Why are bacteria larger in the deep hypolimnion? An hypothesis comparing temperate and tropical lakes

Bacterial size and morphology are controlled by several factors including predation, viral lysis, UV radiation, and inorganic nutrients. We observed that bacteria biovolume from the hypolimnion of a deep lake in the temperate region (Lake Maggiore, Northern Italy) is larger than that of bacteria living in the layer from surface to 20 m, roughly corresponding to the euphotic/epilimnetic zone. Our first hypothesis was that low temperature could have an important effect on cellular division in the deep layer, resulting in cell enlargement. Data of bacterial size from an high altitude lake in the tropical region (Lake Alchichica, Mexico) revealed a similar pattern, with smaller bacteria present in the 0-20 m layer. In this case, temperature, unlike in Lake Maggiore, could not be invoked as an important factor explaining the different bacterial size in the two zones, as the hypolimnion in the tropics is always near 14°C. Also, the anaerobic conditions of the hypolimnion (characteristic of Lake Alchichica) could not be a determining factor of bacteria biovolume increase because in Lake Maggiore the hypolimnion is always well oxygenated. It is clear, then, that if we consider the two lakes separately we risk reducing the explanation of bacterial size variation in the water column to merely regional factors. Only if we compare the two lakes can we offer a more general explanation for bacterial biovolume variation. Indeed, pooled data from the two lakes showed that the limiting phosphorus could favour the smaller cells with a lower surface:volume ratio present in the 0-20 m in both lakes. Therefore we propose that bacteria in the euphotic zone of oligotrophic lakes are smaller than those in the deeper layer to outcompete algae. On the other hand, with an higher phosphorus concentration and with lower or absent algae, the necessity to be small is not so compelling in the hypolimnion.