An elementary example: from photosynthesis to air

Well over two billion years ago, the Earth went through the most marked environmental change in its history, a change that transformed it from a planet resembling Saturn's moon Titan in its methane-rich atmospheric haze to one with a clear, breathable, oxygen-rich envelope (Kasting and Siefert 2002; Johnson et al. 2013; and references therein). This dramatic rise in atmospheric oxygen concentration was brought about by some of the earliest-evolved, unicellular organisms, the cyanobacteria that inhabited the Earth's oceans (Beerling and Berner 2005; Taylor and McElwain 2010). Their metabolic repertoire included the complex process of photosynthesis, which fixes solar energy in a chemically usable form while generating molecular oxygen as a byproduct. The collective photosynthetic activity of these countless microscopic organisms gradually and irreversibly transformed the chemistry of the aerial environment (Figure 5.1a), such that subsequent evolution and ecology have unfolded in the context of adaptation to an oxygenated atmosphere (Lyons et al. 2014). For instance, the symbiotic origin of the eukaryotic cell, with its new capacity to carry out the far more efficient oxidative (rather than anaerobic) metabolism, is believed to have led to the evolution of multicellular organisms (Taylor and McElwain 2010 and references therein).

The ongoing activities of photosynthetic organisms continue to determine the composition of the Earth's atmosphere in the present day (although very recently human activities have also taken a major role). Because the ancestor of the biotically dominant clade that includes green algae and land plants had incorporated cyanobacteria symbioti- cally, both unicellular marine plankton "pastures"

Organism and Environment. First Edition. Sonia E. Sultan.

© Sonia E. Sultan. 2015. Published 2015 by Oxford University Press.

The photosynthetic activity of organisms has collectively transformed the Earth's aerial environment to one rich in oxygen

Figure 5.1 The photosynthetic activity of organisms has collectively transformed the Earth's aerial environment to one rich in oxygen. (a) A schematic showing the change in atmospheric oxygen concentration over geological time. The general association between estimated oxygen concentration changes and major evolutionary events is shown based on currently available data, with approximate dates in billions of years (Ga). Figure reproduced (with updated information) from D. Shevela, R. Y. pishchalinikov, L. A. Eichacker and Govindjee (2013) "Oxygenic Photosynthesis in Cyanobacteria", in "Stress Biology of Cyanobacteria," edited by A. K. Srivastava, A. N. Rai, and B. A. Neilan, CRC Publishers. pp. 3-40.

Amended version provided courtesy of the corresponding authors D. Shevela and Govindjee. (b) The release of oxygen from plants into the atmosphere is regulated at the cellular level by the opening and closing of stomates. Photomicrograph of a live Brassica rapa leaf shows several open stomates on the lower epidermal surface (400x magnification; field of view = 0.037 mm2). Image courtesy of Dana L. Royer and Peter Martin.

and later-evolved multicellular taxa in this clade are able to carry out photosynthetic carbon fixation and produce oxygen (Kasting and Siefert 2002). Indeed, it is estimated that these organismic activities draw approximately 120 x 1015 g of carbon annually from the atmosphere's 730 x 1015 g into the primary production of plant tissues (Hetherington and Woodward 2003), leading to a profound feedback on the function and evolutionary diversification of plants and, through them, terrestrial faunas (Beerling and Berner 2005; Baldwin 2010). In addition, the release of water vapor via transpiration that is coupled to photosynthesis (an estimated 30-40 x 1018 g of water vapor) makes an enormous contribution to atmospheric moisture content and hence to the global water cycle (Lake et al. 2002; Gerten et al. 2004; and references therein).

These vast global effects are governed by developmental and behavioral events that scale down spatially to the individual cell, and temporally to the microsecond. These events in turn are governed by environmental conditions which elicit responses at both immediate and selective timescales. Stomates (from the Greek stoma, or mouth; see Chapter 3, Section 3.2) are tiny epidermal structures distributed on plant shoots, primarily on leaf undersides and surfaces. Each stomate consists of a pair of guard cells that alternately swell shut or relax to reveal a central pore through which carbon dioxide enters the leaf tissue and water vapor exits (Figure 5.1b). The size and duration of stomatal opening thus regulates both the uptake of carbon dioxide for photosynthesis and the loss of water from the plant's tissues. This aperture is determined both by developmental "decisions" within the leaf epidermis regarding the size and density of stomates and by the rapid behavioral responses of the guard cells to immediate atmospheric conditions such as concentrations of carbon dioxide and water vapor.

Both these longer-term developmental effects and the moment-to-moment behavior of stomates are conditioned in highly complex ways by the plant's external and resulting internal environment (see Chapter 3, Section 3.2). The precise ways in which environmental factors enter into the developmental pathways that determine stomate size and density in the epidermis are not yet known, although both light and carbon dioxide appear to play roles (possibly integrated by MAP kinase signaling pathways; D. Bergmann 2006; Casson and Hetherington 2010). On a scale of seconds to hours, stomates open and close in response to a complex, interacting set of environmental inputs (Hethering- ton and Woodward 2003; also see Chapter 3, Section 3.2). Larger-scale patterns of environmental variation participate in the selective evolution of traits that affect photosynthetic function, including norms of reaction for biochemical, anatomical, al- locational, and morphological responses to ambient carbon dioxide and humidity conditions, at the population and species levels. Clearly, the "intimate and bidirectional relationship" between living organisms and the composition of the planet's atmosphere (Taylor and McElwain 2010, 272) consists of feedback cycles at cellular, individual, and macro— or even mega—evolutionary and ecological scales, exemplifying the "vice versa" of environmental impact on organisms.

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