al. present the first genome sequence of a green seaweed, a dominant group of primary producers in coastal environments. The Ulva genome informs on an independent acquisition of multicellularity, sheds light on adaptations to life in intertidal habitats, and identifies candidate genes involved in DMSP biosynthesis and conversion to DMS.

Phytoplankton organisms are, by definition, photosynthetic, and include cyanobacteria as well as algae. Most of the phytoplankton in inland waters obtain all of their energy from light, and their chemical requirements from dissolved inorganic solutes, i.e. are photolithotrophs. Growth rate limitation by resource supply in photolithotrophic phytoplankton can involve light, phosphate, combined nitrogen (except in cyanobacterial nitrogen-fixers), inorganic carbon and iron. Some phytoplankton organisms are mixotrophs, i.e. can obtain energy and carbon from organic compounds in the environment as well as from light and inorganic carbon. This category includes sapromixotrophs (using dissolved organic carbon) and phagomixotrophs (using particulate organic matter). Sapromixotrophy in phytoplankton is rarer in the natural environment that is the loss of organic matter by photolithotrophs. Phagomixotrophy also occurs in the natural environment, and can relate to the acquisition of phosphorus, iron and nitrogen as well as of organic carbon. Even in photolithotrophic phytoplankton cells phosphorus and combined nitrogen can be obtained from dissolved organic phosphate and organic nitrogen.

Publisher Summary This chapter examines the differences in physiology among embryophytes and their algal ancestors, with particular emphasis on their water relations. The embryophytes have very significant variations in water relations and the chapter considers their evolution within the embryophytes as well as the evolution of embryophyte water relations from those of their algal ancestors. The chapter also examines the relationship of the likely evolution of embryophyte water relations to cladistic analyses of embryophyte phylogeny and to the fossil record. The physiological changes that occurred in the evolution from algal ancestors to the different grades of organization of embryophytes has been determined from the physiology of extant plants in relation to their phylogeny as determined by cladistic analysis and from the order in which anatomical features appeared in the fossil record. The fossil record of embryophytes also reveals certain characteristics of organisms that are not found today. The fossil record is not helpful in providing some information such as desiccation tolerance or intolerance, except by applying an empirical correlation from extant plants that no embryophyte more than 1 meter in height is desiccation tolerant in the vegetative phase. Overall, lines of evidence indicate that the earliest embryophytes were desiccation tolerant and poikilohydric.

Publisher Summary This chapter discusses chemistry of the ocean in relation to the origin and early evolution of life. The chapter emphasizes on how the early ocean could supply the energy and materials needed for extant life and also of putative ancestral organisms and prebiotic components. The chapter begins with an outline of the likely chemical composition of the Hadean Ocean, including changes to ocean chemistry because of interactions with the earth's crust and with the atmosphere. Furthermore, the chapter also considers the ways in which the Hadean Ocean could have supplied the materials and the energy needed for the origin and early evolution of life. This view focuses on ocean and atmosphere interactions as most significant in providing the energy for early life to the ocean. Later the direct use of light energy by organisms at the ocean surface became the major energy transforming reaction, providing energy for almost all living organisms. The chemical elements that were used by the earliest organisms were determined by their availability to early organisms as well as by their physicochemical appropriateness for particular biological functions. Modification of ocean chemistry by the activities of living organisms—for example, the accumulation of photosynthetically produced oxygen—changed the availability of several biologically essential elements. The chapter concludes with a discussion on how early life could have modified the chemistry of the ocean, and how these changes could have fed back to the evolution of the early organisms.

Photosynthesis, or building with light, involves the use of electromagnetic energy to bring about endergonic reactions. The concept is generally extended to the use of the more immediate photochemical products to reduce CO 2 and (in most photosynthetic organisms) oxidize H 2 O and evolve O 2 , this extended definition is used here. This article deals with the partial processes of photosynthesis in the order in which they occur subsequent to initial photon absorption.

The predominant silicon (Si) compound in the soil solution is silicic acid, and the baseline condition for Si transport into and within a plant with no membrane channels or transporters which can move Si compounds is the movement of silicic acid across membranes by dissolving in the lipid phase of the membrane (‘lipid solution’ transport). Based on the best current estimates of ‘lipid solution’ permeability of membranes to silicic acid (∼10 −10 m s −1 ), even the lowest Si contents in plants cannot be explained in terms of the soil solution silicic acid concentration and the lipid solution mechanism, and a component of silicic acid entry coupled to transpiratory water uptake is required. For Oryza (rice) and, under some conditions, Hordeum (barley), and Phaseolus (bean), active influx of silicic acid is needed to account for the observed silica content. Further work is needed as to the mechanism of active tranport of silicic acid following the lead of the characterization of Na + -coupled transport in a diatom, and on how silicic acid is coupled to water transport (involving aquaporins?), and on the phloem mobility of silicic acid.

Publisher Summary This chapter discusses the structure and functions of vacuole. A number of functions have been demonstrated, or suggested, for the aqueous vacuoles of plants and fungi, as well as vacuole–like structures in prokaryotes. In order to understand the distribution of vacuoles, then functions of the vacuole that yield quantifiable benefits are considered in the context of the costs of synthesizing and maintaining the vacuoles, including any consequential requirements of vacuolation such as increased resource allocation to cell walls. Such cost–benefit analyses are predicated on maximizing the inclusive fitness of the organisms by optimal acquisition and allocation of resources. The costs and benefits must be expressed in the same resource units; the appropriate units in a given environment are determined by the resource that is most limiting for growth as determined by sensitivity analysis. An inescapable effect of vacuolation of a cell of a given shape is to increase the surface area per unit volume of cytoplasm. This increases the rate of resource acquisition from resource–limited environments on a unit cytoplasm basis. At the risk of being too Panglossian, it would be possible to conclude that the semiquantitative analyses currently possible show that the evolutionary costs of vacuoles are outweighed by their evolutionary benefits. However, much more quantitative analysis is needed to test the suggestion that all of the costs and benefits of vacuolation and their quantitative importance have been currently identified.

Publisher Summary This chapter discusses inorganic carbon (C) acquisition by marine autotrophs. This chapter discusses the characteristics of inorganic C acquisition process responsible for major biogeochemical process. The chapter discusses the transport of inorganic C species from seawater to the site of carboxylation; the interconversions of inorganic C species that occur en route ; and the mechanism of the carboxylations, which occur in parallel to yielding carboxylic acids usable in biosynthesis leading to the production of all organic C in the organism, including any “photorespiratory” decarboxylation processes. There is great genetic and ecological diversity among marine autotrophs; even when chemolithotrophs and non-O 2 -evolving phototrophs are not considered, there is much more genetic diversity among marine photolithotrophic O 2 -evolvers than among terrestrial O 2 -evolvers. O 2 -evolvers bring about more than 90% of gross inorganic C assimilation in the oceans, and essentially all of this inorganic C is routed through Ribulose-1,5-bisphosphate carboxylase oxygenase (RUBISCO), although in some instances some or all of this inorganic C has been subject to a prior carboxylation–decarboxylation cycle using enzymes other than RUBISCO. There have been variable results from attempts to demonstrate a higher inorganic C concentration inside the cells of marine O 2 -evolvers than in normal seawater for organisms, while gas-exchange characteristics suggest the occurrence of a CO 2 -concentrating mechanism. The evolution of inorganic C-assimilation systems in marine autotrophs will only be better understood when more is known of the present day mechanisms of inorganic C transport at the molecular genetic level.

Publisher Summary This chapter describes several aspects of the significance of transport processes in the evolution of vascular land plants from their putative ancestors—the green algae. The vascular land plants have, in addition to the transport processes at the cell level, which are common to all organisms, important transport processes at the supracellular level, which involve complex anatomical features. The homoiohydric vascular land plant has three major transport systems at the supracellular level—the apoplast, the symplast, and the intercellular gas space system. The aquatic green algal ancestors of these plants use only the symplast for transport at the supracellular level. The other two transport systems, together with the elaboration of the symplast into the much more efficient phloem, are essential components of the homoiohydric land plants.