Ecology and Diversity of Extremophiles
our culture studies we also found relatively rapid growth rates at 0°C, provided substrate
concentration was high [17]. Similarly, Pomeroy and Deibel [10] and Pomeroy et al. [11]
demonstrated that the respiration of natural populations, while minimal or unmeasureable
in unamended samples, was substantial if amended with substrate, or it the temperature
was elevated from the spring minimum temperature of -1 or 0° to +2 or 3.5°C. Nedwell
and Rutter [1990] have provided evidence for a mechanism that may explain these data.
They found that at temperatures approaching the minimum for growth of two
psychrotolerant Antarctic bacteria, there was a decrease of specific affinity for substrate.
Thus at low temperatures, higher substrate concentrations are required for growth than at
higher temperatures. Figure 1 presents a graphical example of this statement. We find
this same relationship in many cultures of marine bacteria. The problem appears to be not
the lack of growth per se at minimal temperatures, but the lack of sufficient substrate in
the natural environment.
Studies in cold sediments [9] and in sea ice [4], in which bacterial activities approach
those in warmer waters, have also been used to dispute the hypothesis. However, these
environments contain relatively high natural substrate concentrations and thus are not
comparable to the conditions in the water column (It should be noted that the authors
cited do not make this claim.).
A second problem involves the source of substrates for bacteria. As reviewed by
Pomeroy and Wiebe [12], a wide variety of potential sources of substrates exist, among
them the exudates from algae, which should be maximal during blooms. However, since
actively growing phytoplankton usually release ≤10% of their photosynthate as dissolved
products (e. g. [5]), bacteria could not be expected to process more than this amount
unless there were algal cell lysis or cell breakage due to grazing. In the initial period of
the spring blooms in Conception Bay, few zooplankters were present and their
development time at 0° approaches 70 days [6]. There was no evidence of algal cells
lysing. Thus there appears to be a time window at the start of the spring bloom when the
algae are growing rapidly and accumulating, but most of their production is not available
to the bacteria.
The third problem is one of data interpretation. Investigators studying bacterial activity
in cold temperate spring blooms have arrived at different conclusions using apparently
similar methods: direct bacterial counts as a measure of biomass and thymidine or leucine
incorporation as a measure of bacterial activity. For example, van Boekel et al. [15]
reported that the biomass of bacteria and protozoa remained low during a
Phaeocystis
bloom in the North Sea until the bloom started to decline. However, Rivkin et al. [14],
while noting that the biomass of bacteria was low in a Conception Bay spring bloom,
found rapid per-cell growth rates for the bacteria, comparable to those in temperate
waters. Rivkin et al. [14] have compared their bacterial productivity data with those in
Pomeroy et al. [13] and concluded that the discrepancy between the two data sets involves
the conversion factors for uptake of radioactive thymidine to bacterial growth rates rather
than in the data themselves.
Since the bacterial biomass in both studies [14,13] was equally low, the bacterial
productivity data would not necessarily dispute the respiration results of Pomeroy and
Deibel [10] or Pomeroy et al. [13]. Thymidine uptake is an extremely sensitive assay
(nanomoles of uptake), while respiration measurements are orders of magnitude less
