Tải bản đầy đủ - 0 (trang)
IV. Blue–Green Algae in Flooded Soils

IV. Blue–Green Algae in Flooded Soils

Tải bản đầy đủ - 0trang

ENHANCING NITROGEN FIXATION



215



floodwater. As the algae multiply, the large biomass they create serves as a

food source for invertebrates, and the populations of these grazers often

become quite large. The grazers may build up on either the blue-green algae

or on the diatoms or unicellular green algae that precede them in the

succession of organisms in the water. At times, the unicellular green algae that

flourish support large numbers of waterfleas (cladocera), and these animals

may be so dense that they eliminate a blue-green alga that is introduced as an

inoculum (Watanabe et al., 1955a). Ostracods may sometimes be very

abundant in the paddy field, and numbers of 5,000 to 15,000 per square meter

have been recorded. An active population of such ostracods could potentially

consume 12 tons of algae per crop of rice (Grant and Alexander, 1981). These

microcrustaceans are often common in rice fields of Southeast Asia (Fernando, 1977). Grazing by Cypris sp. can prevent the development of or

suppress inocula of blue-green algae tested under laboratory conditions, and

these effects are evident in terms of the biomass or nitrogen-fixing activity of

the algae. The extent of grazing and the influence on nitrogen fixation vary

with the alga available to Cypris sp., the ostracod exhibiting a marked

preference in its feeding habits (Osa-Afiana and Alexander, 1981; Wilson et

al., 1980).

Both cladocerans and ostracods are susceptible to an array of insecticides,

and grazing by these invertebrates can be markedly reduced or totally

abolished by the use of low levels of several insecticides. As early as 1967,

Raghu and MacRae reported that the application of lindane (y-isomer of

hexachlorocyclohexane) in rice fields killed indigenous Cypris, thereby

allowing the flourishing of native blue-green algae. As little as 0.1 pg/ml of

lindane totally prevented the feeding by Cypris sp. on Tolypothrix tenuis in

laboratory tests (Grant and Alexander, 1981), and 0.1 pg of parathion per

milliliter of floodwater completely abolished the feeding on Aulosira by the

same ostracod (Osa-Afiana and Alexander, 1981). Low concentrations of

methyllindane and carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranol

carbamate) brought about the cessation of grazing by the ostracods Cyprinotus carolinensis and Heterocypris luzonensis (Grant et al., 1983a). The

cladoceran Daphnia is also easy to control, and both lindane (Maity and

Saxena, 1979) and parathion (Watanabe et al., 1955b) are effective in this

regard. Field tests in the Philippines demonstrated that Perthane [1,ldichloro-2,2-bis(p-ethylphenyl)ethane] suppressed ostracods and resulted

not only in a 10-fold increase in biomass of blue-green algae and nitrogen

fixation but an increase in rice yield (Grant et al., 1983b). Hence, a significant

factor that often appears to reduce nitrogen fixation by blue-green algae can

be overcome by the use of insecticides. The insecticides themselves, at the low

concentrations needed to suppress grazing by the invertebrates, usually have

no significant effect on algae (Roger and Kulasooriya, 1980).



276



MARTIN ALEXANDER



In the floodwater, algae other than the blue-green algae appear, sometimes

in thick growths and often before the onset of significant development of the

nitrogen-fixing blue-green algae. Because growth of the nonfixing algae is

often limited by the same inorganic nutrients that limit growth of the

blue-green algae, it is likely that algae early in succession may preempt much

of the supply of a common nutrient element, leaving little for the blue-green

algae that appear later in the succession. Alternatively, there may be a direct

competition between nitrogen-fixing and nonfixing algae for a common

limiting nutrient. Support for the view that nonfixing species affect bluegreen algae in such ways comes from a report that suppression of indigenous

algae was necessary to permit the colonization of flooded rice fields by

Tolypothrix tenuis (Hirano et al., 1955; Watanabe, 1962). The finding that the

growth, the nitrogen-fixing activity, or both of T. tenuis or Aulosira sp. is

reduced by indigenous algae or an experimentally introduced green alga adds

weight to the view that indigenous algae may significantly interfere with the

fixation of nitrogen by photosynthetic microorganisms in lowland rice

production (Wilson et al., 1979). Control of indigenous algae is not difficult

because they are quite susceptible to a variety of herbicides (McCann and

Cullimore, 1979; Wright, 1978). Hence, a potentially useful approach to

increasing nitrogen fixation in lowland rice production is the control of

indigenous algae, which often have low or no capacity for nitrogen fixation,

with a suitable herbicide and inoculation of the field with a herbicideresistant alga that has high nitrogenase activity and a rapid growth rate.

Repeated reinoculation would not be necessary if the alga is also one that can

survive the period when the soil is dried following harvest of the rice.

Obviously, the pesticide must be one that does not injure the rice plant.

This approach has been tested in the laboratory. The herbicide used was

simetryne [2,4-bis(ethylamino)-6-(methylthio)-1,3,5-triazine], and the alga

was a variant of Aulosira sp. that was resistant to the pesticide at levels that

suppressed proliferation of indigenous algae. When added to flooded soil,

simetryne controlled the indigenous algae, and this suppression of potential

competitors allowed the herbicide-tolerant Aulosira sp. to proliferate and

reach high levels of nitrogen fixation (Wilson et al., 1979). Strains of

blue-green algae resistant to other pesticides have also been obtained

(Sharma and Gaur, 1981 ; Vaishampayan, 1984).



V. RESISTANT ISOLATES

Several methods have been used to obtain isolates of microorganisms that

are resistant to the chemicals that are introduced for the control of predators

or possible competitors that affect the nitrogen fixers. To obtain a fungicide-



ENHANCING NITROGEN FIXATION



277



resistant strain of Rhizobium, the original sensitive culture may be serially

transferred in media with increasing concentrations of the chemical until a

suitably resistant organism is obtained. This approach has been used to

obtain rhizobia resistant to thiram, Spergon (2,3,5,6-tetrachloro-p-benzoquinone), Phygon (2,3-dichloro- 1 ,Qnaphthoquinone) (Odeyemi and Alexander, 1977), benomyl, streptomycin, and erythromycin (Hossain and

Alexander, unpublished data), and the identical procedure has been used to

obtain an isolate of Pseudomonus sp. resistant to mancozeb (Mendez-Castro

and Alexander, 1983). In contrast, isolates of Rhizobium resistant to streptomycin and erythromycin have been obtained in a single step by plating a

large number of cells on an agar medium containing the toxicant and

selecting the colonies that appeared on the agar (Pena-Cabriales and

Alexander, 1983). On the other hand, the herbicide-resistant alga used in such

studies was obtained by exposing cells to a mutagen and then allowing the

phenotypic expression of the resistance trait to become evident following

introduction of the cells into a herbicide-free medium (Wilson et al., 1979).

No special procedures are needed to obtain insecticide-tolerant blue-green

algae, because they are resistant to the pesticides used to date for the control

of invertebrates.

Each isolate should be tested to be sure that the cultures finally evaluated

for usefulness as inoculants retain their resistance when grown in the absence

of the pesticide and also maintain their ability to fix nitrogen as rapidly as the

wild-type culture. For Rhizobium, the cultures also must be tested to

determine that they still nodulate their leguminous hosts. Fungicide-resistant

rhizobia generally nodulate the same hosts as the wild type and fix nitrogen

in symbiosis with the host as well as the parent culture; indeed, these strains

can be used for inoculation when there is concern that fungicides needed for

the control of pathogens attacking seeds or seedlings may prevent nodulation

(Odeyemi and Alexander, 1977). The herbicide-resistant alga that has been

evaluated retained its nitrogenase activity (Wilson et al., 1979). On occasion,

however, rhizobia that gain the ability to grow in the presence of an inhibitor

simultaneously lose their capacity to fix nitrogen (Schwinghamer, 1964).



VI. CHOICE OF PESTICIDES AND INOCULA

In some instances, the enhanced nitrogen fixation arising from the joint use

of pesticides and pesticide-resistant inocula will be appreciable. As pointed

out above, these inocula need not be used often if they become established in

the soil. Indeed, their establishment will benefit from the repeated use of the

chemical so that persistence in soil of the introduced resistant organism may

be greater than has been observed with nonresistant organisms, for which the



278



MARTIN ALEXANDER



selective advantage of the toxicant is not present. Whether the fixation is

markedly or only modestly enhanced, a savings in cost can be achieved if the

pesticide used to promote nitrogen fixation is also part of the farmer’s system

of pest control. From the viewpoint of the marketing of pesticides, a chemical

that has two functions would have special attraction.

Many of the compounds now widely sold for pest control can be the basis

for the selective enhancement of nitrogen fixation. For example, legume seeds

are often treated with antifungal agents to prevent or minimize such fungal

diseases as damping-off and seed and seedling rots, and these protective

compounds applied to the seeds are often ideal choices for the inhibition of

protozoa preying on rhizobia and bacteria competing with the root-nodule

microsymbionts. The very fact that many fungicides at recommended rates

are deleterious to rhizobia (Fisher, 1976; Fisher and Hayes, 1981; Tu, 1980)

indicates they are good antibacterial as well as antifungal agents, and the data

cited above also show that widely used fungicides have antiprotozoan

activity. Similarly, the insecticide used to control ostracods, cladocerans, or

other invertebrate grazers could be one already being applied to control pests

of rice. Thus, compounds such as lindane or carbofuran would have a dual

purpose. Not only would they control grazing on the algae, but they would

also have the already desired function, lindane being important for protection

of rice against stem borers and carbofuran being widely used to control a

variety of insects, mites, and nematodes. Similarly, in regions where herbicides are already used to control weeds in rice fields, the herbicide might also

be selected for its toxicity to the indigenous algae that otherwise would

reduce nitrogen fixation. Such dual functions might also make pesticide use

more attractive to farmers not already applying them for the control of

particular groups of pests.

The pesticide-resistant inoculum must be carefully chosen, however. Not

only must it be active in nitrogen fixation but it should tolerate stresses of the

habitat, and it should survive well when the crop is no longer being grown or,

for lowland rice, during drying of the soil. Furthermore, the mechanism of its

resistance to the pesticide must be checked to be sure the organism does not

owe its tolerance to its ability to degrade the toxicant, because then the

desired control of both the pests and the species affecting the nitrogen-fixing

inoculum would be lost.



VII. LIMITATIONS

One of the major limitations of the proposed approach is the cost of the

pesticide. This expense, although real, can be absorbed as part of the cost of



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

IV. Blue–Green Algae in Flooded Soils

Tải bản đầy đủ ngay(0 tr)

×