34 . SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



Mechanisms of Calcification 



The mechanisms of calcification in Clathromorphum also 

 need to be fully understood with intensive laboratory research if 

 sources of measurement variation are to be removed from these 

 archives. In this genus multiple calcification sites and autogenic 

 dissolution are general capabilities of the organism. Whether 

 secondary calcification (e.g., sporangial wall calcification) is 

 metabolic or, as is more likely, simply carbonate chemistry in 

 the mostly enclosed conceptacle cavity environment remains 

 unknown. Although the precise chemistry of carbonate disso- 

 lution by both vegetative cells (fusions) and sporangia remains 

 unknown, there is little question of its routine occurrence. This 

 is also true for the interfilament (outer cell wall) formation of 

 calcite crystals, although a role for light in directly controlling 

 chemistry seems possible in this case. 



As we noted in the Introduction, all coralline genera have 

 cell walls with prismatic, high-magnesium calcite crystals in- 

 serted in the cell wall perpendicular to the cell lumen. Since 

 cell elongation in some genera is progressive with depth in the 

 perithallium and in many genera each filament has independent 

 cell division and growth, cells have a very thin outer wall of flat 

 crystals parallel to the filament axis that functions as a ghde 

 plane (Adey et al., 2005). On the other hand, two very different, 

 concurrent mechanisms for calcification in Clathromorphum are 

 distinguished in this volume. Although developed specifically 

 on the basis of the abundant information available for C. com- 

 pactum, there is little obvious difference found in the tissues of 

 C. nereostratum. Primary calcification is likely an ion pump- 

 driven process that forms inner cell walls that include high- 

 magnesium calcite crystals. These crystals are embedded in an 

 organic matrix that provides a mineralogical template. At winter 

 temperatures (0°C to -1.8° C) on the southern Labrador Coast, 

 cells of C. compacttim are typically 9-10 \\m long; at summer 

 temperatures (3°C-4°C), they are longer (12-13 pm) and have a 

 length-based increase of inner-wall carbonate volume. 



The production of new inner cell walls with calcite crystals 

 appears to be directly controlled by temperature; the higher the 

 temperature is, the greater the vertical extension of the organic 

 wall is. Vertical extension (growth) occurs with crystal emplace- 

 ment in the inner-wall matrix of a meristem cell, along with or- 

 ganic wall material. In the Labrador winter, not only is there a 

 roughly 20% reduction in cell length, there is also an approxi- 

 mately 30% reduction in the number of cells produced per month. 

 These factors are reflected in the 60% reduction in yearly growth 

 from the south northward on the North American coast and a 

 further 75% reduction continuing northward to northern Baffin 

 Island (Figures 21, 22). The thickness of inner cell walls, with 

 their embedded prismatic calcite crystals being radially oriented 

 and less than a micron in length, is constant with temperature, 

 light, and time. It would appear that Clathromorphum species 

 and perhaps crustose corallines in general have "boxed" them- 

 selves into a carbonate framework that is limited by temperature 

 ccjntrol of metabolic calcite crystal emplacement and not by po- 

 tential productivity. If the growth curve of Figure 22 is extended 



to a tropical temperature of 25°C, yearly accretion would be 1-2 

 mm yr~', approximately the rate of algal ridge accretion in the 

 Caribbean by Porolithon pachydermum (Adey, 1978). 



Except for a few obligate parasites, crustose corallines are 

 photoautotrophs with plastids, chlorophyll ti, and phycobilipro- 

 tein pigments; photosynthesis is required at least at some point 

 in a plant's life cycle. Whereas Adey (1998) had presented a 

 model for crustose coralline calcification that is driven by ion 

 pumps, some recent researchers (Ries, 2010) have suggested 

 that all calcification in corallines is directly driven by the pho- 

 tosynthetic removal of CO, followed by a corresponding pH 

 increase and chemical precipitation. The calcified tropical green 

 algae Halhneda is a well-published model for CO, removal- 

 induced aragonite formation in enclosed algal tissues. However, 

 as we have described, at least in Clathromorphum, where winter 

 growth in Arctic darkness and additionally beneath sea ice is a 

 significant part of total calcification, the formation of high- 

 magnesium calcite crystals must be metabolically driven using 

 stored photosynthate. 



Precipitation might be the dominant form of calcification in 

 non melobesioid tropical genera of corallines, which have been the 

 subject of most calcification research. However, a survey of high- 

 magnification SEM scans of several tropical genera has shown 

 that inner-wall calcification morphology is quite similar to that 

 described here. It is likely that other Arctic-Subarctic genera (e.g., 

 Lithothamnion and Leptophytum), which lack significant interfila- 

 ment calcification (see Adey et al., 2005), also calcify metabolically, 

 not only during the dark of Arctic winters but also year-round. 



Priaaary Production and Growth Rates 



Crustose corallines, particularly at higher latitudes, are very 

 low-level primary producers (Adey, 1970, 1973) and commonly 

 are the deepest abundant algae at all latitudes. In laboratory ex- 

 periments, Clathromorphum circumscriptum, an intertidal and 

 shallow sublittoral plant, demonstrated a compensation point 

 of 35 lux at 0.3°C; at the highest temperatures usually encoun- 

 tered by this species (10°C-16°C), the compensation point was 

 200-300 lux. In the same experiments, the deeper-water Subarc- 

 tic Leptophytum leave showed compensation points at optimum 

 growth temperatures (5°C-12°C) of 35 lux. Both C. compac- 

 tum and C. nereostratum are mid-depth species. It is likely that 

 the compensation points at the temperatures that both of these 

 species encounter range from approximately 10 to 50 lux. As 

 we demonstrated above, the growth rate of C. compactum in 

 the northwestern Atlantic ranged from roughly 100 to 400 pm 

 yr"' latitudinally; our analysis suggests a direct relationship with 

 temperature. One could argue that light could also be a factor 

 over that north-south range. However, Gulf of Maine waters are 

 highly turbid because of tidal mixing; also, in the years that WHA 

 collected the Gulf of Maine samples from which the growth data 

 were extracted, Saccharina kelp was abundant in most C. com- 

 pactum habitats. In contrast, the Labrador C. compactum sam- 

 ples used for growth analysis were taken from very clear waters, 

 with near-tropical visibilities, and from savanna-like habitats of 



