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heterotrophs (animals, fungi, bacteria). An increase of temperature can change (or
enhance) microbial activities including bacterial production, respiration, photosyn-
thesis and growth efficiency, as well as bacterial-grazer trophic interactions, which
can result in the rapid mineralization of organic matter in natural waters, particu-
larly in Arctic and Antarctic ecosystems (Norf et al. 2007 ; Vázquez-Domínguez
et al. 2007 ; Falkowski and Oliver 2007 , 2008 ; Peters 2008 ; Norf and Weitere
2010 ; Sarmento et al. 2010 ; Sawicka et al. 2010 ; Nedwell and Rutter 1994 ; Ochs
et al. 1995 ; Felip et al. 1996 ; Nedwell 1999 ; Reay et al. 1999 ; Vrede 2005 ; Morán
et al. 2006 ; López-Urrutia and Morán 2007 ). These studies show that an increase
in temperature may enhance the availability of labile substrates, which is responsi-
ble for an increase of microbial activity at elevated temperature.
The response to temperature of a species or microorganism is characterized by
a number of 'cardinal temperatures': upper and lower limits of temperature for
growth, and an optimum growth temperature included between the two extremes
(Morita 1975 ). Microorganisms living near the lower temperature limit of a spe-
cies can be stimulated either by higher temperature or by higher concentrations
of added substrates in natural waters (Pomeroy et al. 1991 ; Wiebe et al. 1992 ,
1993 ). The microbial metabolism modifies organic nutrients such as glucose and
the functional groups of macromolecules such as fulvic and humic acids of vas-
cular plant origin or autochthonous fulvic acids of algal origin. The consequence
of microbial processing may be the release in water of a variety of end products
such as H 2 O 2 , CO 2 , DIC, PO 4 3 , NH 4
+
and CH 4 (Mostofa and Sakugawa 2009 ;
Ma and Green 2004 ; Fu et al. 2010 ; Palenik and Morel 1988 ; Lovley et al. 1996 ;
Zhang et al. 2004 , 2009 ; Kim et al. 2006 ; Li et al. 2008 ). Algae or phytoplank-
ton biomass can release autochthonous DOM by microbial degradation or assimi-
lation (Mostofa et al. 2009a , b , 2011 ; Stedmon et al. 2007a , b ; Rochelle-Newall
and Fisher 2002 ; Fu et al. 2010 ; Zhang et al. 2009 ; Biddanda and Benner 1997 ;
Yamashita and Tanoue 2004 , 2008 ; Stedmon and Markager 2005 ), and an increase
in temperature can accelerate the bacterial degradation of phytoplankton-derived
organic matter (Wohlers et al. 2009 ; Hoppe et al. 2008 ). Small algae carry out
40-95 % of total grazing on bacteria in the euphotic layer of the temperate North
Atlantic Ocean in summer (Zubkov and Tarran 2008 ). A similar range (37-70 %)
has been observed in the surface waters of the tropical Northeast Atlantic Ocean
(Zubkov and Tarran 2008 ).
In Lake La Caldera it has been observed that at the lower temperature values
(5.0-7.0 ºC) one finds higher bacterial abundance (3.9-7.9 × 10 5 cells ml 1 ,
mean = 6.4) and higher bacterial biomass (4.0-6.7 μ g C L 1 , mean = 5.2) com-
pared to the higher temperature values (7.5-11.1 ºC), which yielded 1.3-2.5 × 10 5
cells ml 1 (mean = 1.8) and 1.3-2.4 μ g C L 1 (mean = 1.7) for bacterial abun-
dance and biomass, respectively (Carrillo et al. 2002 ). The grazing on bacteria
increases with increasing temperature, but the rate of the increase is maximum at
temperatures lower than 2 ºC, whilst bacterial production increases at higher rates
at temperatures higher than 2 ºC. Such a finding, obtained in a microcosm experi-
ment with temperature manipulation ( 1 to 5 ºC) of Antarctic waters, suggests
that bacterial production and bacterial grazing could become uncoupled processes
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