Complementary ecophysiological strategies combine to facilitate survival in the hostile conditions of a deep chlorophyll maximum

Abstract

In the deep, cooler layers of clear, nutrient-poor, stratified water bodies, phytoplankton often accumulate to form a thin band or "deep chlorophyll maximum" (DCM) of ecological importance. Under such conditions, these photosynthetic microorganisms may be close to their physiological compensation points and to the boundaries of their ecological tolerance. To grow and survive any resulting energy limitation, DCM species are thought to exhibit highly specialised or flexible acclimation strategies. In this study, we investigated several of the adaptable ecophysiological strategies potentially employed by one such species, Chlamydomonas acidophila: a motile, unicellular, phytoplanktonic flagellate that often dominates the DCM in stratified, acidic lakes. Physiological and behavioural responses were measured in laboratory experiments and were subsequently related to field observations. Results showed moderate light compensation points for photosynthesis and growth at 22°C, relatively low maintenance costs, a behavioural preference for low to moderate light, and a decreased compensation point for photosynthesis at 8°C. Even though this flagellated alga exhibited a physiologically mediated diel vertical migration in the field, migrating upwards slightly during the day, the ambient light reaching the DCM was below compensation points, and so calculations of daily net photosynthetic gain showed that survival by purely autotrophic means was not possible. Results suggested that strategies such as low-light acclimation, small-scale directed movements towards light, a capacity for mixotrophic growth, acclimation to low temperature, in situ exposure to low O(2), high CO(2) and high P concentrations, and an avoidance of predation, could combine to help overcome this energetic dilemma and explain the occurrence of the DCM. Therefore, corroborating the deceptive ecophysiological complexity of this and similar organisms, only a suite of complementary strategies can facilitate the survival of C. acidophila in this DCM.

Photosynthetic and fatty acid acclimation of four phytoplankton species in response to light intensity and phosphorus availability

“…ovata had a combination of a low P max and a high α value, suggesting that it is a typical LL-adapted species (e.g. Richardson et al, 1983;Clegg et al, 2012). These results are in line with the fact that cryptophytes often contribute substantially to the phytoplankton biomass in the deep chlorophyll maximum (Gervais, 1998).…”

“…in the deep chlorophyll maximum), phytoplankton continuously live in LL but with enhanced P i conditions. Photosynthetic characteristics such as P max have been described as a hyperbolic function of the cellular P quota in several species (Smith, 1983;Spijkerman, 2010), but also as correlating with light intensity (Clegg et al, 2012), and in both cases they have been described in a direct positive correlation with growth. The importance of P i as the limiting nutrient for freshwater phytoplankton and its role as structural components of phospholipids should thus be taken into account when analysing photosynthetic properties of algae or when analysing primary production (Van Mooy et al, 2009).…”

“…Surviving in the deep chlorophyll maximum often means a trade-off between higher accessibility of nutrients from deeper layers and lower energy in the form of light from above (e.g. Clegg et al, 2012). In this context, different challenges arise for species that permanently live under LL conditions, species that generally are adapted to HL intensities and those that migrate throughout the water column and face fluctuating light intensities ranging between 1 and 2500 µmol photons m −2 s −1 .…”

Blowing in the wind: how many roads can a phytoplanktont walk down? A synthesis on phytoplankton biogeography and spatial processes

“…(e.g., Heaney & Furnass, 1980) or Microcystis spp. (e.g., Hunter et al, 2008); (ii) the abundance and compositional differences along trophic gradients (Istváno-vics et al, 2007); (iii) the wind-driven horizontal patchiness of bloom-forming cyanobacteria (e.g., Rolland et al, 2013) or other ephemeral blooms (Reynolds et al, 1993); (iv) the development of dense phytoplankton layers in the metalimnion or upper hypolimnion(DCM; e.g., Clegg et al, 2012). Since Nauwerck's (1963) seminal work, Lake Erken became a kind of etalon for phytoplankton research and numerous important mechanisms were explored by studying the Erken phytoplankton such as the importance of P-storage and internal loads (Pettersson et al, 1993) and the key role of alkaline phosphatases in P-uptake (Pettersson, 1980).…”

The flow of nitrogen into the euphotic zone of the Baltic Proper as a result of the vertical migration of phytoplankton: An analysis of the long-term observations and ecohydrodynamic model simulation

“…Such computations are performed assuming a statistical model of vertical chlorophyll distributions (Ostrowska et al, 2007). The VM of phytoplankton can, however, give rise to bimodal vertical phytoplankton distributions (Lips and Lips, Bimodal distribution patterns of motile phytoplankton in relation to physical processes and stratification (Gulf of Finland, Baltic Sea), 2014) and to deep chlorophyll maxima (Clegg et al, 2012;Kononen et al, 2003). In the case of chlorophyll concentration maxima located below the euphotic layer, satellite methods may underestimate primary production.…”