Photosynthesis in the Marine Environment

Sven Beer, Mats Björk, John Beardall

• Éditeur: Wiley-Blackwell
• Année: 2014
• ISBN:
  9781118803387
• Édition: 1
• Sujet: Biowissenschaften
• Sous-sujet: Meeresbiologie

"Marine photosynthesis provides for at least half of the primary production worldwide..."

Photosynthesis in the Marine Environment constitutes a comprehensive explanation of photosynthetic processes as related to the special environment in which marine plants live. The first part of the book introduces the different photosynthesising organisms of the various marine habitats: the phytoplankton (both cyanobacteria and eukaryotes) in open waters, and macroalgae, marine angiosperms and photosymbiont-containing invertebrates in those benthic environments where there is enough light for photosynthesis to support growth, and describes how these organisms evolved. The special properties of seawater for sustaining primary production are then considered, and the two main differences between terrestrial and marine environments in supporting photosynthesis and plant growth are examined, namely irradiance and inorganic carbon. The second part of the book outlines the general mechanisms of photosynthesis, and then points towards the differences in light-capturing and carbon acquisition between terrestrial and marine plants. This is followed by discussing the need for a CO 2 concentrating mechanism in most of the latter, and a description of how such mechanisms function in different marine plants. Part three deals with the various ways in which photosynthesis can be measured for marine plants, with an emphasis on novel in situ measurements, including discussions of the extent to which such measurements can serve as a proxy for plant growth and productivity. The final chapters of the book are devoted to ecological aspects of marine plant photosynthesis and growth, including predictions for the future.

Part I
Plants and the Oceans

Introduction

Our planet as viewed from space is largely blue. This is because it is largely covered by water, mainly in the form of oceans (and we explain why the oceans are blue in Box 3.2, i.e. Box 2 in Chapter 3). So why then call this planet Earth? In our view, Planet Ocean, or Oceanus, would be a more suitable name; since the primary hit on Google for ‘Planet Ocean’ is a watch brand, let's meanwhile stick with Oceanus. Not only is the area covered by oceans larger than that covered by earth, rocks, cities and other dry places, but the volume of the oceans that can sustain life is vastly larger than that of their terrestrial counterparts. Thus, if we approximate that the majority of terrestrial life extends from just beneath the soil surface (where bacteria and some worms live) to some tens of metres (m) up (where the birds soar), and given the fact that, on the other hand, life in the oceans extends to their deepest depths of some 11 000 m, then a simple multiplication of the surface areas (∼70% for oceans and ∼30% for land) with the average depth of the oceans (3800 m), or height of terrestrial environments, gives the result that the oceans constitute a life-sustaining volume that is some 1000 times larger than that of the land. If we do the same calculation for plants only under the assumptions that the average terrestrial-plant height is 1 m (including the roots) and that there is enough light to drive diel positive apparent (or net) photosynthesis¹ down to an average depth of 100 m, then the ‘life volume’ for plant growth in the oceans is 250 times that provided by the terrestrial environments.

It has been estimated that photosynthesis by aquatic plants² provides roughly half of the global primary production³ of organic matter (see, e.g. the book Aquatic Photosynthesis by Paul Falkowski and John Raven). Given that the salty seas occupy much larger areas of our planet than the freshwater lakes and rivers, there is no doubt that the vastly greater part of that aquatic productivity stems from photosynthetic organisms of the oceans. Other researchers have indicated that the marine primary (i.e. photosynthetic) productivity may be even higher than the terrestrial one: Woodward in a 2007 paper estimated the global marine primary production to be 65 Gt⁴ carbon year⁻¹ while the terrestrial one was 60 Gt year⁻¹. This, and recently increasing realisations of the important contributions from very small, cyanobacterial, organisms previously missed by researchers (see Figure I.1a), lends thought to the realistic possibility that photosynthesis in the marine environment contributes to the major part of the primary productivity worldwide. Some of the players contributing to the marine primary production are depicted in Fig. I.1, while many others are described in Chapter 2.

Figure I.1 Some of the ‘players’ in marine photosynthetic productivity. (a) The tiniest cyanobacterium (Prochlorococcus marinus) – the cyanobacteria may provide close to half of the oceanic primary production, at least in nutrient-poor waters. (b) A eukaryotic phytoplankter (the microalga Chaetoceros sp.) – the microalgae are probably the main primary producers of the seas. (c) Two photosymbiont- (zooxanthellae-)containing corals from the Red Sea (Millepora sp., left, and Stylophora pistillata, right) – while quantitatively minor providers to global primary production, these photosymbionts keep the coral reefs alive. (d) The temperate macroalga Laminaria digitata – macroalgae provide maybe up to 10% of the marine primary production. (e) A meadow of the Mediterranean seagrass Posidonia oceanica – even though their primary productivity is amongst the highest in the world on an area basis, the contribution of seagrasses to the global primary production is small (but they form beautiful meadows!). Photos with permission from, and thanks to, William K. W. Li (Bedford Institute of Oceanography, Dartmouth, NS, Canada) and Frédéric Partensky (Station Biologique, CNRS, Roscoff, France) (a), Olivia Sackett (b), Sven Beer (c), Katrin Österlund (d) and Mats Björk (e).

Because marine plants are the basis for generating energy for virtually every marine food web⁵, and given that the oceans may be even more productive than all terrestrial environments together, it is logical that the process of marine photosynthesis should be of interest to every biologist. But what about non-biologists? Why should the average person care about marine photosynthesis or marine plants? One of SB's in-laws has never eaten algae or any other products stemming from the sea; he hates even the smell of fish! However, after telling him that the oxygen (O₂) we breathe is (virtually, see Box 1.2) exclusively generated by plants through the process of photosynthesis, and that approximately half of the global photosynthetic activity takes place in the seas, even he agreed that any interference with the oceans that would lower their photosynthetic production would indeed also jeopardise his own wellbeing. And for those who like fish as part of their diet, the primary production of phytoplankton⁶ is directly related to global fisheries' catches. So, for whatever reason, the need to maintain a healthy marine environment that promotes high rates of photosynthesis and, accordingly, plant growth in the oceans should be of high concern for everyone, just as it is of more intuitive concern to maintain healthy terrestrial environments.

This book will in this, its first, part initially review what we think we know about the evolution of marine photosynthetic organisms. Then, the different photosynthetic ‘players’ in the various marine environments will be introduced: the cyanobacteria and microalgae that constitute the bulk of the phytoplankton in sunlit open waters, the photosymbionts that inhabit many marine invertebrates, and the different macroalgae as well as seagrasses in those benthic⁷ environments where there is enough light during the day for dielly positive net (or ‘apparent’, see, e.g. Section 8.1) photosynthesis to take place. Finally, the third chapter of this part will outline the different properties of seawater that are conducive to plant photosynthesis and growth, and in doing so will especially focus on what we view as the two main differences between the marine and terrestrial environments: the availability of light and inorganic carbon⁸. In doing so, this first part will, hopefully, become the basis for understanding the other two parts, which deal more specifically with photosynthesis in the marine environment.

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Personal Note: Why algae are important

Most of the students taking my (JB) Marine Biology class are more interested in the animals (especially the ‘charismatic’ mega-fauna like dolphins, turtles and whales) than in those plants that ultimately provide their food. Indeed, when I first started my PhD, my supervisor advised me to “…never admit at parties to what you are studying (i.e. algae), or no-one will talk to you…”. However, algae (as a major proportion of the marine flora) are far more important than ‘just those things that cause green scum in swimming pools’. I deal with this in the very first lecture of my course by asking the students to put down their pens (or these days their laptops) and take a deep breath. They do this and I then I ask them to take a second deep breath – by this time they are wondering if I've finally lost it, but then I explain that the second breath they have just taken is using oxygen from photosynthesis by marine plants and that if it wasn't for this group of organisms, not only would our fisheries be more depleted than they already are, but also we'd only have half the oxygen to breath! This tends to focus their attention on those organisms at the bottom of the marine food chain and the fact that algae are responsible for around half of the biological CO₂ draw-down that occurs on...