SOIL ORGANISMS - Soil Ecology - page 215

Despite the relative mobility due to their growth patterns, many fungi are dependent on
invertebrates, especially earthworms (Reddell and Spain, 199la), microarthropods and
Diptera larvae, for their dispersal through the soil sometimes involving selective grazing
and spread (Arpin et al., 1980; Swift and Boddy, 1984; Hanlon and Anderson, 1980).

2.3.2 SURVIVAL

Bacterial cells and colonies and fungi may live for more than 50 years in vegetative
form, or as spores (Hattori, 1973). Sneath (1962) surveyed the organisms present on
plant roots in herbaria and observed that while few micro-organisms survived more than
50 years, it would take 1000 years to sterilise the roots. Micro-organisms spend most of
their lives just surviving. Estimates of the turnover time of microbial biomass in soils are
1,000 to 10,000 times less than the values measured in optimal laboratory conditions ( ca.
one day). This means that, on average, bacteria may be inactive for several successive
months or even years after a period of activity and may thus be fully active only every 3
to 30 years. Environmental conditions seldom meet their often-specific requirements.
Unable to move and search for suitable conditions, they possess adaptations to permit
them to survive long periods of inactivity.
Four main mechanisms are involved in this survival:
(i) cannibalism;
(ii) cell maintenance with minimal energy expenditure;
(iii) physical protection in favourable micro-environments; and
(iv) the production of resistant spores.
When nutritional conditions decline and micro-organisms start to die, part of the colony
may survive by ingesting the products of the lysis of dead cells. This cannibalism may
be efficient and 13 dead cells were found to be sufficient to allow the doubling of one
living cell (Nioh and Furusaka, 1968 quoted in Hattori, 1973). The liberation of endoen-
zymes after lysis of dead cells may also allow the production of assimilable substances
in the remaining live cells. Short-term survival of colonies through these processes is
obviously dependent on their initial size.
The maintenance energy of dormant cells is in the range of 1.6-4.3 mg glucose
biomass dry wt Equivalent values for active cells are in the range 0.004-0.17, i.e.,
three orders of magnitude smaller than those measured for growing cells in laboratory
cultures (Anderson and Domsch, 1985a, b). Jenkinson and Ladd (1981) estimated that the
average maintenance energy for inactive microbial biomass in a cropped soil at Rothamsted
(England) was 21 % of biomass per year. Such low values can only be explained by
assuming that starving cells have a different metabolism and may use endocellular rather
than exocellular substrates for their respiration.
Long-term survival involves either endospore production or physical protection in
suitable micro-environments. When environmental conditions are no longer suitable for
growth, some individuals produce endospores, for example, the Gram-positive bacteria
which live outside microaggregates (Hattori, 1973). However, Kilbertus et al. (1977),
observed that, in laboratory cultures, few individuals were able to produce these endospores
because sporulation requires specific chemical compounds which may be absent.