Photosynthesis in the Marine Environment (III)

This will be my last post about the book. After having spent a few hours on the post I started to realize the post would become very long if I were to cover all the remaining chapters, and so in the end I decided not to discuss material from chapter 12 (‘How some marine plants modify the environment for other organisms’) here, even though I actually thought some of that stuff was quite interesting. I may decide to talk briefly about some of the stuff in that chapter in another blogpost later on (but most likely I won’t). For a few general remarks about the book, see my second post about it.

Some stuff from the last half of the book below:

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“The light reactions of marine plants are similar to those of terrestrial plants […], except that pigments other than chlorophylls a and b and carotenoids may be involved in the capturing of light […] and that special arrangements between the two photosystems may be different […]. Similarly, the CO₂-fixation and -reduction reactions are also basically the same in terrestrial and marine plants. Perhaps one should put this the other way around: Terrestrial-plant photosynthesis is similar to marine-plant photosynthesis, which is not surprising since plants have evolved in the oceans for 3.4 billion years and their descendants on land for only 350–400 million years. […] In underwater marine environments, the accessibility to CO₂ is low mainly because of the low diffusivity of solutes in liquid media, and for CO₂ this is exacerbated by today’s low […] ambient CO₂ concentrations. Therefore, there is a need for a CCM also in marine plants […] CCMs in cyanobacteria are highly active and accumulation factors (the internal vs. external CO₂ concentrations ratio) can be of the order of 800–900 […] CCMs in eukaryotic microalgae are not as effective at raising internal CO₂ concentrations as are those in cyanobacteria, but […] microalgal CCMs result in CO₂ accumulation factors as high as 180 […] CCMs are present in almost all marine plants. These CCMs are based mainly on various forms of HCO₃⁻ [bicarbonate] utilisation, and may raise the intrachloroplast (or, in cyanobacteria, intracellular or intra-carboxysome) CO₂ to several-fold that of seawater. Thus, Rubisco is in effect often saturated by CO₂, and photorespiration is therefore often absent or limited in marine plants.”

“we view the main difference in photosynthesis between marine and terrestrial plants as the latter’s ability to acquire Ci [inorganic carbon] (in most cases HCO₃⁻) from the external medium and concentrate it intracellularly in order to optimise their photosynthetic rates or, in some cases, to be able to photosynthesise at all. […] CO₂ dissolved in seawater is, under air-equilibrated conditions and given today’s seawater pH, in equilibrium with a >100 times higher concentration of HCO₃⁻, and it is therefore not surprising that most marine plants utilise the latter Ci form for their photosynthetic needs. […] any plant that utilises bulk HCO₃⁻ from seawater must convert it to CO₂ somewhere along its path to Rubisco. This can be done in different ways by different plants and under different conditions”

“The conclusion that macroalgae use HCO₃⁻ stems largely from results of experiments in which concentrations of CO₂ and HCO₃⁻ were altered (chiefly by altering the pH of the seawater) while measuring photosynthetic rates, or where the plants themselves withdrew these Ci forms as they photosynthesised in a closed system as manifested by a pH increase (so-called pH-drift experiments) […] The reason that the pH in the surrounding seawater increases as plants photosynthesise is first that CO₂ is in equilibrium with carbonic acid (H₂CO₃), and so the acidity decreases (i.e. pH rises) as CO₂ is used up. At higher pH values (above ∼9), when all the CO₂ is used up, then a decrease in HCO₃⁻ concentrations will also result in increased pH since the alkalinity is maintained by the formation of OH […] some algae can also give off OH⁻ to the seawater medium in exchange for HCO₃⁻ uptake, bringing the pH up even further (to >10).”

“Carbonic anhydrase (CA) is a ubiquitous enzyme, found in all organisms investigated so far (from bacteria, through plants, to mammals such as ourselves). This may be seen as remarkable, since its only function is to catalyse the inter-conversion between CO₂ and HCO₃⁻ in the reaction CO₂ + H₂O ↔ H₂CO₃; we can exchange the latter Ci form to HCO₃⁻ since this is spontaneously formed by H₂CO₃ and is present at a much higher equilibrium concentration than the latter. Without CA, the equilibrium between CO₂ and HCO₃⁻ is a slow process […], but in the presence of CA the reaction becomes virtually instantaneous. Since CO₂ and HCO₃⁻ generate different pH values of a solution, one of the roles of CA is to regulate intracellular pH […] another […] function is to convert HCO₃⁻ to CO₂ somewhere en route towards the latter’s final fixation by Rubisco.”

“with very few […] exceptions, marine macrophytes are not C 4 plants. Also, while a CAM-like [Crassulacean acid metabolism-like, see my previous post about the book for details] feature of nightly uptake of Ci may complement that of the day in some brown algal kelps, this is an exception […] rather than a rule for macroalgae in general. Thus, virtually no marine macroalgae are C 4 or CAM plants, and instead their CCMs are dependent on HCO₃⁻ utilization, which brings about high concentrations of CO₂ in the vicinity of Rubisco. In Ulva, this type of CCM causes the intra-cellular CO₂ concentration to be some 200 μM, i.e. ∼15 times higher than that in seawater.“

“deposition of calcium carbonate (CaCO₃) as either calcite or aragonite in marine organisms […] can occur within the cells, but for macroalgae it usually occurs outside of the cell membranes, i.e. in the cell walls or other intercellular spaces. The calcification (i.e. CaCO₃ formation) can sometimes continue in darkness, but is normally greatly stimulated in light and follows the rate of photosynthesis. During photosynthesis, the uptake of CO₂ will lower the total amount of dissolved inorganic carbon (Ci) and, thus, increase the pH in the seawater surrounding the cells, thereby increasing the saturation state of CaCO₃. This, in turn, favours calcification […]. Conversely, it has been suggested that calcification might enhance the photosynthetic rate by increasing the rate of conversion of HCO₃⁻ to CO₂ by lowering the pH. Respiration will reduce calcification rates when released CO₂ increases Ci and/but lowers intercellular pH.”

“photosynthesis is most efficient at very low irradiances and increasingly inefficient as irradiances increase. This is most easily understood if we regard ‘efficiency’ as being dependent on quantum yield: At low ambient irradiances (the light that causes photosynthesis is also called ‘actinic’ light), almost all the photon energy conveyed through the antennae will result in electron flow through (or charge separation at) the reaction centres of photosystem II […]. Another way to put this is that the chances for energy funneled through the antennae to encounter an oxidised (or ‘open’) reaction centre are very high. Consequently, almost all of the photons emitted by the modulated measuring light will be consumed in photosynthesis, and very little of that photon energy will be used for generating fluorescence […] the higher the ambient (or actinic) light, the less efficient is photosynthesis (quantum yields are lower), and the less likely it is for photon energy funnelled through the antennae (including those from the measuring light) to find an open reaction centre, and so the fluorescence generated by the latter light increases […] Alpha (α), which is a measure of the maximal photosynthetic efficiency (or quantum yield, i.e. photosynthetic output per photons received, or absorbed […] by a specific leaf/thallus area, is high in low-light plants because pigment levels (or pigment densities per surface area) are high. In other words, under low-irradiance conditions where few photons are available, the probability that they will all be absorbed is higher in plants with a high density of photosynthetic pigments (or larger ‘antennae’ […]). In yet other words, efficient photon absorption is particularly important at low irradiances, where the higher concentration of pigments potentially optimises photosynthesis in low-light plants. In high-irradiance environments, where photons are plentiful, their efficient absorption becomes less important, and instead it is reactions downstream of the light reactions that become important in the performance of optimal rates of photosynthesis. The CO₂-fixing capability of the enzyme Rubisco, which we have indicated as a bottleneck for the entire photosynthetic apparatus at high irradiances, is indeed generally higher in high-light than in low-light plants because of its higher concentration in the former. So, at high irradiances where the photon flux is not limiting to photosynthetic rates, the activity of Rubisco within the CO₂-fixation and -reduction part of photosynthesis becomes limiting, but is optimised in high-light plants by up-regulation of its formation. […] photosynthetic responses have often been explained in terms of adaptation to low light being brought about by alterations in either the number of ‘photosynthetic units’ or their size […] There are good examples of both strategies occurring in different species of algae”.

“In general, photoinhibition can be defined as the lowering of photosynthetic rates at high irradiances. This is mainly due to the rapid (sometimes within minutes) degradation of […] the D1 protein. […] there are defense mechanisms [in plants] that divert excess light energy to processes different from photosynthesis; these processes thus cause a downregulation of the entire photosynthetic process while protecting the photosynthetic machinery from excess photons that could cause damage. One such process is the xanthophyll cycle. […] It has […] been suggested that the activity of the CCM in marine plants […] can be a source of energy dissipation. If CO₂ levels are raised inside the cells to improve Rubisco activity, some of that CO₂ can potentially leak out of the cells, and so raising the net energy cost of CO₂ accumulation and, thus, using up large amounts of energy […]. Indirect evidence for this comes from experiments in which CCM activity is down-regulated by elevated CO₂”

“Photoinhibition is often divided into dynamic and chronic types, i.e. the former is quickly remedied (e.g. during the day[…]) while the latter is more persistent (e.g. over seasons […] the mechanisms for down-regulating photosynthesis by diverting photon energies and the reducing power of electrons away from the photosynthetic systems, including the possibility of detoxifying oxygen radicals, is important in high-light plants (that experience high irradiances during midday) as well as in those plants that do see significant fluctuations in irradiance throughout the day (e.g. intertidal benthic plants). While low-light plants may lack those systems of down-regulation, one must remember that they do not live in environments of high irradiances, and so seldom or never experience high irradiances. […] If plants had a mind, one could say that it was worth it for them to invest in pigments, but unnecessary to invest in high amounts of Rubisco, when growing under low-light conditions, and necessary for high-light growing plants to invest in Rubisco, but not in pigments. Evolution has, of course, shaped these responses”.

“shallow-growing corals […] show two types of photoinhibition: a dynamic type that remedies itself at the end of each day and a more chronic type that persists over longer time periods. […] Bleaching of corals occurs when they expel their zooxanthellae to the surrounding water, after which they either die or acquire new zooxanthellae of other types (or clades) that are better adapted to the changes in the environment that caused the bleaching. […] Active Ci acquisition mechanisms, whether based on localised active H⁺ extrusion and acidification and enhanced CO₂ supply, or on active transport of HCO₃⁻, are all energy requiring. As a consequence it is not surprising that the CCM activity is decreased at lower light levels […] a whole spectrum of light-responses can be found in seagrasses, and those are often in co-ordinance with the average daily irradiances where they grow. […] The function of chloroplast clumping in Halophila stipulacea appears to be protection of the chloroplasts from high irradiances. Thus, a few peripheral chloroplasts ‘sacrifice’ themselves for the good of many others within the clump that will be exposed to lower irradiances. […] While water is an effective filter of UV radiation (UVR)², many marine organisms are sensitive to UVR and have devised ways to protect themselves against this harmful radiation. These ways include the production of UV-filtering compounds called mycosporine-like amino acids (MAAs), which is common also in seagrasses”.

“Many algae and seagrasses grow in the intertidal and are, accordingly, exposed to air during various parts of the day. On the one hand, this makes them amenable to using atmospheric CO₂, the diffusion rate of which is some 10 000 times higher in air than in water. […] desiccation is […] the big drawback when growing in the intertidal, and excessive desiccation will lead to death. When some of the green macroalgae left the seas and formed terrestrial plants some 400 million years ago (the latter of which then ‘invaded’ Earth), there was a need for measures to evolve that on the one side ensured a water supply to the above-ground parts of the plants (i.e. roots¹) and, on the other, hindered the water entering the plants to evaporate (i.e. a water-impermeable cuticle). Macroalgae lack those barriers against losing intracellular water, and are thus more prone to desiccation, the rate of which depends on external factors such as heat and humidity and internal factors such as thallus thickness. […] the mechanisms of desiccation tolerance in macroalgae is not well understood on the cellular level […] there seems to be a general correlation between the sensitivity of the photosynthetic apparatus (more than the respiratory one) to desiccation and the occurrence of macroalgae along a vertical gradient in the intertidal: the less sensitive (i.e. the more tolerant), the higher up the algae can grow. This is especially true if the sensitivity to desiccation is measured as a function of the ability to regain photosynthetic rates following rehydration during re-submergence. While this correlation exists, the mechanism of protecting the photosynthetic system against desiccation is largely unknown”.