Effect of temperature on diatom volume, growth rate, and carbon and nitrogen content: Reconsidering some paradigms

We examined the response of diatoms to naturally experienced temperatures and tested these hypotheses: (1) diatoms follow the rule that organism size decreases with increasing temperature; (2) diatom growth rate follows a Q10-like response; (3) diatom carbon (C) and nitrogen (N) content per unit volume (V) decrease with increasing size, and changes in temperature affect this relationship; and (4) diatom C : V is the same as that of other phytoplankton. We also present, as predictive equations, relationships between (1) growth rate, temperature, and size; (2) C content and V; and (3) N content and V. Eight diatoms and two flagellates were acclimated for approximately five generations and grown for approximately five more generations at five temperatures (9–258C) on a 14 : 10 light : dark cycle at ;50 mmol photons m22 s21. Growth rate, cell V, and C and N content per cell were measured; relationships between these parameters and temperature were determined. For five diatoms and both flagellates, cell V decreased with increasing temperature; cells decrease by ;4% of their mean V per 8C. Growth rate appeared to increase linearly with temperature in all cases. The literature suggests that a linear response is the rule, not the exception. Temperature did not significantly affect C or N per V of diatom species. When all diatoms were considered, both C and N per V decreased with increasing cell size; our data support the argument that diatoms differ from other protists in this respect, but the difference is less pronounced than stated in previous reports. As diatoms are indisputably a major component of many food webs, estimating their abundance, biomass, and growth rate has been, and will be, an essential component of marine studies. Like all organisms, diatoms are influenced by ambient temperatures, a point that has long been accepted (e.g., Eppley 1972; Goldman and Carpenter 1974). There is now an increasing awareness of global-warming impacts and other anthropogenic and natural changes in marine systems. Concomitantly, there is a need to better understand the influence of temperature on phytoplankton in general and on diatoms specifically. This study improves our ability to assess the effect of temperature change on diatoms by making estimates of how their size, biomass, and growth rate vary over naturally occurring ranges. Furthermore, three biological paradigms are examined: the rule of diminishing size with increasing temperature (Atkinson 1994); the Arrhenius/Q10 relationship (e.g., Cossins and Bowler 1987); and the difference in carbon : volume (C : V) ratio between diatoms and other phytoplankton (Strathmann 1967). Paradigm i—Reviews by Atkinson (1994, 1995) indicated that for ectotherms, size decreases with increasing temperature. One of the few exceptions to this rule was the diatom Phaeodactylum tricornutum (Atkinson 1994), but there are 1 Corresponding author ([email protected]). 2 Present address: School of Biology and Biochemistry, The Queens University of Belfast, Belfast, Northern Ireland BT7 1NN.