Sven Beer

Biodiversity of marine plants in an era of climate change: some predictions based on physiological performance

There are too few data to allow any confident statements on the effects of global climate change on the diversity of marine plant life. However, on the basis of information available in the literature, it is possible to make predictions about the physiological responses of plants under situations of anticipated increases in CO2 concentrations, temperature and UV-B fluxes and point out how differences in the responses of major marine plant groups might lead to changes in performance and distribution of these organisms. For instance we may predict that macrophytes such as seagrasses will show enhanced photosynthetic rates and growth as atmospheric CÜ2 levels continue to rise whilst many intertidal macroalgae are already at CO2 saturation and may not show any enhanced performance as COi increases. Decreasing ozone concentrations in the stratosphere will lead to enhanced UV-B fluxes and could consequently favour those species with UV tolerance or repair mechanisms. It has been suggested that interactions between temperature range and photoperiod can be responsible for excluding species from particular regions of the worlds oceans. Other species might be affected in this way as temperatures at a given latitude change. Temperature will also influence the relationship between atmospheric and dissolved CC>2 and the proportions of the various components of dissolved inorganic carbon available for growth. Climate change may well have other effects on the efficiency with which marine plants use other resources such as N, Fe or Zn and these will also be discussed.

Photosynthetic rates of Ulva (Chlorophyta) measured by pulse amplitude modulated (PAM) fluorometry

In this work, we attempt to quantify pulse amplitude modulated (PAM) chlorophyll fluorescence measurements in marine macroalgae in terms of photosynthetic rates. For this, the effective electron transfer quantum yield of photosystem II measured for two Ulva species, at various irradiances and inorganic carbon (Ci) concentrations, was multiplied by the estimated flux of photons absorbed by the photosynthetic pigments associated with this photosystem. The rates of electron transport (ETR) calculated in this way were then compared with rates of photosynthetic O2 evolution as measured in association with the fluorescence measurements. It was found that the calculated ETRs correlated linearly with rates of ‘gross’ O2 evolution (net O2 exchange corrected for dark respiration as measured immediately after turning off each irradiance level) within the range of irradiances applied (up to 608 µmol photons m−2s−1). The average molar O2/ETR ratio was 0.238 for Ulva lactuca and 0.261 for Ulva fasciata, which is close to the theoretical maximal value of 0.25. Rates of O2 evolution at various concentrations of Ci also showed linear correlations with ETR, and the average molar O2/ETR ratio was 0.249. These results show that PAM fluorometry can be used as a practical tool for quantifying photosynthetic rates at least under moderate irradiances in thin-bladed macroalgae such as Ulva possessing a CO2-concentrating system. A comparison between the PAM-101 (which was used in Sweden for the light- and Ci-response measurements of Ulva lactuca) and the newly developed portable Diving-PAM (used for Ulva fasciata in Israel) showed that such fluorescence-based photosynthetic rate measurements can also be carried out in situ.

TIDES, LIGHT AND THE DISTRIBUTION OF ZOSTERA MARINA IN LONG ISLAND SOUND, USA

The disappearance of Zostera marina L. (eelgrass) in western Long Island Sound has been attributed to the eutrophication-induced increase in light attenuation in the waters of that area. In this work we explore whether the much higher tidal range in the western (3 m) than in the eastern (1 m) Sound could further reduce light availability and, therefore, restrict the vertical distribution of eelgrass. Assuming that the spring low water level determines the upper limit of distribution and the depth of minimum light required for growth determines the lower limit, then the vertical zone for growth in the western Sound is limited to a 1 m fringe. Eelgrass within this narrow range would be vulnerable to exposure during storm events. In the eastern Sound, the viable range for eelgrass growth is 4 m, and similar disturbances would be less likely to affect the seagrass population (since deeper growing shoots may provide energy for shallow-growing ones). A further evaluation of tidal effects on the light availability for Z. marina in Long Island Sound was pursued by allowing surface irradiance and depth of the water column above seagrass canopies to fluctuate over 24 h periods in a Lambert-Beer Law based model. It revealed that the diel benthic light curves were skewed or had a bimodal (rather than sinusoidal) shape and that the number of hours of growth-saturating (about 300 μmol quanta m−2 s−1) light was smaller as light attenuation and tidal ranges increased in the western Sound. Therefore, the large tidal ranges may have contributed to the disappearance of eelgrass in the western Sound. We suggest that, due to the significant influence of tides on light availability resulting in light restrictions for benthic vegetation, tides should be taken into account when managing coastal waters with the aim of allowing for the successful growth of seagrasses.

Chapter 9 – Measurements of photosynthetic rates in seagrasses

There are several ways of measuring the photosynthetic rates of seagrasses. Until a few years ago, gas exchange measurements were used most frequently. In these, O 2 concentrations are measured either by electrodes or by Winkler titrations, and because of practical considerations, mostly in the laboratory. However, even when applied in the field, these methods require incubations in enclosures (“bottle experiments”). In addition to O 2 , it is also possible to measure CO 2 exchange, but this requires more sophisticated and expensive instrumentation, and is mainly applicable to seagrasses exposed to air. Another way of measuring CO 2 uptake is by the use of 14 C. One limitation in gas exchange measurements lies in the intrinsic need to enclose the leaves; the conditions in such enclosures invariably differ from those of the surrounding seawater in terms of water movement and irradiance. Therefore, a trend for using less “intrusive” methods has evolved. One such method is pulse amplitude modulated (PAM) fluorometry, in which the quenching of chlorophyll fluorescence by photosynthetic quantum yield is utilized.

Limitations in the use of PAM fluorometry for measuring photosynthetic rates of macroalgae at high irradiances

Pulse amplitude modulated (PAM) fluorometry can be used for measuring photosynthetic electron transport rates (ETR) of marine angiosperms and macroalgae both in the laboratory and in situ. Regarding macroalgae, quantitative values and linear correlations between ETR and rates of photosynthetic O2 evolution have so far been shown only for a few species under low irradiances. As a logical continuation of such work, the aim of the present study was to (a) assess to what degree high irradiances would limit such measurements and (b) evaluate whether PAM fluorometry could be used quantitatively also for other marine macroalgae from different phyla. This was done by comparing ETR with rates of gross O2 evolution (net O2 exhcange corrected for dark respiration) at various irradiances for the green alga Ulva lactuca grown at two irradiances, the brown algae Fucus serratus and Laminaria saccharina and the red algae Palmaria palmata and Porphyra umbilicalis. At low irradiances, there was a clear positive correlation between O2 evolution and fluorescence-based ETR. At high irradiances, however, all algae featured an apparent decrease in ETR while O2 evolution remained relatively constant, and this resulted in markedly increasing O2/ETR ratios. This anomaly could be nicely illustrated in plots of O2/ETR as a function of the effective quantum yield of photosystem II (Y). Such plots showed that the O2/ETR ratio generally started to increase when Y reached a critical low value of c. 0.1. It was further found that the irradiance at which this value was reached varied with species and previous light histories. Thus, it is the Y value, rather than the irradiance per se

The acquisition of inorganic carbon by the seagrass Zostera marina

Abstract In this work, it is elucidated to what degree the seagrass Zostera marina L. can utilise HCO 3 − as an external inorganic-carbon source for photosynthesis, and which of two possible systems for its acquisition is in effect. It was found that HCO 3 − was used as a major source of inorganic carbon at the normal seawater-pH of 8.2, and that bulk CO 2 contributed only marginally (less than 20%) to photosynthesis at the pH. By comparing photosynthetic rates at pH 8.2 and 9.0, it was deduced that CO 3 2− could not be utilised. It was further found that HCO 3 − could be acquired via extracellular dehydration to CO 2 , as catalysed by external/surface-bound carbonic anhydrase, prior to inorganic-carbon uptake. Indications for active, ATPase-mediated, HCO 3 − transport was also found, but an inhibitor of extracellular carbonic anhydrase affected photosynthetic rates more than did the less specific ATPase inhibitors. The rationale for HCO 3 − dehydration versus its direct uptake is discussed with regard to the photosynthetic performance of seagrasses, many of which are inorganic-carbon limited in their natural habitats.

Photosynthetic carbon utilization by Enteromorpha intestinalis (Chlorophyta) from a Swedish rockpool

Enteromorpha intestinalis grows along the Swedish west coast in rockpools which are isolated from the open seawater for long time periods and where, therefore, the inorganic carbon content is low and the pH is high during the day. In order to investigate how E. intestinalis could grow under such apparently CO2-constraining conditions, we measured its photosynthetic responses to inorganic carbon in the presence of an inhibitor of external/surface-bound carbonic anhydrase (acetazolamide) as well as an inhibitor of HCO3 − transport via anion exchange (4,4′-diisothiocyanatostilbene-2,2′-disulfonate). The results show that both HCO3 − dehydration via surface-bound carbonic anhydrase and HCO3 − transport via a 4,4′-diisothiocyanatostilbene-2,2′-disulfonate-sensitive mechanism were present in E. intestinalis, but only HCO3 − uptake via the putative transporter was operative in rockpool water during most of the photic period (pH > 9.4, inorganic carbon <0.45 mol m−3 and CO2 < 0.05 mmol m−3). The advantage of such...

Bicarbonate uptake in the marine macroalga Ulva sp. is inhibited by classical probes of anion exchange by red blood cells

We demonstrate in this work that HCOinf3sup−uptake in the marine macroalga Ulva sp. features functional resemblances to anion transport mediated by anion exchangers of mammalian cell membranes. The evidence is based on (i) competitive inhibition of photosynthesis by the classical red-blood-cell anion-exchange blockers 4,4′-dinitrostilbene-2,2′-disulfonate and 4-nitro-4′-isothiocyanostilbene-2,2′-disulfonate under conditions where HCOinf3sup−, but not CO2, was the inorganic carbon form taken up; (ii) inhibition of HCOinf3−uptake by pyridoxal phospate, indicating the involvement of lysine residues in the binding/translocation of HCOinf3sup−; and (iii) inhibition of HCOinf3sup−(but not of CO2) uptake by exofacial trypsin treatments, indicating the functional involvement of a plasmalemma protein. It is suggested that HCOinf3sup−uptake mediated by such a putative anion transporter can be a fundamental step in providing inorganic carbon for the CO2-concentrating system of marine marcoalgae in an environment where the HCOinf3sup−concentration is high, but the CO2 concentration and rates of uncatalyzed HCOinf3sup−dehydration are low.

Photosynthesis and photorespiration of marine angiosperms

The marine angiosperms, or seagrasses, constitute a small but important plant group, common to many coastal habitats. In spite of their high productivity within near-shore ecosystems, the photosynthetic mechanisms of these plants have received relatively little investigation. The exogenous inorganic carbon form utilized by seagrasses is either CO2 and HCO3− or, according to another view and/or depending on species, CO2 only. In both cases, ambient CO2 concentrations limit photosynthetic rates at saturating light. In species using HCO3−, this is owing to a rather ineffective HCO3−-utilization system; although seawater contains sufficient HCO3− to saturate photosynthesis, photosynthetic rates are strongly enhanced by additional CO2. Seagrasses are characterized by relatively high 13C/12C ratios, and net photosynthetic rates do not appear to be strongly influenced by photorespiration. However, photosynthetic C4 acid metabolism is not common within this plant group; most species investigated show a more or less typical C3 incorporation pattern of inorganic carbon. Such an apparently contradictory behaviour could be explained by the photosynthetic carbon assimilation system being “enclosed” by the unstirred water layer surrounding the leaves and/or by an alternate CO2 concentrating mechanism. This would lead to efficient refixation of photorespired CO2 and alleviate the ability of ribulose bisphosphate carboxylase-oxygenase both to act as an oxygenase and to discriminate against 13C. Effects of environmental parameters such as light and temperature on biochemical pathways can presently not be evaluated; instead, these parameters are discussed only as affecting photosynthetic rates and productivity. Many species feature low light compensation and saturation levels such as are found in shade-adapted plants. Others show higher saturation levels suggestive of light limitations even at shallow depths. It seems that most seagrasses have temperature optima for both photosynthesis and growth at around 30°C. High ambient salt concentrations are reduced in the metabolically active epidermal cells by compartmentalization into underlying cell layers. In cases where light is not a limiting factor for growth, high hydrostatic pressure at increasing depths may limit growth by deviating the flow of photosynthetically derived O2 out of the lacunae rather than downwards to sustain root growth.

EFFECTS OF PHOTON FLUENCE RATE AND LIGHT SPECTRUM COMPOSITION ON GROWTH, PHOTOSYNTHESIS AND PIGMENT RELATIONS IN GRACILARIA SP.1

The optimal photon fluence rate for growth of tha llus tips of Gracilaria sp. was low (about 100 μE·–2·1); higher photon fluence rates inhibited growth. Both phycoerythrin (PE) and chlorophyll (chl) contents decreased with increasing photon fluence rates (up to 100 μE·–m–2s–1) in a fashion inverse to the growth response. Chl/PE ratios varied directly as the growth response over a larger photon fluence rate range. The peak chl/PE ratios were obtained at a photon fluence rate optimal for growth, suggesting that this parameter may be used to estimate in situ growth rates. A low compensation point (about 7 μE·–2s–1) was observed for low light (15 μE·–2s–1) grown plants. This compensation point was also obtained for growth in the long–term (5–6 weeks) experiments. Plants grown at 60 and 140 μE·–2s–1 showed higher light compensation and saturation points, suggesting that the variations in pigment composition found between the different treatments determine the photosynthetic responses at sub–optimal photon fluence rates. Photosynthetic rates at light saturation were the same, on a biomass basis, for plants grown at the various photon fluence rates. Thus, the photosynthetic dark reactions were not influenced by previous light regimes. It is suggested that maximal photosynthetic rates expressed on a biomass basis better reflect the potential productivity at tight saturation than if expressed on a pigment basis. Gracilaria sp. grew better under non–filtered fluorescent and greenish than under reddish and blue–enriched light of equal and sub–optimal photon, fluence rate. However, the pigment relations of the algae did not change in a direction complementary to the light composition at which they grew. This, together with the relatively higher photosynthetic rates under reddish and blueish light for plants previously grown under reddish and blueish light, suggests that adaptations to variouslight spectra are based on mechanisms different from complementary chromatic adaptation of the pigments.