Temperature

In considering the effect of temperature on aquatic photosynthesis, it is necessary to distinguish between the effects immediately following a change in temperature and the effects obtained if the plants are allowed to adjust to the new temperature for one to several days. Considering the immediate effects of temperature change first, if the photosynthetic rate of a phyto-plankton population or a marine or freshwater macrophyte is measured under saturating light, at a series of temperatures covering the range from just above freezing to temperatures becoming unfavourable for life, say 5 to 40° C, it will be found that photosynthetic rate per unit biomass at first increases with temperature, then levels off, and finally begins to decrease again, i.e. there is a temperature optimum. The temperature optimum determined in the laboratory varies in accordance with the temperature of the normal habitat of the aquatic species. For example, benthic marine algae from tidal pools exposed to high temperatures in the summer have somewhat higher optimum temperatures for photosynthesis than algae, sometimes even of the same species, obtained from the subtidal zone.921 Figure 11.5

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16 20 25 13U 35 40

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Fig. 11.5 Photosynthetic rate as a function of temperature in samples of the marine benthic alga Ulva pertusa (Chlorophyceae) collected from the sub-tidal zone and from a tidal pool (after Mizusawa et al, 1978).

compares the dependence of photosynthetic rate on temperature for specimens of the green alga Ulva pertusa isolated from these two habitats: the temperature optimum for the subtidal sample is about 5°C lower than that for the pool sample. Yokohama (1973) found that for several species of green, brown and red benthic algae collected around the shore of the Izu peninsula, Japan (34°40' N), the temperature optimum was 5 to 10°C higher for material collected in summer than for samples obtained in winter. The reasons for the decrease in photosynthetic rate at temperatures above the optimum are not well understood: denaturation of enzymes, a runaway increase in respiration and other forms of thermal damage are likely to be involved.

If photosynthesis as a function of irradiance is measured at a series of temperatures (not exceeding the optimum), then it is observed that the maximum photosynthetic rate at light saturation increases with temperature: this is shown for the marine diatom Skeletonema costatum and the lower-littoral-zone red alga Gigartina stellata in Fig. 11.6. It is, however, generally found in experiments of this type (i.e. in which the plants are not given time to adapt to each temperature) that at low light intensities, in the linear part of the P versus Ed curve, variation in temperature has little effect on the rate (i.e. the initial slope, a, is essentially constant over the physiological temperature range. We may reasonably attribute the fall-off in the light-saturated rate (Pm) with decreasing temperature to the enzymic reactions of the dark carboxylation system working progressively more slowly as temperature decreases. The irradiance value, Ek, corresponding to onset of light saturation also decreases together with the temperature: since Ek = Pm/a, then since a is unaffected by temperature, Ek must decrease in parallel with Pm. When, due to low temperature, the photosynthetic dark reactions are working slowly, less light is required to produce NADPH2 and ATP fast enough to saturate the dark reaction system. The comparative insensitivity of the photosynthetic rate at low light intensity to temperature is commonly attributed to the fact that photochemical processes are not very sensitive to temperature, and it is mainly the light reactions that determine rate in the light-limited part of the curve. Respiration, however, does increase markedly with temperature. The failure of the light-limited photosynthetic rate to respond means that as temperature increases the light intensity required for photosynthesis to equal respiration, i.e. the light compensation point, also increases.

Although the photosynthetic rate of any individual aquatic plant or phytoplankton population sample is positively correlated with

Fig. 11.6 Photosynthetic rate as a function of irradiance at different temperatures. (a) In the marine diatom Skeletonema costatum (after Steemann Nielsen and Jorgensen, 1968, and Steemann Nielsen, 1975). The cells were grown under an irradiance of 75 x 1018 quanta m~2 s_1, and exposed to the

temperature (up to the optimum) in short-term experiments, in the case of the whole aquatic ecosystem undergoing slow seasonal temperature changes, there is time both for physiological adaptation by any given species, and for changes in the dominant species present. Thus the relation between primary production and temperature may not be so simple at the ecosystem level. Considering adaptation first, if samples of a phytoplankton population are exposed to each new temperature for some days before the P versus Ed curve is measured at that temperature, then in the case of some species the cells can adapt and the differences between the light-saturated rates of photosynthesis at the different temperatures is much smaller. For example, Steemann Nielsen and Jorgensen (1968) found that in cells of the diatom Skeletonema costatum, grown at 20° C, an immediate transfer to 8°C reduced the light-saturated photosynthetic rate by about two thirds: in contrast, cells grown and measured at 8°C had a light-saturated photosynthetic rate only about 10% lower than that of cells grown and measured at 20° C.

Given that we are interpreting the reduction in light-saturated photo-synthetic rate brought about by an immediate temperature lowering as being due to a lowered specific activity of the dark reaction enzyme system, then we might expect the adaptation to lower temperature to consist of an increase in the cellular content of this enzyme system.1297 Jorgensen (1968) found that the protein content of S. costatum per cell was twice as high in cells grown at 7°C as in cells grown at 20° C. In Dunaliella, Morris and Farrell (1971) observed that the level of photosynthetic enzymes increased as the growth temperature was lowered.

Skeletonema costatum is a ubiquitous diatom, which in nature grows well over a wide range of temperature. It is therefore not surprising that marked adaptive abilities can be demonstrated in the laboratory in this species. Yentsch (1974), however, doubts if most phytoplankton species have this ability. Oceanic measurements from various parts of the world indicate that the photosynthetic rate per unit phytoplankton chlorophyll does not vary in any systematic manner with the water temperature:1484 in all temperature regions, areas with high and low rates of total primary

experimental temperature for 30 min before measurements were carried out. (b) In the multicellular red alga Gigartina stellata (plotted from data of Mathieson and Burns, 1971). Plants were collected from the lower littoral zone of the coast of New Hampshire, USA. The original irradiance values (in foot-candles) have been converted to quantum units.

production are found.1297 This could be due to adaptation but more recent studies suggest that there is little temperature adaptation of most oceanic phytoplankton species in nature, and that the lack of a clear dependence of photosynthetic rate on temperature in the oceans is due to the dominance of different (presumably genetically adapted) species in different temperature regimes.1484 On the Nova Scotian Shelf, Bouman et al. (2005) found that the seasonal changes in temperature were closely followed by changes in phytoplankton community structure.

Notwithstanding the apparent lack of a systematic effect of temperature on photosynthesis in the oceans, it is generally found for shallow, productive coastal waters and for inland waters that both the in situ photosynthetic capacity (the maximum specific photosynthetic rate per unit phytoplankton or macrophyte) and the integral (areal) rate of photosynthesis are positively correlated with the prevailing temperature. For example, in Nova Scotia (Canada) coastal waters, Platt and Jassby (1976) found that in the period July 1973 to March 1975 (temperature range 0-15°C) the photosynthetic capacity of the phytoplankton was linearly dependent on ambient temperature, with a regression slope of 0.53 mg C mg chl a hr_1 °C_1. For the waters of the Hudson estuary and the New York Bight, through an 11-month period, Malone (1977a, b) found that for both netplankton and nanoplankton, the photosynthetic capacity was an exponential function of ambient temperature from 8 to 20°C, and 8 to 24°C, respectively. The Q10 values (proportionate increase in rate per 10°C rise in temperature) were 2.0 to 2.6 for the nanoplankton and 4.0 for the netplankton. Below 8°C the rates for both fractions were higher than expected on the basis of the exponential dependence on temperature. Above 20°C the photosynthetic capacity of the netplankton declined with temperature. In a number of temperate-zone eutrophic lakes, dominated by blue-green algae, the photosynthetic capacity of the phytoplankton has been found to vary throughout the year in an exponential manner with the prevailing temperature,117,444,641 the Q10 being about 2 over the approximate range 4 to 20°C. It should be noted that the changes in coastal and inland waters we have just been considering are seasonal changes, which must include changes in the character (species composition, physiological adaptation) of the phytoplankton as well as in the temperature itself, and this may account for the very high seasonal Q10 of 4.0 observed by Malone (1977b) in the New York Bight. If a given phytoplankton population is exposed to a given series of temperatures (i.e. allowing no time for adaptation or species succession), photosynthetic capacity varies exponentially with temperature with a Q10 of about 2.3.1333

It is commonly observed in the ocean that the productivity of the phytoplankton population, expressed as specific photosynthetic rate, P* (mmoles CO2 mg chl a-1 hr-1) tends to decrease as phytoplankton cell size increases. Shiomoto et al. (1997), from their measurements in the North Pacific Ocean and the Bering Sea, found that this generalization held in the southern parts of this region, where the in situ temperature was >10°C. In the northern subarctic region, where the water temperature was <10°C, picoplankton (<2 mm) productivity was generally about equal to, or lower than that of the larger sized phytoplankton (2-10 and 10-200 mm). They concluded that picoplankton productivity is more sensitive to in situ temperature than is that of larger sized phytoplankton, and that this accounts for the observation that the productivity of pico-plankton is not significantly higher than that of the larger phytoplankton in the subarctic region.

We noted earlier that when a given phytoplankton population or macrophyte sample is exposed to different temperatures, Ek, the irradi-ance value corresponding to onset of saturation, increases as temperature increases (Fig. 11.6). This also appears to be true on a seasonal basis (i.e. including any changes in the phytoplankton population). In Lough Neagh, N. Ireland, Jewson (1976) found the in situ Ek to be correlated with prevailing temperature throughout a two-year period, rising 2.5- to 4.5-fold between the winter (temperature 3-4°C) and late summer (18.5°C). This rise in Ek is best seen as being part of the rise in photosyn-thetic capacity with temperature, than as a separate phenomenon. Given the increase in maximum specific photosynthetic rate, due to the increased rate of operation of the carboxylation system, it simply means that more light is required to produce NADPH2 and ATP fast enough to saturate the dark reaction system.

In summary, there is no doubt that in coastal and inland waters temperature, operating mainly through its effect on photosynthetic capacity, is a major limiting factor for photosynthesis. In simple terms we may say that in any optically deep water body at a temperature below the photosynthetic optimum, photosynthesis per unit volume is likely to be limited by light in the lower part of the euphotic zone, by temperature in the upper part of the euphotic zone and by CO2 everywhere. In water bodies so shallow that adequate light penetrates to the bottom, temperature (apart from CO2) can become the dominant factor controlling photosynthesis in the whole system. For a number of stratified lakes with high light penetration in Ontario (Canada), Dale (1986) concluded that depth of colonization by macrophytes was limited by temperature. In the

Beaufort Channel, North Carolina, USA - a shallow estuary of average depth 1m - Williams and Murdoch (1966) observed a pronounced seasonal cycle in phytoplankton primary production, which appeared to follow the water temperature cycle but not to be related to insolation. They point out that such temperature-driven annual cycles in the productivity of marine phytoplankton are likely to be characteristic of shallow embayments in temperate regions.

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